Spring Cloud Stream Core
1. Quick Start
You can try Spring Cloud Stream in less then 5 min even before you jump into any details by following this three-step guide.
We show you how to create a Spring Cloud Stream application that receives messages coming from the messaging middleware of your choice (more on this later) and logs received messages to the console.
We call it LoggingConsumer
.
While not very practical, it provides a good introduction to some of the main concepts
and abstractions, making it easier to digest the rest of this user guide.
The three steps are as follows:
1.1. Creating a Sample Application by Using Spring Initializr
To get started, visit the Spring Initializr. From there, you can generate our LoggingConsumer
application. To do so:
-
In the Dependencies section, start typing
stream
. When the “Cloud Stream” option should appears, select it. -
Start typing either 'kafka' or 'rabbit'.
-
Select “Kafka” or “RabbitMQ”.
Basically, you choose the messaging middleware to which your application binds. We recommend using the one you have already installed or feel more comfortable with installing and running. Also, as you can see from the Initilaizer screen, there are a few other options you can choose. For example, you can choose Gradle as your build tool instead of Maven (the default).
-
In the Artifact field, type 'logging-consumer'.
The value of the Artifact field becomes the application name. If you chose RabbitMQ for the middleware, your Spring Initializr should now be as follows:
-
Click the Generate Project button.
Doing so downloads the zipped version of the generated project to your hard drive.
-
Unzip the file into the folder you want to use as your project directory.
We encourage you to explore the many possibilities available in the Spring Initializr. It lets you create many different kinds of Spring applications. |
1.2. Importing the Project into Your IDE
Now you can import the project into your IDE. Keep in mind that, depending on the IDE, you may need to follow a specific import procedure. For example, depending on how the project was generated (Maven or Gradle), you may need to follow specific import procedure (for example, in Eclipse or STS, you need to use File → Import → Maven → Existing Maven Project).
Once imported, the project must have no errors of any kind. Also, src/main/java
should contain com.example.loggingconsumer.LoggingConsumerApplication
.
Technically, at this point, you can run the application’s main class. It is already a valid Spring Boot application. However, it does not do anything, so we want to add some code.
1.3. Adding a Message Handler, Building, and Running
Modify the com.example.loggingconsumer.LoggingConsumerApplication
class to look as follows:
@SpringBootApplication
@EnableBinding(Sink.class)
public class LoggingConsumerApplication {
public static void main(String[] args) {
SpringApplication.run(LoggingConsumerApplication.class, args);
}
@StreamListener(Sink.INPUT)
public void handle(Person person) {
System.out.println("Received: " + person);
}
public static class Person {
private String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String toString() {
return this.name;
}
}
}
As you can see from the preceding listing:
-
We have enabled
Sink
binding (input-no-output) by using@EnableBinding(Sink.class)
. Doing so signals to the framework to initiate binding to the messaging middleware, where it automatically creates the destination (that is, queue, topic, and others) that are bound to theSink.INPUT
channel. -
We have added a
handler
method to receive incoming messages of typePerson
. Doing so lets you see one of the core features of the framework: It tries to automatically convert incoming message payloads to typePerson
.
You now have a fully functional Spring Cloud Stream application that does listens for messages.
From here, for simplicity, we assume you selected RabbitMQ in step one.
Assuming you have RabbitMQ installed and running, you can start the application by running its main
method in your IDE.
You should see following output:
--- [ main] c.s.b.r.p.RabbitExchangeQueueProvisioner : declaring queue for inbound: input.anonymous.CbMIwdkJSBO1ZoPDOtHtCg, bound to: input
--- [ main] o.s.a.r.c.CachingConnectionFactory : Attempting to connect to: [localhost:5672]
--- [ main] o.s.a.r.c.CachingConnectionFactory : Created new connection: rabbitConnectionFactory#2a3a299:0/SimpleConnection@66c83fc8. . .
. . .
--- [ main] o.s.i.a.i.AmqpInboundChannelAdapter : started inbound.input.anonymous.CbMIwdkJSBO1ZoPDOtHtCg
. . .
--- [ main] c.e.l.LoggingConsumerApplication : Started LoggingConsumerApplication in 2.531 seconds (JVM running for 2.897)
Go to the RabbitMQ management console or any other RabbitMQ client and send a message to input.anonymous.CbMIwdkJSBO1ZoPDOtHtCg
.
The anonymous.CbMIwdkJSBO1ZoPDOtHtCg
part represents the group name and is generated, so it is bound to be different in your environment.
For something more predictable, you can use an explicit group name by setting spring.cloud.stream.bindings.input.group=hello
(or whatever name you like).
The contents of the message should be a JSON representation of the Person
class, as follows:
{"name":"Sam Spade"}
Then, in your console, you should see:
Received: Sam Spade
You can also build and package your application into a boot jar (by using ./mvnw clean install
) and run the built JAR by using the java -jar
command.
Now you have a working (albeit very basic) Spring Cloud Stream application.
2. What’s New in 2.0?
Spring Cloud Stream introduces a number of new features, enhancements, and changes. The following sections outline the most notable ones:
2.1. New Features and Components
-
Polling Consumers: Introduction of polled consumers, which lets the application control message processing rates. See “Using Polled Consumers” for more details. You can also read this blog post for more details.
-
Micrometer Support: Metrics has been switched to use Micrometer.
MeterRegistry
is also provided as a bean so that custom applications can autowire it to capture custom metrics. See “Metrics Emitter” for more details. -
New Actuator Binding Controls: New actuator binding controls let you both visualize and control the Bindings lifecycle. For more details, see Binding visualization and control.
-
Configurable RetryTemplate: Aside from providing properties to configure
RetryTemplate
, we now let you provide your own template, effectively overriding the one provided by the framework. To use it, configure it as a@Bean
in your application.
2.2. Notable Enhancements
This version includes the following notable enhancements:
2.2.1. Both Actuator and Web Dependencies Are Now Optional
This change slims down the footprint of the deployed application in the event neither actuator nor web dependencies required. It also lets you switch between the reactive and conventional web paradigms by manually adding one of the following dependencies.
The following listing shows how to add the conventional web framework:
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
The following listing shows how to add the reactive web framework:
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-webflux</artifactId>
</dependency>
The following list shows how to add the actuator dependency:
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-actuator</artifactId>
</dependency>
2.2.2. Content-type Negotiation Improvements
One of the core themes for verion 2.0 is improvements (in both consistency and performance) around content-type negotiation and message conversion. The following summary outlines the notable changes and improvements in this area. See the “Content Type Negotiation” section for more details. Also this blog post contains more detail.
-
All message conversion is now handled only by
MessageConverter
objects. -
We introduced the
@StreamMessageConverter
annotation to provide customMessageConverter
objects. -
We introduced the default
Content Type
asapplication/json
, which needs to be taken into consideration when migrating 1.3 application or operating in the mixed mode (that is, 1.3 producer → 2.0 consumer). -
Messages with textual payloads and a
contentType
oftext/…
or…/json
are no longer converted toMessage<String>
for cases where the argument type of the providedMessageHandler
can not be determined (that is,public void handle(Message<?> message)
orpublic void handle(Object payload)
). Furthermore, a strong argument type may not be enough to properly convert messages, so thecontentType
header may be used as a supplement by someMessageConverters
.
2.3. Notable Deprecations
As of version 2.0, the following items have been deprecated:
2.3.1. Java Serialization (Java Native and Kryo)
JavaSerializationMessageConverter
and KryoMessageConverter
remain for now. However, we plan to move them out of the core packages and support in the future.
The main reason for this deprecation is to flag the issue that type-based, language-specific serialization could cause in distributed environments, where Producers and Consumers may depend on different JVM versions or have different versions of supporting libraries (that is, Kryo).
We also wanted to draw the attention to the fact that Consumers and Producers may not even be Java-based, so polyglot style serialization (i.e., JSON) is better suited.
2.3.2. Deprecated Classes and Methods
The following is a quick summary of notable deprecations. See the corresponding {spring-cloud-stream-javadoc-current}[javadoc] for more details.
-
SharedChannelRegistry
. UseSharedBindingTargetRegistry
. -
Bindings
. Beans qualified by it are already uniquely identified by their type — for example, providedSource
,Processor
, or custom bindings:
public interface Sample { String OUTPUT = "sampleOutput"; @Output(Sample.OUTPUT) MessageChannel output(); }
-
HeaderMode.raw
. Usenone
,headers
orembeddedHeaders
-
ProducerProperties.partitionKeyExtractorClass
in favor ofpartitionKeyExtractorName
andProducerProperties.partitionSelectorClass
in favor ofpartitionSelectorName
. This change ensures that both components are Spring configured and managed and are referenced in a Spring-friendly way. -
BinderAwareRouterBeanPostProcessor
. While the component remains, it is no longer aBeanPostProcessor
and will be renamed in the future. -
BinderProperties.setEnvironment(Properties environment)
. UseBinderProperties.setEnvironment(Map<String, Object> environment)
.
This section goes into more detail about how you can work with Spring Cloud Stream. It covers topics such as creating and running stream applications.
3. Introducing Spring Cloud Stream
Spring Cloud Stream is a framework for building message-driven microservice applications. Spring Cloud Stream builds upon Spring Boot to create standalone, production-grade Spring applications and uses Spring Integration to provide connectivity to message brokers. It provides opinionated configuration of middleware from several vendors, introducing the concepts of persistent publish-subscribe semantics, consumer groups, and partitions.
You can add the @EnableBinding
annotation to your application to get immediate connectivity to a message broker, and you can add @StreamListener
to a method to cause it to receive events for stream processing.
The following example shows a sink application that receives external messages:
@SpringBootApplication
@EnableBinding(Sink.class)
public class VoteRecordingSinkApplication {
public static void main(String[] args) {
SpringApplication.run(VoteRecordingSinkApplication.class, args);
}
@StreamListener(Sink.INPUT)
public void processVote(Vote vote) {
votingService.recordVote(vote);
}
}
The @EnableBinding
annotation takes one or more interfaces as parameters (in this case, the parameter is a single Sink
interface).
An interface declares input and output channels.
Spring Cloud Stream provides the Source
, Sink
, and Processor
interfaces. You can also define your own interfaces.
The following listing shows the definition of the Sink
interface:
public interface Sink {
String INPUT = "input";
@Input(Sink.INPUT)
SubscribableChannel input();
}
The @Input
annotation identifies an input channel, through which received messages enter the application.
The @Output
annotation identifies an output channel, through which published messages leave the application.
The @Input
and @Output
annotations can take a channel name as a parameter.
If a name is not provided, the name of the annotated method is used.
Spring Cloud Stream creates an implementation of the interface for you. You can use this in the application by autowiring it, as shown in the following example (from a test case):
@RunWith(SpringJUnit4ClassRunner.class)
@SpringApplicationConfiguration(classes = VoteRecordingSinkApplication.class)
@WebAppConfiguration
@DirtiesContext
public class StreamApplicationTests {
@Autowired
private Sink sink;
@Test
public void contextLoads() {
assertNotNull(this.sink.input());
}
}
4. Main Concepts
Spring Cloud Stream provides a number of abstractions and primitives that simplify the writing of message-driven microservice applications. This section gives an overview of the following:
4.1. Application Model
A Spring Cloud Stream application consists of a middleware-neutral core. The application communicates with the outside world through input and output channels injected into it by Spring Cloud Stream. Channels are connected to external brokers through middleware-specific Binder implementations.
4.1.1. Fat JAR
Spring Cloud Stream applications can be run in stand-alone mode from your IDE for testing. To run a Spring Cloud Stream application in production, you can create an executable (or “fat”) JAR by using the standard Spring Boot tooling provided for Maven or Gradle. See the Spring Boot Reference Guide for more details.
4.2. The Binder Abstraction
Spring Cloud Stream provides Binder implementations for Kafka and Rabbit MQ. Spring Cloud Stream also includes a TestSupportBinder, which leaves a channel unmodified so that tests can interact with channels directly and reliably assert on what is received. You can also use the extensible API to write your own Binder.
Spring Cloud Stream uses Spring Boot for configuration, and the Binder abstraction makes it possible for a Spring Cloud Stream application to be flexible in how it connects to middleware.
For example, deployers can dynamically choose, at runtime, the destinations (such as the Kafka topics or RabbitMQ exchanges) to which channels connect.
Such configuration can be provided through external configuration properties and in any form supported by Spring Boot (including application arguments, environment variables, and application.yml
or application.properties
files).
In the sink example from the Introducing Spring Cloud Stream section, setting the spring.cloud.stream.bindings.input.destination
application property to raw-sensor-data
causes it to read from the raw-sensor-data
Kafka topic or from a queue bound to the raw-sensor-data
RabbitMQ exchange.
Spring Cloud Stream automatically detects and uses a binder found on the classpath. You can use different types of middleware with the same code. To do so, include a different binder at build time. For more complex use cases, you can also package multiple binders with your application and have it choose the binder( and even whether to use different binders for different channels) at runtime.
4.3. Persistent Publish-Subscribe Support
Communication between applications follows a publish-subscribe model, where data is broadcast through shared topics. This can be seen in the following figure, which shows a typical deployment for a set of interacting Spring Cloud Stream applications.
Data reported by sensors to an HTTP endpoint is sent to a common destination named raw-sensor-data
.
From the destination, it is independently processed by a microservice application that computes time-windowed averages and by another microservice application that ingests the raw data into HDFS (Hadoop Distributed File System).
In order to process the data, both applications declare the topic as their input at runtime.
The publish-subscribe communication model reduces the complexity of both the producer and the consumer and lets new applications be added to the topology without disruption of the existing flow. For example, downstream from the average-calculating application, you can add an application that calculates the highest temperature values for display and monitoring. You can then add another application that interprets the same flow of averages for fault detection. Doing all communication through shared topics rather than point-to-point queues reduces coupling between microservices.
While the concept of publish-subscribe messaging is not new, Spring Cloud Stream takes the extra step of making it an opinionated choice for its application model. By using native middleware support, Spring Cloud Stream also simplifies use of the publish-subscribe model across different platforms.
4.4. Consumer Groups
While the publish-subscribe model makes it easy to connect applications through shared topics, the ability to scale up by creating multiple instances of a given application is equally important. When doing so, different instances of an application are placed in a competing consumer relationship, where only one of the instances is expected to handle a given message.
Spring Cloud Stream models this behavior through the concept of a consumer group.
(Spring Cloud Stream consumer groups are similar to and inspired by Kafka consumer groups.)
Each consumer binding can use the spring.cloud.stream.bindings.<channelName>.group
property to specify a group name.
For the consumers shown in the following figure, this property would be set as spring.cloud.stream.bindings.<channelName>.group=hdfsWrite
or spring.cloud.stream.bindings.<channelName>.group=average
.
All groups that subscribe to a given destination receive a copy of published data, but only one member of each group receives a given message from that destination. By default, when a group is not specified, Spring Cloud Stream assigns the application to an anonymous and independent single-member consumer group that is in a publish-subscribe relationship with all other consumer groups.
4.5. Consumer Types
Two types of consumer are supported:
-
Message-driven (sometimes referred to as Asynchronous)
-
Polled (sometimes referred to as Synchronous)
Prior to version 2.0, only asynchronous consumers were supported. A message is delivered as soon as it is available and a thread is available to process it.
When you wish to control the rate at which messages are processed, you might want to use a synchronous consumer.
4.5.1. Durability
Consistent with the opinionated application model of Spring Cloud Stream, consumer group subscriptions are durable. That is, a binder implementation ensures that group subscriptions are persistent and that, once at least one subscription for a group has been created, the group receives messages, even if they are sent while all applications in the group are stopped.
Anonymous subscriptions are non-durable by nature. For some binder implementations (such as RabbitMQ), it is possible to have non-durable group subscriptions. |
In general, it is preferable to always specify a consumer group when binding an application to a given destination. When scaling up a Spring Cloud Stream application, you must specify a consumer group for each of its input bindings. Doing so prevents the application’s instances from receiving duplicate messages (unless that behavior is desired, which is unusual).
4.6. Partitioning Support
Spring Cloud Stream provides support for partitioning data between multiple instances of a given application. In a partitioned scenario, the physical communication medium (such as the broker topic) is viewed as being structured into multiple partitions. One or more producer application instances send data to multiple consumer application instances and ensure that data identified by common characteristics are processed by the same consumer instance.
Spring Cloud Stream provides a common abstraction for implementing partitioned processing use cases in a uniform fashion. Partitioning can thus be used whether the broker itself is naturally partitioned (for example, Kafka) or not (for example, RabbitMQ).
Partitioning is a critical concept in stateful processing, where it is critical (for either performance or consistency reasons) to ensure that all related data is processed together. For example, in the time-windowed average calculation example, it is important that all measurements from any given sensor are processed by the same application instance.
To set up a partitioned processing scenario, you must configure both the data-producing and the data-consuming ends. |
5. Programming Model
To understand the programming model, you should be familiar with the following core concepts:
-
Destination Binders: Components responsible to provide integration with the external messaging systems.
-
Destination Bindings: Bridge between the external messaging systems and application provided Producers and Consumers of messages (created by the Destination Binders).
-
Message: The canonical data structure used by producers and consumers to communicate with Destination Binders (and thus other applications via external messaging systems).
5.1. Destination Binders
Destination Binders are extension components of Spring Cloud Stream responsible for providing the necessary configuration and implementation to facilitate integration with external messaging systems. This integration is responsible for connectivity, delegation, and routing of messages to and from producers and consumers, data type conversion, invocation of the user code, and more.
Binders handle a lot of the boiler plate responsibilities that would otherwise fall on your shoulders. However, to accomplish that, the binder still needs some help in the form of minimalistic yet required set of instructions from the user, which typically come in the form of some type of configuration.
While it is out of scope of this section to discuss all of the available binder and binding configuration options (the rest of the manual covers them extensively), Destination Binding does require special attention. The next section discusses it in detail.
5.2. Destination Bindings
As stated earlier, Destination Bindings provide a bridge between the external messaging system and application-provided Producers and Consumers.
Applying the @EnableBinding annotation to one of the application’s configuration classes defines a destination binding.
The @EnableBinding
annotation itself is meta-annotated with @Configuration
and triggers the configuration of the Spring Cloud Stream infrastructure.
The following example shows a fully configured and functioning Spring Cloud Stream application that receives the payload of the message from the INPUT
destination as a String
type (see Content Type Negotiation section), logs it to the console and sends it to the OUTPUT
destination after converting it to upper case.
@SpringBootApplication
@EnableBinding(Processor.class)
public class MyApplication {
public static void main(String[] args) {
SpringApplication.run(MyApplication.class, args);
}
@StreamListener(Processor.INPUT)
@SendTo(Processor.OUTPUT)
public String handle(String value) {
System.out.println("Received: " + value);
return value.toUpperCase();
}
}
As you can see the @EnableBinding
annotation can take one or more interface classes as parameters. The parameters are referred to as bindings,
and they contain methods representing bindable components.
These components are typically message channels (see Spring Messaging)
for channel-based binders (such as Rabbit, Kafka, and others). However other types of bindings can
provide support for the native features of the corresponding technology. For example Kafka Streams binder (formerly known as KStream) allows native bindings directly to Kafka Streams
(see Kafka Streams for more details).
Spring Cloud Stream already provides binding interfaces for typical message exchange contracts, which include:
-
Sink: Identifies the contract for the message consumer by providing the destination from which the message is consumed.
-
Source: Identifies the contract for the message producer by providing the destination to which the produced message is sent.
-
Processor: Encapsulates both the sink and the source contracts by exposing two destinations that allow consumption and production of messages.
public interface Sink {
String INPUT = "input";
@Input(Sink.INPUT)
SubscribableChannel input();
}
public interface Source {
String OUTPUT = "output";
@Output(Source.OUTPUT)
MessageChannel output();
}
public interface Processor extends Source, Sink {}
While the preceding example satisfies the majority of cases, you can also define your own contracts by defining your own bindings interfaces and use @Input
and @Output
annotations to identify the actual bindable components.
For example:
public interface Barista {
@Input
SubscribableChannel orders();
@Output
MessageChannel hotDrinks();
@Output
MessageChannel coldDrinks();
}
Using the interface shown in the preceding example as a parameter to @EnableBinding
triggers the creation of the three bound channels named orders
, hotDrinks
, and coldDrinks
,
respectively.
You can provide as many binding interfaces as you need, as arguments to the @EnableBinding
annotation, as shown in the following example:
@EnableBinding(value = { Orders.class, Payment.class })
In Spring Cloud Stream, the bindable MessageChannel
components are the Spring Messaging MessageChannel
(for outbound) and its extension, SubscribableChannel
,
(for inbound).
Pollable Destination Binding
While the previously described bindings support event-based message consumption, sometimes you need more control, such as rate of consumption.
Starting with version 2.0, you can now bind a pollable consumer:
The following example shows how to bind a pollable consumer:
public interface PolledBarista {
@Input
PollableMessageSource orders();
. . .
}
In this case, an implementation of PollableMessageSource
is bound to the orders
“channel”. See Using Polled Consumers for more details.
Customizing Channel Names
By using the @Input
and @Output
annotations, you can specify a customized channel name for the channel, as shown in the following example:
public interface Barista {
@Input("inboundOrders")
SubscribableChannel orders();
}
In the preceding example, the created bound channel is named inboundOrders
.
Normally, you need not access individual channels or bindings directly (other then configuring them via @EnableBinding
annotation). However there may be
times, such as testing or other corner cases, when you do.
Aside from generating channels for each binding and registering them as Spring beans, for each bound interface, Spring Cloud Stream generates a bean that implements the interface. That means you can have access to the interfaces representing the bindings or individual channels by auto-wiring either in your application, as shown in the following two examples:
Autowire Binding interface
@Autowire
private Source source
public void sayHello(String name) {
source.output().send(MessageBuilder.withPayload(name).build());
}
Autowire individual channel
@Autowire
private MessageChannel output;
public void sayHello(String name) {
output.send(MessageBuilder.withPayload(name).build());
}
You can also use standard Spring’s @Qualifier
annotation for cases when channel names are customized or in multiple-channel scenarios that require specifically named channels.
The following example shows how to use the @Qualifier annotation in this way:
@Autowire
@Qualifier("myChannel")
private MessageChannel output;
5.3. Producing and Consuming Messages
You can write a Spring Cloud Stream application by using either Spring Integration annotations or Spring Cloud Stream native annotation.
5.3.1. Spring Integration Support
Spring Cloud Stream is built on the concepts and patterns defined by Enterprise Integration Patterns and relies in its internal implementation on an already established and popular implementation of Enterprise Integration Patterns within the Spring portfolio of projects: Spring Integration framework.
So its only natiural for it to support the foundation, semantics, and configuration options that are already established by Spring Integration
For example, you can attach the output channel of a Source
to a MessageSource
and use the familiar @InboundChannelAdapter
annotation, as follows:
@EnableBinding(Source.class)
public class TimerSource {
@Bean
@InboundChannelAdapter(value = Source.OUTPUT, poller = @Poller(fixedDelay = "10", maxMessagesPerPoll = "1"))
public MessageSource<String> timerMessageSource() {
return () -> new GenericMessage<>("Hello Spring Cloud Stream");
}
}
Similarly, you can use @Transformer or @ServiceActivator while providing an implementation of a message handler method for a Processor binding contract, as shown in the following example:
@EnableBinding(Processor.class)
public class TransformProcessor {
@Transformer(inputChannel = Processor.INPUT, outputChannel = Processor.OUTPUT)
public Object transform(String message) {
return message.toUpperCase();
}
}
While this may be skipping ahead a bit, it is important to understand that, when you consume from the same binding using @StreamListener annotation, a pub-sub model is used.
Each method annotated with @StreamListener receives its own copy of a message, and each one has its own consumer group.
However, if you consume from the same binding by using one of the Spring Integration annotation (such as @Aggregator , @Transformer , or @ServiceActivator ), those consume in a competing model.
No individual consumer group is created for each subscription.
|
5.3.2. Using @StreamListener Annotation
Complementary to its Spring Integration support, Spring Cloud Stream provides its own @StreamListener
annotation, modeled after other Spring Messaging annotations
(@MessageMapping
, @JmsListener
, @RabbitListener
, and others) and provides conviniences, such as content-based routing and others.
@EnableBinding(Sink.class)
public class VoteHandler {
@Autowired
VotingService votingService;
@StreamListener(Sink.INPUT)
public void handle(Vote vote) {
votingService.record(vote);
}
}
As with other Spring Messaging methods, method arguments can be annotated with @Payload
, @Headers
, and @Header
.
For methods that return data, you must use the @SendTo
annotation to specify the output binding destination for data returned by the method, as shown in the following example:
@EnableBinding(Processor.class)
public class TransformProcessor {
@Autowired
VotingService votingService;
@StreamListener(Processor.INPUT)
@SendTo(Processor.OUTPUT)
public VoteResult handle(Vote vote) {
return votingService.record(vote);
}
}
5.3.3. Using @StreamListener for Content-based routing
Spring Cloud Stream supports dispatching messages to multiple handler methods annotated with @StreamListener
based on conditions.
In order to be eligible to support conditional dispatching, a method must satisfy the follow conditions:
-
It must not return a value.
-
It must be an individual message handling method (reactive API methods are not supported).
The condition is specified by a SpEL expression in the condition
argument of the annotation and is evaluated for each message.
All the handlers that match the condition are invoked in the same thread, and no assumption must be made about the order in which the invocations take place.
In the following example of a @StreamListener
with dispatching conditions, all the messages bearing a header type
with the value bogey
are dispatched to the
receiveBogey
method, and all the messages bearing a header type
with the value bacall
are dispatched to the receiveBacall
method.
@EnableBinding(Sink.class)
@EnableAutoConfiguration
public static class TestPojoWithAnnotatedArguments {
@StreamListener(target = Sink.INPUT, condition = "headers['type']=='bogey'")
public void receiveBogey(@Payload BogeyPojo bogeyPojo) {
// handle the message
}
@StreamListener(target = Sink.INPUT, condition = "headers['type']=='bacall'")
public void receiveBacall(@Payload BacallPojo bacallPojo) {
// handle the message
}
}
Content Type Negotiation in the Context of condition
It is important to understand some of the mechanics behind content-based routing using the condition
argument of @StreamListener
, especially in the context of the type of the message as a whole.
It may also help if you familiarize yourself with the Content Type Negotiation before you proceed.
Consider the following scenario:
@EnableBinding(Sink.class)
@EnableAutoConfiguration
public static class CatsAndDogs {
@StreamListener(target = Sink.INPUT, condition = "payload.class.simpleName=='Dog'")
public void bark(Dog dog) {
// handle the message
}
@StreamListener(target = Sink.INPUT, condition = "payload.class.simpleName=='Cat'")
public void purr(Cat cat) {
// handle the message
}
}
The preceding code is perfectly valid. It compiles and deploys without any issues, yet it never produces the result you expect.
That is because you are testing something that does not yet exist in a state you expect. That is becouse the payload of the message is not yet converted from the
wire format (byte[]
) to the desired type.
In other words, it has not yet gone through the type conversion process described in the Content Type Negotiation.
So, unless you use a SPeL expression that evaluates raw data (for example, the value of the first byte in the byte array), use message header-based expressions
(such as condition = "headers['type']=='dog'"
).
At the moment, dispatching through @StreamListener conditions is supported only for channel-based binders (not for reactive programming)
support.
|
5.3.4. Using Polled Consumers
When using polled consumers, you poll the PollableMessageSource
on demand.
Consider the following example of a polled consumer:
public interface PolledConsumer {
@Input
PollableMessageSource destIn();
@Output
MessageChannel destOut();
}
Given the polled consumer in the preceding example, you might use it as follows:
@Bean
public ApplicationRunner poller(PollableMessageSource destIn, MessageChannel destOut) {
return args -> {
while (someCondition()) {
try {
if (!destIn.poll(m -> {
String newPayload = ((String) m.getPayload()).toUpperCase();
destOut.send(new GenericMessage<>(newPayload));
})) {
Thread.sleep(1000);
}
}
catch (Exception e) {
// handle failure (throw an exception to reject the message);
}
}
};
}
The PollableMessageSource.poll()
method takes a MessageHandler
argument (often a lambda expression, as shown here).
It returns true
if the message was received and successfully processed.
As with message-driven consumers, if the MessageHandler
throws an exception, messages are published to error channels, as discussed in “[binder-error-channels]”.
Normally, the poll()
method acknowledges the message when the MessageHandler
exits.
If the method exits abnormally, the message is rejected (not re-queued).
You can override that behavior by taking responsibility for the acknowledgment, as shown in the following example:
@Bean
public ApplicationRunner poller(PollableMessageSource dest1In, MessageChannel dest2Out) {
return args -> {
while (someCondition()) {
if (!dest1In.poll(m -> {
StaticMessageHeaderAccessor.getAcknowledgmentCallback(m).noAutoAck();
// e.g. hand off to another thread which can perform the ack
// or acknowledge(Status.REQUEUE)
})) {
Thread.sleep(1000);
}
}
};
}
You must ack (or nack ) the message at some point, to avoid resource leaks.
|
Some messaging systems (such as Apache Kafka) maintain a simple offset in a log. If a delivery fails and is re-queued with StaticMessageHeaderAccessor.getAcknowledgmentCallback(m).acknowledge(Status.REQUEUE); , any later successfully ack’d messages are redelivered.
|
There is also an overloaded poll
method, for which the definition is as follows:
poll(MessageHandler handler, ParameterizedTypeReference<?> type)
The type
is a conversion hint that allows the incoming message payload to be converted, as shown in the following example:
boolean result = pollableSource.poll(received -> {
Map<String, Foo> payload = (Map<String, Foo>) received.getPayload();
...
}, new ParameterizedTypeReference<Map<String, Foo>>() {});
5.4. Error Handling
Errors happen, and Spring Cloud Stream provides several flexible mechanisms to handle them. The error handling comes in two flavors:
-
application: The error handling is done within the application (custom error handler).
-
system: The error handling is delegated to the binder (re-queue, DL, and others). Note that the techniques are dependent on binder implementation and the capability of the underlying messaging middleware.
Spring Cloud Stream uses the Spring Retry library to facilitate successful message processing. See Retry Template for more details. However, when all fails, the exceptions thrown by the message handlers are propagated back to the binder. At that point, binder invokes custom error handler or communicates the error back to the messaging system (re-queue, DLQ, and others).
Application Error Handling
There are two types of application-level error handling. Errors can be handled at each binding subscription or a global handler can handle all the binding subscription errors. Let’s review the details.
For each input binding, Spring Cloud Stream creates a dedicated error channel with the following semantics <destinationName>.errors
.
The <destinationName> consists of the name of the binding (such as input ) and the name of the group (such as myGroup ).
|
Consider the following:
spring.cloud.stream.bindings.input.group=myGroup
@StreamListener(Sink.INPUT) // destination name 'input.myGroup'
public void handle(Person value) {
throw new RuntimeException("BOOM!");
}
@ServiceActivator(inputChannel = Processor.INPUT + ".myGroup.errors") //channel name 'input.myGroup.errors'
public void error(Message<?> message) {
System.out.println("Handling ERROR: " + message);
}
In the preceding example the destination name is input.myGroup
and the dedicated error channel name is input.myGroup.errors
.
The use of @StreamListener annotation is intended specifically to define bindings that bridge internal channels and external destinations. Given that the destination specific error channel does NOT have an associated external destination, such channel is a prerogative of Spring Integration (SI). This means that the handler for such destination must be defined using one of the SI handler annotations (i.e., @ServiceActivator, @Transformer etc.). |
If group is not specified anonymous group is used (something like input.anonymous.2K37rb06Q6m2r51-SPIDDQ ), which is not suitable for error
handling scenarious, since you don’t know what it’s going to be until the destination is created.
|
Also, in the event you are binding to the existing destination such as:
spring.cloud.stream.bindings.input.destination=myFooDestination
spring.cloud.stream.bindings.input.group=myGroup
the full destination name is myFooDestination.myGroup
and then the dedicated error channel name is myFooDestination.myGroup.errors
.
Back to the example…
The handle(..)
method, which subscribes to the channel named input
, throws an exception. Given there is also a subscriber to the error channel input.myGroup.errors
all error messages are handled by this subscriber.
If you have multiple bindings, you may want to have a single error handler. Spring Cloud Stream automatically provides support for
a global error channel by bridging each individual error channel to the channel named errorChannel
, allowing a single subscriber to handle all errors,
as shown in the following example:
@StreamListener("errorChannel")
public void error(Message<?> message) {
System.out.println("Handling ERROR: " + message);
}
This may be a convenient option if error handling logic is the same regardless of which handler produced the error.
Also, error messages sent to the errorChannel
can be published to the specific destination at the broker by configuring a binding named error
for the outbound target.
This option provides a mechanism to automatically send error messages to another application bound to that destination or for later retrieval (for example, audit).
For example, to publish error messages to a broker destination named myErrors
, set the following property:
spring.cloud.stream.bindings.error.destination=myErrors.
The ability to bridge global error channel to a broker destination essentially provides a mechanism which connects the application-level error handling with the system-level error handling. |
System Error Handling
System-level error handling implies that the errors are communicated back to the messaging system and, given that not every messaging system is the same, the capabilities may differ from binder to binder.
That said, in this section we explain the general idea behind system level error handling and use Rabbit binder as an example. NOTE: Kafka binder provides similar support, although some configuration properties do differ. Also, for more details and configuration options, see the individual binder’s documentation.
If no internal error handlers are configured, the errors propagate to the binders, and the binders subsequently propagate those errors back to the messaging system. Depending on the capabilities of the messaging system such a system may drop the message, re-queue the message for re-processing or send the failed message to DLQ. Both Rabbit and Kafka support these concepts. However, other binders may not, so refer to your individual binder’s documentation for details on supported system-level error-handling options.
Drop Failed Messages
By default, if no additional system-level configuration is provided, the messaging system drops the failed message. While acceptable in some cases, for most cases, it is not, and we need some recovery mechanism to avoid message loss.
DLQ - Dead Letter Queue
DLQ allows failed messages to be sent to a special destination: - Dead Letter Queue.
When configured, failed messages are sent to this destination for subsequent re-processing or auditing and reconciliation.
For example, continuing on the previous example and to set up the DLQ with Rabbit binder, you need to set the following property:
spring.cloud.stream.rabbit.bindings.input.consumer.auto-bind-dlq=true
Keep in mind that, in the above property, input
corresponds to the name of the input destination binding.
The consumer
indicates that it is a consumer property and auto-bind-dlq
instructs the binder to configure DLQ for input
destination, which results in an additional Rabbit queue named input.myGroup.dlq
.
Once configured, all failed messages are routed to this queue with an error message similar to the following:
delivery_mode: 1
headers:
x-death:
count: 1
reason: rejected
queue: input.hello
time: 1522328151
exchange:
routing-keys: input.myGroup
Payload {"name”:"Bob"}
As you can see from the above, your original message is preserved for further actions.
However, one thing you may have noticed is that there is limited information on the original issue with the message processing. For example, you do not see a stack trace corresponding to the original error. To get more relevant information about the original error, you must set an additional property:
spring.cloud.stream.rabbit.bindings.input.consumer.republish-to-dlq=true
Doing so forces the internal error handler to intercept the error message and add additional information to it before publishing it to DLQ. Once configured, you can see that the error message contains more information relevant to the original error, as follows:
delivery_mode: 2
headers:
x-original-exchange:
x-exception-message: has an error
x-original-routingKey: input.myGroup
x-exception-stacktrace: org.springframework.messaging.MessageHandlingException: nested exception is
org.springframework.messaging.MessagingException: has an error, failedMessage=GenericMessage [payload=byte[15],
headers={amqp_receivedDeliveryMode=NON_PERSISTENT, amqp_receivedRoutingKey=input.hello, amqp_deliveryTag=1,
deliveryAttempt=3, amqp_consumerQueue=input.hello, amqp_redelivered=false, id=a15231e6-3f80-677b-5ad7-d4b1e61e486e,
amqp_consumerTag=amq.ctag-skBFapilvtZhDsn0k3ZmQg, contentType=application/json, timestamp=1522327846136}]
at org.spring...integ...han...MethodInvokingMessageProcessor.processMessage(MethodInvokingMessageProcessor.java:107)
at. . . . .
Payload {"name”:"Bob"}
This effectively combines application-level and system-level error handling to further assist with downstream troubleshooting mechanics.
Re-queue Failed Messages
As mentioned earlier, the currently supported binders (Rabbit and Kafka) rely on RetryTemplate
to facilitate successful message processing. See Retry Template for details.
However, for cases when max-attempts
property is set to 1, internal reprocessing of the message is disabled. At this point, you can facilitate message re-processing (re-tries)
by instructing the messaging system to re-queue the failed message. Once re-queued, the failed message is sent back to the original handler, essentially creating a retry loop.
This option may be feasible for cases where the nature of the error is related to some sporadic yet short-term unavailability of some resource.
To accomplish that, you must set the following properties:
spring.cloud.stream.bindings.input.consumer.max-attempts=1
spring.cloud.stream.rabbit.bindings.input.consumer.requeue-rejected=true
In the preceding example, the max-attempts
set to 1 essentially disabling internal re-tries and requeue-rejected
(short for requeue rejected messages) is set to true
.
Once set, the failed message is resubmitted to the same handler and loops continuously or until the handler throws AmqpRejectAndDontRequeueException
essentially allowing you to build your own re-try logic within the handler itself.
Retry Template
The RetryTemplate
is part of the Spring Retry library.
While it is out of scope of this document to cover all of the capabilities of the RetryTemplate
, we will mention the following consumer properties that are specifically related to
the RetryTemplate
:
- maxAttempts
-
The number of attempts to process the message.
Default: 3.
- backOffInitialInterval
-
The backoff initial interval on retry.
Default 1000 milliseconds.
- backOffMaxInterval
-
The maximum backoff interval.
Default 10000 milliseconds.
- backOffMultiplier
-
The backoff multiplier.
Default 2.0.
While the preceding settings are sufficient for majority of the customization requirements, they may not satisfy certain complex requirements at, which
point you may want to provide your own instance of the RetryTemplate
. To do so configure it as a bean in your application configuration. The application provided
instance will override the one provided by the framework. Also, to avoid conflicts you must qualify the instance of the RetryTemplate
you want to be used by the binder
as @StreamRetryTemplate
. For example,
@StreamRetryTemplate
public RetryTemplate myRetryTemplate() {
return new RetryTemplate();
}
As you can see from the above example you don’t need to annotate it with @Bean
since @StreamRetryTemplate
is a qualified @Bean
.
5.5. Reactive Programming Support
Spring Cloud Stream also supports the use of reactive APIs where incoming and outgoing data is handled as continuous data flows.
Support for reactive APIs is available through spring-cloud-stream-reactive
, which needs to be added explicitly to your project.
The programming model with reactive APIs is declarative. Instead of specifying how each individual message should be handled, you can use operators that describe functional transformations from inbound to outbound data flows.
At present Spring Cloud Stream supports the only the Reactor API. In the future, we intend to support a more generic model based on Reactive Streams.
The reactive programming model also uses the @StreamListener
annotation for setting up reactive handlers.
The differences are that:
-
The
@StreamListener
annotation must not specify an input or output, as they are provided as arguments and return values from the method. -
The arguments of the method must be annotated with
@Input
and@Output
, indicating which input or output the incoming and outgoing data flows connect to, respectively. -
The return value of the method, if any, is annotated with
@Output
, indicating the input where data should be sent.
Reactive programming support requires Java 1.8. |
As of Spring Cloud Stream 1.1.1 and later (starting with release train Brooklyn.SR2), reactive programming support requires the use of Reactor 3.0.4.RELEASE and higher.
Earlier Reactor versions (including 3.0.1.RELEASE, 3.0.2.RELEASE and 3.0.3.RELEASE) are not supported.
spring-cloud-stream-reactive transitively retrieves the proper version, but it is possible for the project structure to manage the version of the io.projectreactor:reactor-core to an earlier release, especially when using Maven.
This is the case for projects generated by using Spring Initializr with Spring Boot 1.x, which overrides the Reactor version to 2.0.8.RELEASE .
In such cases, you must ensure that the proper version of the artifact is released.
You can do so by adding a direct dependency on io.projectreactor:reactor-core with a version of 3.0.4.RELEASE or later to your project.
|
The use of term, “reactive”, currently refers to the reactive APIs being used and not to the execution model being reactive (that is, the bound endpoints still use a 'push' rather than a 'pull' model). While some backpressure support is provided by the use of Reactor, we do intend, in a future release, to support entirely reactive pipelines by the use of native reactive clients for the connected middleware. |
Reactor-based Handlers
A Reactor-based handler can have the following argument types:
-
For arguments annotated with
@Input
, it supports the ReactorFlux
type. The parameterization of the inbound Flux follows the same rules as in the case of individual message handling: It can be the entireMessage
, a POJO that can be theMessage
payload, or a POJO that is the result of a transformation based on theMessage
content-type header. Multiple inputs are provided. -
For arguments annotated with
Output
, it supports theFluxSender
type, which connects aFlux
produced by the method with an output. Generally speaking, specifying outputs as arguments is only recommended when the method can have multiple outputs.
A Reactor-based handler supports a return type of Flux
. In that case, it must be annotated with @Output
. We recommend using the return value of the method when a single output Flux
is available.
The following example shows a Reactor-based Processor
:
@EnableBinding(Processor.class)
@EnableAutoConfiguration
public static class UppercaseTransformer {
@StreamListener
@Output(Processor.OUTPUT)
public Flux<String> receive(@Input(Processor.INPUT) Flux<String> input) {
return input.map(s -> s.toUpperCase());
}
}
The same processor using output arguments looks like the following example:
@EnableBinding(Processor.class)
@EnableAutoConfiguration
public static class UppercaseTransformer {
@StreamListener
public void receive(@Input(Processor.INPUT) Flux<String> input,
@Output(Processor.OUTPUT) FluxSender output) {
output.send(input.map(s -> s.toUpperCase()));
}
}
Reactive Sources
Spring Cloud Stream reactive support also provides the ability for creating reactive sources through the @StreamEmitter
annotation.
By using the @StreamEmitter
annotation, a regular source may be converted to a reactive one.
@StreamEmitter
is a method level annotation that marks a method to be an emitter to outputs declared with @EnableBinding
.
You cannot use the @Input
annotation along with @StreamEmitter
, as the methods marked with this annotation are not listening for any input. Rather, methods marked with @StreamEmitter
generate output.
Following the same programming model used in @StreamListener
, @StreamEmitter
also allows flexible ways of using the @Output
annotation, depending on whether the method has any arguments, a return type, and other considerations.
The remainder of this section contains examples of using the @StreamEmitter
annotation in various styles.
The following example emits the Hello, World
message every millisecond and publishes to a Reactor Flux
:
@EnableBinding(Source.class)
@EnableAutoConfiguration
public static class HelloWorldEmitter {
@StreamEmitter
@Output(Source.OUTPUT)
public Flux<String> emit() {
return Flux.intervalMillis(1)
.map(l -> "Hello World");
}
}
In the preceding example, the resulting messages in the Flux
are sent to the output channel of the Source
.
The next example is another flavor of an @StreamEmmitter
that sends a Reactor Flux
.
Instead of returning a Flux
, the following method uses a FluxSender
to programmatically send a Flux
from a source:
@EnableBinding(Source.class)
@EnableAutoConfiguration
public static class HelloWorldEmitter {
@StreamEmitter
@Output(Source.OUTPUT)
public void emit(FluxSender output) {
output.send(Flux.intervalMillis(1)
.map(l -> "Hello World"));
}
}
The next example is exactly same as the above snippet in functionality and style.
However, instead of using an explicit @Output
annotation on the method, it uses the annotation on the method parameter.
@EnableBinding(Source.class)
@EnableAutoConfiguration
public static class HelloWorldEmitter {
@StreamEmitter
public void emit(@Output(Source.OUTPUT) FluxSender output) {
output.send(Flux.intervalMillis(1)
.map(l -> "Hello World"));
}
}
The last example in this section is yet another flavor of writing reacting sources by using the Reactive Streams Publisher API and taking advantage of the support for it in Spring Integration Java DSL.
The Publisher
in the following example still uses Reactor Flux
under the hood, but, from an application perspective, that is transparent to the user and only needs Reactive Streams and Java DSL for Spring Integration:
@EnableBinding(Source.class)
@EnableAutoConfiguration
public static class HelloWorldEmitter {
@StreamEmitter
@Output(Source.OUTPUT)
@Bean
public Publisher<Message<String>> emit() {
return IntegrationFlows.from(() ->
new GenericMessage<>("Hello World"),
e -> e.poller(p -> p.fixedDelay(1)))
.toReactivePublisher();
}
}
6. Binders
Spring Cloud Stream provides a Binder abstraction for use in connecting to physical destinations at the external middleware. This section provides information about the main concepts behind the Binder SPI, its main components, and implementation-specific details.
6.1. Producers and Consumers
The following image shows the general relationship of producers and consumers:
A producer is any component that sends messages to a channel.
The channel can be bound to an external message broker with a Binder
implementation for that broker.
When invoking the bindProducer()
method, the first parameter is the name of the destination within the broker, the second parameter is the local channel instance to which the producer sends messages, and the third parameter contains properties (such as a partition key expression) to be used within the adapter that is created for that channel.
A consumer is any component that receives messages from a channel.
As with a producer, the consumer’s channel can be bound to an external message broker.
When invoking the bindConsumer()
method, the first parameter is the destination name, and a second parameter provides the name of a logical group of consumers.
Each group that is represented by consumer bindings for a given destination receives a copy of each message that a producer sends to that destination (that is, it follows normal publish-subscribe semantics).
If there are multiple consumer instances bound with the same group name, then messages are load-balanced across those consumer instances so that each message sent by a producer is consumed by only a single consumer instance within each group (that is, it follows normal queueing semantics).
6.2. Binder SPI
The Binder SPI consists of a number of interfaces, out-of-the box utility classes, and discovery strategies that provide a pluggable mechanism for connecting to external middleware.
The key point of the SPI is the Binder
interface, which is a strategy for connecting inputs and outputs to external middleware. The following listing shows the definnition of the Binder
interface:
public interface Binder<T, C extends ConsumerProperties, P extends ProducerProperties> {
Binding<T> bindConsumer(String name, String group, T inboundBindTarget, C consumerProperties);
Binding<T> bindProducer(String name, T outboundBindTarget, P producerProperties);
}
The interface is parameterized, offering a number of extension points:
-
Input and output bind targets. As of version 1.0, only
MessageChannel
is supported, but this is intended to be used as an extension point in the future. -
Extended consumer and producer properties, allowing specific Binder implementations to add supplemental properties that can be supported in a type-safe manner.
A typical binder implementation consists of the following:
-
A class that implements the
Binder
interface; -
A Spring
@Configuration
class that creates a bean of typeBinder
along with the middleware connection infrastructure. -
A
META-INF/spring.binders
file found on the classpath containing one or more binder definitions, as shown in the following example:kafka:\ org.springframework.cloud.stream.binder.kafka.config.KafkaBinderConfiguration
6.3. Binder Detection
Spring Cloud Stream relies on implementations of the Binder SPI to perform the task of connecting channels to message brokers. Each Binder implementation typically connects to one type of messaging system.
6.3.1. Classpath Detection
By default, Spring Cloud Stream relies on Spring Boot’s auto-configuration to configure the binding process. If a single Binder implementation is found on the classpath, Spring Cloud Stream automatically uses it. For example, a Spring Cloud Stream project that aims to bind only to RabbitMQ can add the following dependency:
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-stream-binder-rabbit</artifactId>
</dependency>
For the specific Maven coordinates of other binder dependencies, see the documentation of that binder implementation.
6.4. Multiple Binders on the Classpath
When multiple binders are present on the classpath, the application must indicate which binder is to be used for each channel binding.
Each binder configuration contains a META-INF/spring.binders
file, which is a simple properties file, as shown in the following example:
rabbit:\
org.springframework.cloud.stream.binder.rabbit.config.RabbitServiceAutoConfiguration
Similar files exist for the other provided binder implementations (such as Kafka), and custom binder implementations are expected to provide them as well.
The key represents an identifying name for the binder implementation, whereas the value is a comma-separated list of configuration classes that each contain one and only one bean definition of type org.springframework.cloud.stream.binder.Binder
.
Binder selection can either be performed globally, using the spring.cloud.stream.defaultBinder
property (for example, spring.cloud.stream.defaultBinder=rabbit
) or individually, by configuring the binder on each channel binding.
For instance, a processor application (that has channels named input
and output
for read and write respectively) that reads from Kafka and writes to RabbitMQ can specify the following configuration:
spring.cloud.stream.bindings.input.binder=kafka
spring.cloud.stream.bindings.output.binder=rabbit
6.5. Connecting to Multiple Systems
By default, binders share the application’s Spring Boot auto-configuration, so that one instance of each binder found on the classpath is created. If your application should connect to more than one broker of the same type, you can specify multiple binder configurations, each with different environment settings.
Turning on explicit binder configuration disables the default binder configuration process altogether.
If you do so, all binders in use must be included in the configuration.
Frameworks that intend to use Spring Cloud Stream transparently may create binder configurations that can be referenced by name, but they do not affect the default binder configuration.
In order to do so, a binder configuration may have its defaultCandidate flag set to false (for example, spring.cloud.stream.binders.<configurationName>.defaultCandidate=false ).
This denotes a configuration that exists independently of the default binder configuration process.
|
The following example shows a typical configuration for a processor application that connects to two RabbitMQ broker instances:
spring:
cloud:
stream:
bindings:
input:
destination: thing1
binder: rabbit1
output:
destination: thing2
binder: rabbit2
binders:
rabbit1:
type: rabbit
environment:
spring:
rabbitmq:
host: <host1>
rabbit2:
type: rabbit
environment:
spring:
rabbitmq:
host: <host2>
6.6. Binding visualization and control
Since version 2.0, Spring Cloud Stream supports visualization and control of the Bindings through Actuator endpoints.
Starting with version 2.0 actuator and web are optional, you must first add one of the web dependencies as well as add the actuator dependency manually. The following example shows how to add the dependency for the Web framework:
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
The following example shows how to add the dependency for the WebFlux framework:
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-webflux</artifactId>
</dependency>
You can add the Actuator dependency as follows:
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-actuator</artifactId>
</dependency>
To run Spring Cloud Stream 2.0 apps in Cloud Foundry, you must add spring-boot-starter-web and spring-boot-starter-actuator to the classpath. Otherwise, the
application will not start due to health check failures.
|
You must also enable the bindings
actuator endpoints by setting the following property: --management.endpoints.web.exposure.include=bindings
.
Once those prerequisites are satisfied. you should see the following in the logs when application start:
: Mapped "{[/actuator/bindings/{name}],methods=[POST]. . . : Mapped "{[/actuator/bindings],methods=[GET]. . . : Mapped "{[/actuator/bindings/{name}],methods=[GET]. . .
To visualize the current bindings, access the following URL:
<host>:<port>/actuator/bindings
Alternative, to see a single binding, access one of the URLs similar to the following:
<host>:<port>/actuator/bindings/myBindingName
You can also stop, start, pause, and resume individual bindings by posting to the same URL while providing a state
argument as JSON, as shown in the following examples:
curl -d '{"state":"STOPPED"}' -H "Content-Type: application/json" -X POST <host>:<port>/actuator/bindings/myBindingName curl -d '{"state":"STARTED"}' -H "Content-Type: application/json" -X POST <host>:<port>/actuator/bindings/myBindingName curl -d '{"state":"PAUSED"}' -H "Content-Type: application/json" -X POST <host>:<port>/actuator/bindings/myBindingName curl -d '{"state":"RESUMED"}' -H "Content-Type: application/json" -X POST <host>:<port>/actuator/bindings/myBindingName
PAUSED and RESUMED work only when the corresponding binder and its underlying technology supports it. Otherwise, you see the warning message in the logs.
Currently, only Kafka binder supports the PAUSED and RESUMED states.
|
6.7. Binder Configuration Properties
The following properties are available when customizing binder configurations. These properties exposed via org.springframework.cloud.stream.config.BinderProperties
They must be prefixed with spring.cloud.stream.binders.<configurationName>
.
- type
-
The binder type. It typically references one of the binders found on the classpath — in particular, a key in a
META-INF/spring.binders
file.By default, it has the same value as the configuration name.
- inheritEnvironment
-
Whether the configuration inherits the environment of the application itself.
Default:
true
. - environment
-
Root for a set of properties that can be used to customize the environment of the binder. When this property is set, the context in which the binder is being created is not a child of the application context. This setting allows for complete separation between the binder components and the application components.
Default:
empty
. - defaultCandidate
-
Whether the binder configuration is a candidate for being considered a default binder or can be used only when explicitly referenced. This setting allows adding binder configurations without interfering with the default processing.
Default:
true
.
7. Configuration Options
Spring Cloud Stream supports general configuration options as well as configuration for bindings and binders. Some binders let additional binding properties support middleware-specific features.
Configuration options can be provided to Spring Cloud Stream applications through any mechanism supported by Spring Boot. This includes application arguments, environment variables, and YAML or .properties files.
7.1. Binding Service Properties
These properties are exposed via org.springframework.cloud.stream.config.BindingServiceProperties
- spring.cloud.stream.instanceCount
-
The number of deployed instances of an application. Must be set for partitioning on the producer side. Must be set on the consumer side when using RabbitMQ and with Kafka if
autoRebalanceEnabled=false
.Default:
1
. - spring.cloud.stream.instanceIndex
-
The instance index of the application: A number from
0
toinstanceCount - 1
. Used for partitioning with RabbitMQ and with Kafka ifautoRebalanceEnabled=false
. Automatically set in Cloud Foundry to match the application’s instance index. - spring.cloud.stream.dynamicDestinations
-
A list of destinations that can be bound dynamically (for example, in a dynamic routing scenario). If set, only listed destinations can be bound.
Default: empty (letting any destination be bound).
- spring.cloud.stream.defaultBinder
-
The default binder to use, if multiple binders are configured. See Multiple Binders on the Classpath.
Default: empty.
- spring.cloud.stream.overrideCloudConnectors
-
This property is only applicable when the
cloud
profile is active and Spring Cloud Connectors are provided with the application. If the property isfalse
(the default), the binder detects a suitable bound service (for example, a RabbitMQ service bound in Cloud Foundry for the RabbitMQ binder) and uses it for creating connections (usually through Spring Cloud Connectors). When set totrue
, this property instructs binders to completely ignore the bound services and rely on Spring Boot properties (for example, relying on thespring.rabbitmq.*
properties provided in the environment for the RabbitMQ binder). The typical usage of this property is to be nested in a customized environment when connecting to multiple systems.Default:
false
. - spring.cloud.stream.bindingRetryInterval
-
The interval (in seconds) between retrying binding creation when, for example, the binder does not support late binding and the broker (for example, Apache Kafka) is down. Set it to zero to treat such conditions as fatal, preventing the application from starting.
Default:
30
7.2. Binding Properties
Binding properties are supplied by using the format of spring.cloud.stream.bindings.<channelName>.<property>=<value>
.
The <channelName>
represents the name of the channel being configured (for example, output
for a Source
).
To avoid repetition, Spring Cloud Stream supports setting values for all channels, in the format of spring.cloud.stream.default.<property>=<value>
.
In what follows, we indicate where we have omitted the spring.cloud.stream.bindings.<channelName>.
prefix and focus just on the property name, with the understanding that the prefix ise included at runtime.
7.2.1. Common Binding Properties
These properties are exposed via org.springframework.cloud.stream.config.BindingProperties
The following binding properties are available for both input and output bindings and must be prefixed with spring.cloud.stream.bindings.<channelName>.
(for example, spring.cloud.stream.bindings.input.destination=ticktock
).
Default values can be set by using the spring.cloud.stream.default
prefix (for example`spring.cloud.stream.default.contentType=application/json`).
- destination
-
The target destination of a channel on the bound middleware (for example, the RabbitMQ exchange or Kafka topic). If the channel is bound as a consumer, it could be bound to multiple destinations, and the destination names can be specified as comma-separated
String
values. If not set, the channel name is used instead. The default value of this property cannot be overridden. - group
-
The consumer group of the channel. Applies only to inbound bindings. See Consumer Groups.
Default:
null
(indicating an anonymous consumer). - contentType
-
The content type of the channel. See “Content Type Negotiation”.
Default:
application/json
. - binder
-
The binder used by this binding. See “Multiple Binders on the Classpath” for details.
Default:
null
(the default binder is used, if it exists).
7.2.2. Consumer Properties
These properties are exposed via org.springframework.cloud.stream.binder.ConsumerProperties
The following binding properties are available for input bindings only and must be prefixed with spring.cloud.stream.bindings.<channelName>.consumer.
(for example, spring.cloud.stream.bindings.input.consumer.concurrency=3
).
Default values can be set by using the spring.cloud.stream.default.consumer
prefix (for example, spring.cloud.stream.default.consumer.headerMode=none
).
- concurrency
-
The concurrency of the inbound consumer.
Default:
1
. - partitioned
-
Whether the consumer receives data from a partitioned producer.
Default:
false
. - headerMode
-
When set to
none
, disables header parsing on input. Effective only for messaging middleware that does not support message headers natively and requires header embedding. This option is useful when consuming data from non-Spring Cloud Stream applications when native headers are not supported. When set toheaders
, it uses the middleware’s native header mechanism. When set toembeddedHeaders
, it embeds headers into the message payload.Default: depends on the binder implementation.
- maxAttempts
-
If processing fails, the number of attempts to process the message (including the first). Set to
1
to disable retry.Default:
3
. - backOffInitialInterval
-
The backoff initial interval on retry.
Default:
1000
. - backOffMaxInterval
-
The maximum backoff interval.
Default:
10000
. - backOffMultiplier
-
The backoff multiplier.
Default:
2.0
. - instanceIndex
-
When set to a value greater than equal to zero, it allows customizing the instance index of this consumer (if different from
spring.cloud.stream.instanceIndex
). When set to a negative value, it defaults tospring.cloud.stream.instanceIndex
. See “Instance Index and Instance Count” for more information.Default:
-1
. - instanceCount
-
When set to a value greater than equal to zero, it allows customizing the instance count of this consumer (if different from
spring.cloud.stream.instanceCount
). When set to a negative value, it defaults tospring.cloud.stream.instanceCount
. See “Instance Index and Instance Count” for more information.Default:
-1
. - useNativeDecoding
-
When set to
true
, the inbound message is deserialized directly by the client library, which must be configured correspondingly (for example, setting an appropriate Kafka producer value deserializer). When this configuration is being used, the inbound message unmarshalling is not based on thecontentType
of the binding. When native decoding is used, it is the responsibility of the producer to use an appropriate encoder (for example, the Kafka producer value serializer) to serialize the outbound message. Also, when native encoding and decoding is used, theheaderMode=embeddedHeaders
property is ignored and headers are not embedded in the message. See the producer propertyuseNativeEncoding
.Default:
false
.
7.2.3. Producer Properties
These properties are exposed via org.springframework.cloud.stream.binder.ProducerProperties
The following binding properties are available for output bindings only and must be prefixed with spring.cloud.stream.bindings.<channelName>.producer.
(for example, spring.cloud.stream.bindings.input.producer.partitionKeyExpression=payload.id
).
Default values can be set by using the prefix spring.cloud.stream.default.producer
(for example, spring.cloud.stream.default.producer.partitionKeyExpression=payload.id
).
- partitionKeyExpression
-
A SpEL expression that determines how to partition outbound data. If set, or if
partitionKeyExtractorClass
is set, outbound data on this channel is partitioned.partitionCount
must be set to a value greater than 1 to be effective. Mutually exclusive withpartitionKeyExtractorClass
. See “Partitioning Support”.Default: null.
- partitionKeyExtractorClass
-
A
PartitionKeyExtractorStrategy
implementation. If set, or ifpartitionKeyExpression
is set, outbound data on this channel is partitioned.partitionCount
must be set to a value greater than 1 to be effective. Mutually exclusive withpartitionKeyExpression
. See “Partitioning Support”.Default:
null
. - partitionSelectorClass
-
A
PartitionSelectorStrategy
implementation. Mutually exclusive withpartitionSelectorExpression
. If neither is set, the partition is selected as thehashCode(key) % partitionCount
, wherekey
is computed through eitherpartitionKeyExpression
orpartitionKeyExtractorClass
.Default:
null
. - partitionSelectorExpression
-
A SpEL expression for customizing partition selection. Mutually exclusive with
partitionSelectorClass
. If neither is set, the partition is selected as thehashCode(key) % partitionCount
, wherekey
is computed through eitherpartitionKeyExpression
orpartitionKeyExtractorClass
.Default:
null
. - partitionCount
-
The number of target partitions for the data, if partitioning is enabled. Must be set to a value greater than 1 if the producer is partitioned. On Kafka, it is interpreted as a hint. The larger of this and the partition count of the target topic is used instead.
Default:
1
. - requiredGroups
-
A comma-separated list of groups to which the producer must ensure message delivery even if they start after it has been created (for example, by pre-creating durable queues in RabbitMQ).
- headerMode
-
When set to
none
, it disables header embedding on output. It is effective only for messaging middleware that does not support message headers natively and requires header embedding. This option is useful when producing data for non-Spring Cloud Stream applications when native headers are not supported. When set toheaders
, it uses the middleware’s native header mechanism. When set toembeddedHeaders
, it embeds headers into the message payload.Default: Depends on the binder implementation.
- useNativeEncoding
-
When set to
true
, the outbound message is serialized directly by the client library, which must be configured correspondingly (for example, setting an appropriate Kafka producer value serializer). When this configuration is being used, the outbound message marshalling is not based on thecontentType
of the binding. When native encoding is used, it is the responsibility of the consumer to use an appropriate decoder (for example, the Kafka consumer value de-serializer) to deserialize the inbound message. Also, when native encoding and decoding is used, theheaderMode=embeddedHeaders
property is ignored and headers are not embedded in the message. See the consumer propertyuseNativeDecoding
.Default:
false
. - errorChannelEnabled
-
When set to
true
, if the binder supports asynchroous send results, send failures are sent to an error channel for the destination. See “[binder-error-channels]” for more information.Default:
false
.
7.3. Using Dynamically Bound Destinations
Besides the channels defined by using @EnableBinding
, Spring Cloud Stream lets applications send messages to dynamically bound destinations.
This is useful, for example, when the target destination needs to be determined at runtime.
Applications can do so by using the BinderAwareChannelResolver
bean, registered automatically by the @EnableBinding
annotation.
The 'spring.cloud.stream.dynamicDestinations' property can be used for restricting the dynamic destination names to a known set (whitelisting). If this property is not set, any destination can be bound dynamically.
The BinderAwareChannelResolver
can be used directly, as shown in the following example of a REST controller using a path variable to decide the target channel:
@EnableBinding
@Controller
public class SourceWithDynamicDestination {
@Autowired
private BinderAwareChannelResolver resolver;
@RequestMapping(path = "/{target}", method = POST, consumes = "*/*")
@ResponseStatus(HttpStatus.ACCEPTED)
public void handleRequest(@RequestBody String body, @PathVariable("target") target,
@RequestHeader(HttpHeaders.CONTENT_TYPE) Object contentType) {
sendMessage(body, target, contentType);
}
private void sendMessage(String body, String target, Object contentType) {
resolver.resolveDestination(target).send(MessageBuilder.createMessage(body,
new MessageHeaders(Collections.singletonMap(MessageHeaders.CONTENT_TYPE, contentType))));
}
}
Now consider what happens when we start the application on the default port (8080) and make the following requests with CURL:
curl -H "Content-Type: application/json" -X POST -d "customer-1" http://localhost:8080/customers curl -H "Content-Type: application/json" -X POST -d "order-1" http://localhost:8080/orders
The destinations, 'customers' and 'orders', are created in the broker (in the exchange for Rabbit or in the topic for Kafka) with names of 'customers' and 'orders', and the data is published to the appropriate destinations.
The BinderAwareChannelResolver
is a general-purpose Spring Integration DestinationResolver
and can be injected in other components — for example, in a router using a SpEL expression based on the target
field of an incoming JSON message. The following example includes a router that reads SpEL expressions:
@EnableBinding
@Controller
public class SourceWithDynamicDestination {
@Autowired
private BinderAwareChannelResolver resolver;
@RequestMapping(path = "/", method = POST, consumes = "application/json")
@ResponseStatus(HttpStatus.ACCEPTED)
public void handleRequest(@RequestBody String body, @RequestHeader(HttpHeaders.CONTENT_TYPE) Object contentType) {
sendMessage(body, contentType);
}
private void sendMessage(Object body, Object contentType) {
routerChannel().send(MessageBuilder.createMessage(body,
new MessageHeaders(Collections.singletonMap(MessageHeaders.CONTENT_TYPE, contentType))));
}
@Bean(name = "routerChannel")
public MessageChannel routerChannel() {
return new DirectChannel();
}
@Bean
@ServiceActivator(inputChannel = "routerChannel")
public ExpressionEvaluatingRouter router() {
ExpressionEvaluatingRouter router =
new ExpressionEvaluatingRouter(new SpelExpressionParser().parseExpression("payload.target"));
router.setDefaultOutputChannelName("default-output");
router.setChannelResolver(resolver);
return router;
}
}
The Router Sink Application uses this technique to create the destinations on-demand.
If the channel names are known in advance, you can configure the producer properties as with any other destination.
Alternatively, if you register a NewBindingCallback<>
bean, it is invoked just before the binding is created.
The callback takes the generic type of the extended producer properties used by the binder.
It has one method:
void configure(String channelName, MessageChannel channel, ProducerProperties producerProperties,
T extendedProducerProperties);
The following example shows how to use the RabbitMQ binder:
@Bean
public NewBindingCallback<RabbitProducerProperties> dynamicConfigurer() {
return (name, channel, props, extended) -> {
props.setRequiredGroups("bindThisQueue");
extended.setQueueNameGroupOnly(true);
extended.setAutoBindDlq(true);
extended.setDeadLetterQueueName("myDLQ");
};
}
If you need to support dynamic destinations with multiple binder types, use Object for the generic type and cast the extended argument as needed.
|
8. Content Type Negotiation
Data transformation is one of the core features of any message-driven microservice architecture. Given that, in Spring Cloud Stream, such data
is represented as a Spring Message
, a message may have to be transformed to a desired shape or size before reaching its destination. This is required for two reasons:
-
To convert the contents of the incoming message to match the signature of the application-provided handler.
-
To convert the contents of the outgoing message to the wire format.
The wire format is typically byte[]
(that is true for the Kafka and Rabbit binders), but it is governed by the binder implementation.
In Spring Cloud Stream, message transformation is accomplished with an org.springframework.messaging.converter.MessageConverter
.
As a supplement to the details to follow, you may also want to read the following blog post. |
8.1. Mechanics
To better understand the mechanics and the necessity behind content-type negotiation, we take a look at a very simple use case by using the following message handler as an example:
@StreamListener(Processor.INPUT)
@SendTo(Processor.OUTPUT)
public String handle(Person person) {..}
For simplicity, we assume that this is the only handler in the application (we assume there is no internal pipeline). |
The handler shown in the preceding example expects a Person
object as an argument and produces a String
type as an output.
In order for the framework to succeed in passing the incoming Message
as an argument to this handler, it has to somehow transform the payload of the Message
type from the wire format to a Person
type.
In other words, the framework must locate and apply the appropriate MessageConverter
.
To accomplish that, the framework needs some instructions from the user.
One of these instructions is already provided by the signature of the handler method itself (Person
type).
Consequently, in theory, that should be (and, in some cases, is) enough.
However, for the majority of use cases, in order to select the appropriate MessageConverter
, the framework needs an additional piece of information.
That missing piece is contentType
.
Spring Cloud Stream provides three mechanisms to define contentType
(in order of precedence):
-
HEADER: The
contentType
can be communicated through the Message itself. By providing acontentType
header, you declare the content type to use to locate and apply the appropriateMessageConverter
. -
BINDING: The
contentType
can be set per destination binding by setting thespring.cloud.stream.bindings.input.content-type
property.The input
segment in the property name corresponds to the actual name of the destination (which is “input” in our case). This approach lets you declare, on a per-binding basis, the content type to use to locate and apply the appropriateMessageConverter
. -
DEFAULT: If
contentType
is not present in theMessage
header or the binding, the defaultapplication/json
content type is used to locate and apply the appropriateMessageConverter
.
As mentioned earlier, the preceding list also demonstrates the order of precedence in case of a tie. For example, a header-provided content type takes precedence over any other content type. The same applies for a content type set on a per-binding basis, which essentially lets you override the default content type. However, it also provides a sensible default (which was determined from community feedback).
Another reason for making application/json
the default stems from the interoperability requirements driven by distributed microservices architectures, where producer and consumer not only run in different JVMs but can also run on different non-JVM platforms.
When the non-void handler method returns, if the the return value is already a Message
, that Message
becomes the payload. However, when the return value is not a Message
, the new Message
is constructed with the return value as the payload while inheriting
headers from the input Message
minus the headers defined or filtered by SpringIntegrationProperties.messageHandlerNotPropagatedHeaders
.
By default, there is only one header set there: contentType
. This means that the new Message
does not have contentType
header set, thus ensuring that the contentType
can evolve.
You can always opt out of returning a Message
from the handler method where you can inject any header you wish.
If there is an internal pipeline, the Message
is sent to the next handler by going through the same process of conversion. However, if there is no internal pipeline or you have reached the end of it, the Message
is sent back to the output destination.
8.1.1. Content Type versus Argument Type
As mentioned earlier, for the framework to select the appropriate MessageConverter
, it requires argument type and, optionally, content type information.
The logic for selecting the appropriate MessageConverter
resides with the argument resolvers (HandlerMethodArgumentResolvers
), which trigger right before the invocation of the user-defined handler method (which is when the actual argument type is known to the framework).
If the argument type does not match the type of the current payload, the framework delegates to the stack of the
pre-configured MessageConverters
to see if any one of them can convert the payload.
As you can see, the Object fromMessage(Message<?> message, Class<?> targetClass);
operation of the MessageConverter takes targetClass
as one of its arguments.
The framework also ensures that the provided Message
always contains a contentType
header.
When no contentType header was already present, it injects either the per-binding contentType
header or the default contentType
header.
The combination of contentType
argument type is the mechanism by which framework determines if message can be converted to a target type.
If no appropriate MessageConverter
is found, an exception is thrown, which you can handle by adding a custom MessageConverter
(see “User-defined Message Converters”).
But what if the payload type matches the target type declared by the handler method? In this case, there is nothing to convert, and the
payload is passed unmodified. While this sounds pretty straightforward and logical, keep in mind handler methods that take a Message<?>
or Object
as an argument.
By declaring the target type to be Object
(which is an instanceof
everything in Java), you essentially forfeit the conversion process.
Do not expect Message to be converted into some other type based only on the contentType .
Remember that the contentType is complementary to the target type.
If you wish, you can provide a hint, which MessageConverter may or may not take into consideration.
|
8.1.2. Message Converters
MessageConverters
define two methods:
Object fromMessage(Message<?> message, Class<?> targetClass);
Message<?> toMessage(Object payload, @Nullable MessageHeaders headers);
It is important to understand the contract of these methods and their usage, specifically in the context of Spring Cloud Stream.
The fromMessage
method converts an incoming Message
to an argument type.
The payload of the Message
could be any type, and it is
up to the actual implementation of the MessageConverter
to support multiple types.
For example, some JSON converter may support the payload type as byte[]
, String
, and others.
This is important when the application contains an internal pipeline (that is, input → handler1 → handler2 →. . . → output) and the output of the upstream handler results in a Message
which may not be in the initial wire format.
However, the toMessage
method has a more strict contract and must always convert Message
to the wire format: byte[]
.
So, for all intents and purposes (and especially when implementing your own converter) you regard the two methods as having the following signatures:
Object fromMessage(Message<?> message, Class<?> targetClass);
Message<byte[]> toMessage(Object payload, @Nullable MessageHeaders headers);
8.2. Provided MessageConverters
As mentioned earlier, the framework already provides a stack of MessageConverters
to handle most common use cases.
The following list describes the provided MessageConverters
, in order of precedence (the first MessageConverter
that works is used):
-
ApplicationJsonMessageMarshallingConverter
: Variation of theorg.springframework.messaging.converter.MappingJackson2MessageConverter
. Supports conversion of the payload of theMessage
to/from POJO for cases whencontentType
isapplication/json
(DEFAULT). -
TupleJsonMessageConverter
: DEPRECATED Supports conversion of the payload of theMessage
to/fromorg.springframework.tuple.Tuple
. -
ByteArrayMessageConverter
: Supports conversion of the payload of theMessage
frombyte[]
tobyte[]
for cases whencontentType
isapplication/octet-stream
. It is essentially a pass through and exists primarily for backward compatibility. -
ObjectStringMessageConverter
: Supports conversion of any type to aString
whencontentType
istext/plain
. It invokes Object’stoString()
method or, if the payload isbyte[]
, a newString(byte[])
. -
JavaSerializationMessageConverter
: DEPRECATED Supports conversion based on java serialization whencontentType
isapplication/x-java-serialized-object
. -
KryoMessageConverter
: DEPRECATED Supports conversion based on Kryo serialization whencontentType
isapplication/x-java-object
. -
JsonUnmarshallingConverter
: Similar to theApplicationJsonMessageMarshallingConverter
. It supports conversion of any type whencontentType
isapplication/x-java-object
. It expects the actual type information to be embedded in thecontentType
as an attribute (for example,application/x-java-object;type=foo.bar.Cat
).
When no appropriate converter is found, the framework throws an exception. When that happens, you should check your code and configuration and ensure you did not miss anything (that is, ensure that you provided a contentType
by using a binding or a header).
However, most likely, you found some uncommon case (such as a custom contentType
perhaps) and the current stack of provided MessageConverters
does not know how to convert. If that is the case, you can add custom MessageConverter
. See User-defined Message Converters.
8.3. User-defined Message Converters
Spring Cloud Stream exposes a mechanism to define and register additional MessageConverters
.
To use it, implement org.springframework.messaging.converter.MessageConverter
, configure it as a @Bean
, and annotate it with @StreamMessageConverter
.
It is then apended to the existing stack of `MessageConverter`s.
It is important to understand that custom MessageConverter implementations are added to the head of the existing stack.
Consequently, custom MessageConverter implementations take precedence over the existing ones, which lets you override as well as add to the existing converters.
|
The following example shows how to create a message converter bean to support a new content type called application/bar
:
@EnableBinding(Sink.class)
@SpringBootApplication
public static class SinkApplication {
...
@Bean
@StreamMessageConverter
public MessageConverter customMessageConverter() {
return new MyCustomMessageConverter();
}
}
public class MyCustomMessageConverter extends AbstractMessageConverter {
public MyCustomMessageConverter() {
super(new MimeType("application", "bar"));
}
@Override
protected boolean supports(Class<?> clazz) {
return (Bar.class.equals(clazz));
}
@Override
protected Object convertFromInternal(Message<?> message, Class<?> targetClass, Object conversionHint) {
Object payload = message.getPayload();
return (payload instanceof Bar ? payload : new Bar((byte[]) payload));
}
}
Spring Cloud Stream also provides support for Avro-based converters and schema evolution. See “Schema Evolution Support” for details.
9. Schema Evolution Support
Spring Cloud Stream provides support for schema evolution so that the data can be evolved over time and still work with older or newer producers and consumers and vice versa. Most serialization models, especially the ones that aim for portability across different platforms and languages, rely on a schema that describes how the data is serialized in the binary payload. In order to serialize the data and then to interpret it, both the sending and receiving sides must have access to a schema that describes the binary format. In certain cases, the schema can be inferred from the payload type on serialization or from the target type on deserialization. However, many applications benefit from having access to an explicit schema that describes the binary data format. A schema registry lets you store schema information in a textual format (typically JSON) and makes that information accessible to various applications that need it to receive and send data in binary format. A schema is referenceable as a tuple consisting of:
-
A subject that is the logical name of the schema
-
The schema version
-
The schema format, which describes the binary format of the data
This following sections goes through the details of various components involved in schema evolution process.
9.1. Schema Registry Client
The client-side abstraction for interacting with schema registry servers is the SchemaRegistryClient
interface, which has the following structure:
public interface SchemaRegistryClient {
SchemaRegistrationResponse register(String subject, String format, String schema);
String fetch(SchemaReference schemaReference);
String fetch(Integer id);
}
Spring Cloud Stream provides out-of-the-box implementations for interacting with its own schema server and for interacting with the Confluent Schema Registry.
A client for the Spring Cloud Stream schema registry can be configured by using the @EnableSchemaRegistryClient
, as follows:
@EnableBinding(Sink.class)
@SpringBootApplication
@EnableSchemaRegistryClient
public static class AvroSinkApplication {
...
}
The default converter is optimized to cache not only the schemas from the remote server but also the parse() and toString() methods, which are quite expensive.
Because of this, it uses a DefaultSchemaRegistryClient that does not cache responses.
If you intend to change the default behavior, you can use the client directly on your code and override it to the desired outcome.
To do so, you have to add the property spring.cloud.stream.schemaRegistryClient.cached=true to your application properties.
|
9.1.1. Schema Registry Client Properties
The Schema Registry Client supports the following properties:
spring.cloud.stream.schemaRegistryClient.endpoint
-
The location of the schema-server. When setting this, use a full URL, including protocol (
http
orhttps
) , port, and context path. - Default
spring.cloud.stream.schemaRegistryClient.cached
-
Whether the client should cache schema server responses. Normally set to
false
, as the caching happens in the message converter. Clients using the schema registry client should set this totrue
. - Default
-
true
9.2. Avro Schema Registry Client Message Converters
For applications that have a SchemaRegistryClient bean registered with the application context, Spring Cloud Stream auto configures an Apache Avro message converter for schema management. This eases schema evolution, as applications that receive messages can get easy access to a writer schema that can be reconciled with their own reader schema.
For outbound messages, if the content type of the channel is set to application/*+avro
, the MessageConverter
is activated, as shown in the following example:
spring.cloud.stream.bindings.output.contentType=application/*+avro
During the outbound conversion, the message converter tries to infer the schema of each outbound messages (based on its type) and register it to a subject (based on the payload type) by using the SchemaRegistryClient
.
If an identical schema is already found, then a reference to it is retrieved.
If not, the schema is registered, and a new version number is provided.
The message is sent with a contentType
header by using the following scheme: application/[prefix].[subject].v[version]+avro
, where prefix
is configurable and subject
is deduced from the payload type.
For example, a message of the type User
might be sent as a binary payload with a content type of application/vnd.user.v2+avro
, where user
is the subject and 2
is the version number.
When receiving messages, the converter infers the schema reference from the header of the incoming message and tries to retrieve it. The schema is used as the writer schema in the deserialization process.
9.2.1. Avro Schema Registry Message Converter Properties
If you have enabled Avro based schema registry client by setting spring.cloud.stream.bindings.output.contentType=application/*+avro
, you can customize the behavior of the registration by setting the following properties.
- spring.cloud.stream.schema.avro.dynamicSchemaGenerationEnabled
-
Enable if you want the converter to use reflection to infer a Schema from a POJO.
Default:
false
- spring.cloud.stream.schema.avro.readerSchema
-
Avro compares schema versions by looking at a writer schema (origin payload) and a reader schema (your application payload). See the Avro documentation for more information. If set, this overrides any lookups at the schema server and uses the local schema as the reader schema. Default:
null
- spring.cloud.stream.schema.avro.schemaLocations
-
Registers any
.avsc
files listed in this property with the Schema Server.Default:
empty
- spring.cloud.stream.schema.avro.prefix
-
The prefix to be used on the Content-Type header.
Default:
vnd
9.3. Apache Avro Message Converters
Spring Cloud Stream provides support for schema-based message converters through its spring-cloud-stream-schema
module.
Currently, the only serialization format supported out of the box for schema-based message converters is Apache Avro, with more formats to be added in future versions.
The spring-cloud-stream-schema
module contains two types of message converters that can be used for Apache Avro serialization:
-
Converters that use the class information of the serialized or deserialized objects or a schema with a location known at startup.
-
Converters that use a schema registry. They locate the schemas at runtime and dynamically register new schemas as domain objects evolve.
9.4. Converters with Schema Support
The AvroSchemaMessageConverter
supports serializing and deserializing messages either by using a predefined schema or by using the schema information available in the class (either reflectively or contained in the SpecificRecord
).
If you provide a custom converter, then the default AvroSchemaMessageConverter bean is not created. The following example shows a custom converter:
To use custom converters, you can simply add it to the application context, optionally specifying one or more MimeTypes
with which to associate it.
The default MimeType
is application/avro
.
If the target type of the conversion is a GenericRecord
, a schema must be set.
The following example shows how to configure a converter in a sink application by registering the Apache Avro MessageConverter
without a predefined schema.
In this example, note that the mime type value is avro/bytes
, not the default application/avro
.
@EnableBinding(Sink.class)
@SpringBootApplication
public static class SinkApplication {
...
@Bean
public MessageConverter userMessageConverter() {
return new AvroSchemaMessageConverter(MimeType.valueOf("avro/bytes"));
}
}
Conversely, the following application registers a converter with a predefined schema (found on the classpath):
@EnableBinding(Sink.class)
@SpringBootApplication
public static class SinkApplication {
...
@Bean
public MessageConverter userMessageConverter() {
AvroSchemaMessageConverter converter = new AvroSchemaMessageConverter(MimeType.valueOf("avro/bytes"));
converter.setSchemaLocation(new ClassPathResource("schemas/User.avro"));
return converter;
}
}
9.5. Schema Registry Server
Spring Cloud Stream provides a schema registry server implementation.
To use it, you can add the spring-cloud-stream-schema-server
artifact to your project and use the @EnableSchemaRegistryServer
annotation, which adds the schema registry server REST controller to your application.
This annotation is intended to be used with Spring Boot web applications, and the listening port of the server is controlled by the server.port
property.
The spring.cloud.stream.schema.server.path
property can be used to control the root path of the schema server (especially when it is embedded in other applications).
The spring.cloud.stream.schema.server.allowSchemaDeletion
boolean property enables the deletion of a schema. By default, this is disabled.
The schema registry server uses a relational database to store the schemas. By default, it uses an embedded database. You can customize the schema storage by using the Spring Boot SQL database and JDBC configuration options.
The following example shows a Spring Boot application that enables the schema registry:
@SpringBootApplication
@EnableSchemaRegistryServer
public class SchemaRegistryServerApplication {
public static void main(String[] args) {
SpringApplication.run(SchemaRegistryServerApplication.class, args);
}
}
9.5.1. Schema Registry Server API
The Schema Registry Server API consists of the following operations:
-
POST /
— see “Registering a New Schema” -
'GET /{subject}/{format}/{version}' — see “Retrieving an Existing Schema by Subject, Format, and Version”
-
GET /{subject}/{format}
— see “Retrieving an Existing Schema by Subject and Format” -
GET /schemas/{id}
— see “Retrieving an Existing Schema by ID” -
DELETE /{subject}/{format}/{version}
— see “Deleting a Schema by Subject, Format, and Version” -
DELETE /schemas/{id}
— see “Deleting a Schema by ID” -
DELETE /{subject}
— see “Deleting a Schema by Subject”
Registering a New Schema
To register a new schema, send a POST
request to the /
endpoint.
The /
accepts a JSON payload with the following fields:
-
subject
: The schema subject -
format
: The schema format -
definition
: The schema definition
Its response is a schema object in JSON, with the following fields:
-
id
: The schema ID -
subject
: The schema subject -
format
: The schema format -
version
: The schema version -
definition
: The schema definition
Retrieving an Existing Schema by Subject, Format, and Version
To retrieve an existing schema by subject, format, and version, send GET
request to the /{subject}/{format}/{version}
endpoint.
Its response is a schema object in JSON, with the following fields:
-
id
: The schema ID -
subject
: The schema subject -
format
: The schema format -
version
: The schema version -
definition
: The schema definition
Retrieving an Existing Schema by Subject and Format
To retrieve an existing schema by subject and format, send a GET
request to the /subject/format
endpoint.
Its response is a list of schemas with each schema object in JSON, with the following fields:
-
id
: The schema ID -
subject
: The schema subject -
format
: The schema format -
version
: The schema version -
definition
: The schema definition
Retrieving an Existing Schema by ID
To retrieve a schema by its ID, send a GET
request to the /schemas/{id}
endpoint.
Its response is a schema object in JSON, with the following fields:
-
id
: The schema ID -
subject
: The schema subject -
format
: The schema format -
version
: The schema version -
definition
: The schema definition
Deleting a Schema by Subject, Format, and Version
To delete a schema identified by its subject, format, and version, send a DELETE
request to the /{subject}/{format}/{version}
endpoint.
Deleting a Schema by ID
To delete a schema by its ID, send a DELETE
request to the /schemas/{id}
endpoint.
Deleting a Schema by Subject
DELETE /{subject}
Delete existing schemas by their subject.
This note applies to users of Spring Cloud Stream 1.1.0.RELEASE only.
Spring Cloud Stream 1.1.0.RELEASE used the table name, schema , for storing Schema objects. Schema is a keyword in a number of database implementations.
To avoid any conflicts in the future, starting with 1.1.1.RELEASE, we have opted for the name SCHEMA_REPOSITORY for the storage table.
Any Spring Cloud Stream 1.1.0.RELEASE users who upgrade should migrate their existing schemas to the new table before upgrading.
|
9.5.2. Using Confluent’s Schema Registry
The default configuration creates a DefaultSchemaRegistryClient
bean.
If you want to use the Confluent schema registry, you need to create a bean of type ConfluentSchemaRegistryClient
, which supersedes the one configured by default by the framework. The following example shows how to create such a bean:
@Bean
public SchemaRegistryClient schemaRegistryClient(@Value("${spring.cloud.stream.schemaRegistryClient.endpoint}") String endpoint){
ConfluentSchemaRegistryClient client = new ConfluentSchemaRegistryClient();
client.setEndpoint(endpoint);
return client;
}
The ConfluentSchemaRegistryClient is tested against Confluent platform version 4.0.0. |
9.6. Schema Registration and Resolution
To better understand how Spring Cloud Stream registers and resolves new schemas and its use of Avro schema comparison features, we provide two separate subsections:
9.6.1. Schema Registration Process (Serialization)
The first part of the registration process is extracting a schema from the payload that is being sent over a channel.
Avro types such as SpecificRecord
or GenericRecord
already contain a schema, which can be retrieved immediately from the instance.
In the case of POJOs, a schema is inferred if the spring.cloud.stream.schema.avro.dynamicSchemaGenerationEnabled
property is set to true
(the default).
Ones a schema is obtained, the converter loads its metadata (version) from the remote server. First, it queries a local cache. If no result is found, it submits the data to the server, which replies with versioning information. The converter always caches the results to avoid the overhead of querying the Schema Server for every new message that needs to be serialized.
With the schema version information, the converter sets the contentType
header of the message to carry the version information — for example: application/vnd.user.v1+avro
.
9.6.2. Schema Resolution Process (Deserialization)
When reading messages that contain version information (that is, a contentType
header with a scheme like the one described under “Schema Registration Process (Serialization)”), the converter queries the Schema server to fetch the writer schema of the message.
Once it has found the correct schema of the incoming message, it retrieves the reader schema and, by using Avro’s schema resolution support, reads it into the reader definition (setting defaults and any missing properties).
You should understand the difference between a writer schema (the application that wrote the message) and a reader schema (the receiving application).
We suggest taking a moment to read the Avro terminology and understand the process.
Spring Cloud Stream always fetches the writer schema to determine how to read a message.
If you want to get Avro’s schema evolution support working, you need to make sure that a readerSchema was properly set for your application.
|
10. Inter-Application Communication
Spring Cloud Stream enables communication between applications. Inter-application communication is a complex issue spanning several concerns, as described in the following topics:
10.1. Connecting Multiple Application Instances
While Spring Cloud Stream makes it easy for individual Spring Boot applications to connect to messaging systems, the typical scenario for Spring Cloud Stream is the creation of multi-application pipelines, where microservice applications send data to each other. You can achieve this scenario by correlating the input and output destinations of “adjacent” applications.
Suppose a design calls for the Time Source application to send data to the Log Sink application. You could use a common destination named ticktock
for bindings within both applications.
Time Source (that has the channel name output
) would set the following property:
spring.cloud.stream.bindings.output.destination=ticktock
Log Sink (that has the channel name input
) would set the following property:
spring.cloud.stream.bindings.input.destination=ticktock
10.2. Instance Index and Instance Count
When scaling up Spring Cloud Stream applications, each instance can receive information about how many other instances of the same application exist and what its own instance index is.
Spring Cloud Stream does this through the spring.cloud.stream.instanceCount
and spring.cloud.stream.instanceIndex
properties.
For example, if there are three instances of a HDFS sink application, all three instances have spring.cloud.stream.instanceCount
set to 3
, and the individual applications have spring.cloud.stream.instanceIndex
set to 0
, 1
, and 2
, respectively.
When Spring Cloud Stream applications are deployed through Spring Cloud Data Flow, these properties are configured automatically; when Spring Cloud Stream applications are launched independently, these properties must be set correctly.
By default, spring.cloud.stream.instanceCount
is 1
, and spring.cloud.stream.instanceIndex
is 0
.
In a scaled-up scenario, correct configuration of these two properties is important for addressing partitioning behavior (see below) in general, and the two properties are always required by certain binders (for example, the Kafka binder) in order to ensure that data are split correctly across multiple consumer instances.
10.3. Partitioning
Partitioning in Spring Cloud Stream consists of two tasks:
10.3.1. Configuring Output Bindings for Partitioning
You can configure an output binding to send partitioned data by setting one and only one of its partitionKeyExpression
or partitionKeyExtractorName
properties, as well as its partitionCount
property.
For example, the following is a valid and typical configuration:
spring.cloud.stream.bindings.output.producer.partitionKeyExpression=payload.id spring.cloud.stream.bindings.output.producer.partitionCount=5
Based on that example configuration, data is sent to the target partition by using the following logic.
A partition key’s value is calculated for each message sent to a partitioned output channel based on the partitionKeyExpression
.
The partitionKeyExpression
is a SpEL expression that is evaluated against the outbound message for extracting the partitioning key.
If a SpEL expression is not sufficient for your needs, you can instead calculate the partition key value by providing an implementation of org.springframework.cloud.stream.binder.PartitionKeyExtractorStrategy
and configuring it as a bean (by using the @Bean
annotation).
If you have more then one bean of type org.springframework.cloud.stream.binder.PartitionKeyExtractorStrategy
available in the Application Context, you can further filter it by specifying its name with the partitionKeyExtractorName
property, as shown in the following example:
--spring.cloud.stream.bindings.output.producer.partitionKeyExtractorName=customPartitionKeyExtractor
--spring.cloud.stream.bindings.output.producer.partitionCount=5
. . .
@Bean
public CustomPartitionKeyExtractorClass customPartitionKeyExtractor() {
return new CustomPartitionKeyExtractorClass();
}
In previous versions of Spring Cloud Stream, you could specify the implementation of org.springframework.cloud.stream.binder.PartitionKeyExtractorStrategy by setting the spring.cloud.stream.bindings.output.producer.partitionKeyExtractorClass property.
Since version 2.0, this property is deprecated, and support for it will be removed in a future version.
|
Once the message key is calculated, the partition selection process determines the target partition as a value between 0
and partitionCount - 1
.
The default calculation, applicable in most scenarios, is based on the following formula: key.hashCode() % partitionCount
.
This can be customized on the binding, either by setting a SpEL expression to be evaluated against the 'key' (through the partitionSelectorExpression
property) or by configuring an implementation of org.springframework.cloud.stream.binder.PartitionSelectorStrategy
as a bean (by using the @Bean annotation).
Similar to the PartitionKeyExtractorStrategy
, you can further filter it by using the spring.cloud.stream.bindings.output.producer.partitionSelectorName
property when more than one bean of this type is available in the Application Context, as shown in the following example:
--spring.cloud.stream.bindings.output.producer.partitionSelectorName=customPartitionSelector
. . .
@Bean
public CustomPartitionSelectorClass customPartitionSelector() {
return new CustomPartitionSelectorClass();
}
In previous versions of Spring Cloud Stream you could specify the implementation of org.springframework.cloud.stream.binder.PartitionSelectorStrategy by setting the spring.cloud.stream.bindings.output.producer.partitionSelectorClass property.
Since version 2.0, this property is deprecated and support for it will be removed in a future version.
|
10.3.2. Configuring Input Bindings for Partitioning
An input binding (with the channel name input
) is configured to receive partitioned data by setting its partitioned
property, as well as the instanceIndex
and instanceCount
properties on the application itself, as shown in the following example:
spring.cloud.stream.bindings.input.consumer.partitioned=true spring.cloud.stream.instanceIndex=3 spring.cloud.stream.instanceCount=5
The instanceCount
value represents the total number of application instances between which the data should be partitioned.
The instanceIndex
must be a unique value across the multiple instances, with a value between 0
and instanceCount - 1
.
The instance index helps each application instance to identify the unique partition(s) from which it receives data.
It is required by binders using technology that does not support partitioning natively.
For example, with RabbitMQ, there is a queue for each partition, with the queue name containing the instance index.
With Kafka, if autoRebalanceEnabled
is true
(default), Kafka takes care of distributing partitions across instances, and these properties are not required.
If autoRebalanceEnabled
is set to false, the instanceCount
and instanceIndex
are used by the binder to determine which partition(s) the instance subscribes to (you must have at least as many partitions as there are instances).
The binder allocates the partitions instead of Kafka.
This might be useful if you want messages for a particular partition to always go to the same instance.
When a binder configuration requires them, it is important to set both values correctly in order to ensure that all of the data is consumed and that the application instances receive mutually exclusive datasets.
While a scenario in which using multiple instances for partitioned data processing may be complex to set up in a standalone case, Spring Cloud Dataflow can simplify the process significantly by populating both the input and output values correctly and by letting you rely on the runtime infrastructure to provide information about the instance index and instance count.
11. Testing
Spring Cloud Stream provides support for testing your microservice applications without connecting to a messaging system.
You can do that by using the TestSupportBinder
provided by the spring-cloud-stream-test-support
library, which can be added as a test dependency to the application, as shown in the following example:
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-stream-test-support</artifactId>
<scope>test</scope>
</dependency>
The TestSupportBinder uses the Spring Boot autoconfiguration mechanism to supersede the other binders found on the classpath.
Therefore, when adding a binder as a dependency, you must make sure that the test scope is being used.
|
The TestSupportBinder
lets you interact with the bound channels and inspect any messages sent and received by the application.
For outbound message channels, the TestSupportBinder
registers a single subscriber and retains the messages emitted by the application in a MessageCollector
.
They can be retrieved during tests and have assertions made against them.
You can also send messages to inbound message channels so that the consumer application can consume the messages. The following example shows how to test both input and output channels on a processor:
@RunWith(SpringRunner.class)
@SpringBootTest(webEnvironment= SpringBootTest.WebEnvironment.RANDOM_PORT)
public class ExampleTest {
@Autowired
private Processor processor;
@Autowired
private MessageCollector messageCollector;
@Test
@SuppressWarnings("unchecked")
public void testWiring() {
Message<String> message = new GenericMessage<>("hello");
processor.input().send(message);
Message<String> received = (Message<String>) messageCollector.forChannel(processor.output()).poll();
assertThat(received.getPayload(), equalTo("hello world"));
}
@SpringBootApplication
@EnableBinding(Processor.class)
public static class MyProcessor {
@Autowired
private Processor channels;
@Transformer(inputChannel = Processor.INPUT, outputChannel = Processor.OUTPUT)
public String transform(String in) {
return in + " world";
}
}
}
In the preceding example, we create an application that has an input channel and an output channel, both bound through the Processor
interface.
The bound interface is injected into the test so that we can have access to both channels.
We send a message on the input channel, and we use the MessageCollector
provided by Spring Cloud Stream’s test support to capture that the message has been sent to the output channel as a result.
Once we have received the message, we can validate that the component functions correctly.
11.1. Disabling the Test Binder Autoconfiguration
The intent behind the test binder superseding all the other binders on the classpath is to make it easy to test your applications without making changes to your production dependencies.
In some cases (for example, integration tests) it is useful to use the actual production binders instead, and that requires disabling the test binder autoconfiguration.
To do so, you can exclude the org.springframework.cloud.stream.test.binder.TestSupportBinderAutoConfiguration
class by using one of the Spring Boot autoconfiguration exclusion mechanisms, as shown in the following example:
@SpringBootApplication(exclude = TestSupportBinderAutoConfiguration.class)
@EnableBinding(Processor.class)
public static class MyProcessor {
@Transformer(inputChannel = Processor.INPUT, outputChannel = Processor.OUTPUT)
public String transform(String in) {
return in + " world";
}
}
When autoconfiguration is disabled, the test binder is available on the classpath, and its defaultCandidate
property is set to false
so that it does not interfere with the regular user configuration. It can be referenced under the name, test
, as shown in the following example:
spring.cloud.stream.defaultBinder=test
12. Health Indicator
Spring Cloud Stream provides a health indicator for binders.
It is registered under the name binders
and can be enabled or disabled by setting the management.health.binders.enabled
property.
By default management.health.binders.enabled
is set to false
.
Setting management.health.binders.enabled
to true
enables the health indicator, allowing you to access the /health
endpoint to retrieve the binder health indicators.
Health indicators are binder-specific and certain binder implementations may not necessarily provide a health indicator.
13. Metrics Emitter
Spring Boot Actuator provides dependency management and auto-configuration for Micrometer, an application metrics facade that supports numerous monitoring systems.
Spring Cloud Stream provides support for emitting any available micrometer-based metrics to a binding destination, allowing for periodic collection of metric data from stream applications without relying on polling individual endpoints.
Metrics Emitter is activated by defining the spring.cloud.stream.bindings.applicationMetrics.destination
property,
which specifies the name of the binding destination used by the current binder to publish metric messages.
For example:
spring.cloud.stream.bindings.applicationMetrics.destination=myMetricDestination
The preceding example instructs the binder to bind to myMetricDestination
(that is, Rabbit exchange, Kafka topic, and others).
The following properties can be used for customizing the emission of metrics:
- spring.cloud.stream.metrics.key
-
The name of the metric being emitted. Should be a unique value per application.
Default:
${spring.application.name:${vcap.application.name:${spring.config.name:application}}}
- spring.cloud.stream.metrics.properties
-
Allows white listing application properties that are added to the metrics payload
Default: null.
- spring.cloud.stream.metrics.meter-filter
-
Pattern to control the 'meters' one wants to capture. For example, specifying
spring.integration.*
captures metric information for meters whose name starts withspring.integration.
Default: all 'meters' are captured.
- spring.cloud.stream.metrics.schedule-interval
-
Interval to control the rate of publishing metric data.
Default: 1 min
Consider the following:
java -jar time-source.jar \
--spring.cloud.stream.bindings.applicationMetrics.destination=someMetrics \
--spring.cloud.stream.metrics.properties=spring.application** \
--spring.cloud.stream.metrics.meter-filter=spring.integration.*
The following example shows the payload of the data published to the binding destination as a result of the preceding command:
{
"name": "application",
"createdTime": "2018-03-23T14:48:12.700Z",
"properties": {
},
"metrics": [
{
"id": {
"name": "spring.integration.send",
"tags": [
{
"key": "exception",
"value": "none"
},
{
"key": "name",
"value": "input"
},
{
"key": "result",
"value": "success"
},
{
"key": "type",
"value": "channel"
}
],
"type": "TIMER",
"description": "Send processing time",
"baseUnit": "milliseconds"
},
"timestamp": "2018-03-23T14:48:12.697Z",
"sum": 130.340546,
"count": 6,
"mean": 21.72342433333333,
"upper": 116.176299,
"total": 130.340546
}
]
}
Given that the format of the Metric message has slightly changed after migrating to Micrometer, the published message will also have
a STREAM_CLOUD_STREAM_VERSION header set to 2.x to help distinguish between Metric messages from the older versions of the Spring Cloud Stream.
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14. Samples
For Spring Cloud Stream samples, see the spring-cloud-stream-samples repository on GitHub.
14.1. Deploying Stream Applications on CloudFoundry
On CloudFoundry, services are usually exposed through a special environment variable called VCAP_SERVICES.
When configuring your binder connections, you can use the values from an environment variable as explained on the dataflow Cloud Foundry Server docs.
Binder Implementations
15. Apache Kafka Binder
15.1. Usage
To use Apache Kafka binder, you need to add spring-cloud-stream-binder-kafka
as a dependency to your Spring Cloud Stream application, as shown in the following example for Maven:
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-stream-binder-kafka</artifactId>
</dependency>
Alternatively, you can also use the Spring Cloud Stream Kafka Starter, as shown inn the following example for Maven:
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-stream-kafka</artifactId>
</dependency>
15.2. Apache Kafka Binder Overview
The following image shows a simplified diagram of how the Apache Kafka binder operates:
The Apache Kafka Binder implementation maps each destination to an Apache Kafka topic. The consumer group maps directly to the same Apache Kafka concept. Partitioning also maps directly to Apache Kafka partitions as well.
The binder currently uses the Apache Kafka kafka-clients
1.0.0 jar and is designed to be used with a broker of at least that version.
This client can communicate with older brokers (see the Kafka documentation), but certain features may not be available.
For example, with versions earlier than 0.11.x.x, native headers are not supported.
Also, 0.11.x.x does not support the autoAddPartitions
property.
15.3. Configuration Options
This section contains the configuration options used by the Apache Kafka binder.
For common configuration options and properties pertaining to binder, see the core documentation.
Kafka Binder Properties
- spring.cloud.stream.kafka.binder.brokers
-
A list of brokers to which the Kafka binder connects.
Default:
localhost
. - spring.cloud.stream.kafka.binder.defaultBrokerPort
-
brokers
allows hosts specified with or without port information (for example,host1,host2:port2
). This sets the default port when no port is configured in the broker list.Default:
9092
. - spring.cloud.stream.kafka.binder.configuration
-
Key/Value map of client properties (both producers and consumer) passed to all clients created by the binder. Due to the fact that these properties are used by both producers and consumers, usage should be restricted to common properties — for example, security settings.
Default: Empty map.
- spring.cloud.stream.kafka.binder.headers
-
The list of custom headers that are transported by the binder. Only required when communicating with older applications (⇐ 1.3.x) with a
kafka-clients
version < 0.11.0.0. Newer versions support headers natively.Default: empty.
- spring.cloud.stream.kafka.binder.healthTimeout
-
The time to wait to get partition information, in seconds. Health reports as down if this timer expires.
Default: 10.
- spring.cloud.stream.kafka.binder.requiredAcks
-
The number of required acks on the broker. See the Kafka documentation for the producer
acks
property.Default:
1
. - spring.cloud.stream.kafka.binder.minPartitionCount
-
Effective only if
autoCreateTopics
orautoAddPartitions
is set. The global minimum number of partitions that the binder configures on topics on which it produces or consumes data. It can be superseded by thepartitionCount
setting of the producer or by the value ofinstanceCount * concurrency
settings of the producer (if either is larger).Default:
1
. - spring.cloud.stream.kafka.binder.replicationFactor
-
The replication factor of auto-created topics if
autoCreateTopics
is active. Can be overridden on each binding.Default:
1
. - spring.cloud.stream.kafka.binder.autoCreateTopics
-
If set to
true
, the binder creates new topics automatically. If set tofalse
, the binder relies on the topics being already configured. In the latter case, if the topics do not exist, the binder fails to start.This setting is independent of the auto.topic.create.enable
setting of the broker and does not influence it. If the server is set to auto-create topics, they may be created as part of the metadata retrieval request, with default broker settings.Default:
true
. - spring.cloud.stream.kafka.binder.autoAddPartitions
-
If set to
true
, the binder creates new partitions if required. If set tofalse
, the binder relies on the partition size of the topic being already configured. If the partition count of the target topic is smaller than the expected value, the binder fails to start.Default:
false
. - spring.cloud.stream.kafka.binder.transaction.transactionIdPrefix
-
Enables transactions in the binder. See
transaction.id
in the Kafka documentation and Transactions in thespring-kafka
documentation. When transactions are enabled, individualproducer
properties are ignored and all producers use thespring.cloud.stream.kafka.binder.transaction.producer.*
properties.Default
null
(no transactions) - spring.cloud.stream.kafka.binder.transaction.producer.*
-
Global producer properties for producers in a transactional binder. See
spring.cloud.stream.kafka.binder.transaction.transactionIdPrefix
and Kafka Producer Properties and the general producer properties supported by all binders.Default: See individual producer properties.
- spring.cloud.stream.kafka.binder.headerMapperBeanName
-
The bean name of a
KafkaHeaderMapper
used for mappingspring-messaging
headers to and from Kafka headers. Use this, for example, if you wish to customize the trusted packages in aDefaultKafkaHeaderMapper
that uses JSON deserialization for the headers.Default: none.
Kafka Consumer Properties
The following properties are available for Kafka consumers only and
must be prefixed with spring.cloud.stream.kafka.bindings.<channelName>.consumer.
.
- admin.configuration
-
A
Map
of Kafka topic properties used when provisioning topics — for example,spring.cloud.stream.kafka.bindings.input.consumer.admin.configuration.message.format.version=0.9.0.0
Default: none.
- admin.replicas-assignment
-
A Map<Integer, List<Integer>> of replica assignments, with the key being the partition and the value being the assignments. Used when provisioning new topics. See the
NewTopic
Javadocs in thekafka-clients
jar.Default: none.
- admin.replication-factor
-
The replication factor to use when provisioning topics. Overrides the binder-wide setting. Ignored if
replicas-assignments
is present.Default: none (the binder-wide default of 1 is used).
- autoRebalanceEnabled
-
When
true
, topic partitions is automatically rebalanced between the members of a consumer group. Whenfalse
, each consumer is assigned a fixed set of partitions based onspring.cloud.stream.instanceCount
andspring.cloud.stream.instanceIndex
. This requires both thespring.cloud.stream.instanceCount
andspring.cloud.stream.instanceIndex
properties to be set appropriately on each launched instance. The value of thespring.cloud.stream.instanceCount
property must typically be greater than 1 in this case.Default:
true
. - ackEachRecord
-
When
autoCommitOffset
istrue
, this setting dictates whether to commit the offset after each record is processed. By default, offsets are committed after all records in the batch of records returned byconsumer.poll()
have been processed. The number of records returned by a poll can be controlled with themax.poll.records
Kafka property, which is set through the consumerconfiguration
property. Setting this totrue
may cause a degradation in performance, but doing so reduces the likelihood of redelivered records when a failure occurs. Also, see the binderrequiredAcks
property, which also affects the performance of committing offsets.Default:
false
. - autoCommitOffset
-
Whether to autocommit offsets when a message has been processed. If set to
false
, a header with the keykafka_acknowledgment
of the typeorg.springframework.kafka.support.Acknowledgment
header is present in the inbound message. Applications may use this header for acknowledging messages. See the examples section for details. When this property is set tofalse
, Kafka binder sets the ack mode toorg.springframework.kafka.listener.AbstractMessageListenerContainer.AckMode.MANUAL
and the application is responsible for acknowledging records. Also seeackEachRecord
.Default:
true
. - autoCommitOnError
-
Effective only if
autoCommitOffset
is set totrue
. If set tofalse
, it suppresses auto-commits for messages that result in errors and commits only for successful messages. It allows a stream to automatically replay from the last successfully processed message, in case of persistent failures. If set totrue
, it always auto-commits (if auto-commit is enabled). If not set (the default), it effectively has the same value asenableDlq
, auto-committing erroneous messages if they are sent to a DLQ and not committing them otherwise.Default: not set.
- resetOffsets
-
Whether to reset offsets on the consumer to the value provided by startOffset.
Default:
false
. - startOffset
-
The starting offset for new groups. Allowed values:
earliest
andlatest
. If the consumer group is set explicitly for the consumer 'binding' (throughspring.cloud.stream.bindings.<channelName>.group
), 'startOffset' is set toearliest
. Otherwise, it is set tolatest
for theanonymous
consumer group. Also seeresetOffsets
(earlier in this list).Default: null (equivalent to
earliest
). - enableDlq
-
When set to true, it enables DLQ behavior for the consumer. By default, messages that result in errors are forwarded to a topic named
error.<destination>.<group>
. The DLQ topic name can be configurable by setting thedlqName
property. This provides an alternative option to the more common Kafka replay scenario for the case when the number of errors is relatively small and replaying the entire original topic may be too cumbersome. See Dead-Letter Topic Processing processing for more information. Starting with version 2.0, messages sent to the DLQ topic are enhanced with the following headers:x-original-topic
,x-exception-message
, andx-exception-stacktrace
asbyte[]
.Default:
false
. - configuration
-
Map with a key/value pair containing generic Kafka consumer properties.
Default: Empty map.
- dlqName
-
The name of the DLQ topic to receive the error messages.
Default: null (If not specified, messages that result in errors are forwarded to a topic named
error.<destination>.<group>
). - dlqProducerProperties
-
Using this, DLQ-specific producer properties can be set. All the properties available through kafka producer properties can be set through this property.
Default: Default Kafka producer properties.
- standardHeaders
-
Indicates which standard headers are populated by the inbound channel adapter. Allowed values:
none
,id
,timestamp
, orboth
. Useful if using native deserialization and the first component to receive a message needs anid
(such as an aggregator that is configured to use a JDBC message store).Default:
none
- converterBeanName
-
The name of a bean that implements
RecordMessageConverter
. Used in the inbound channel adapter to replace the defaultMessagingMessageConverter
.Default:
null
- idleEventInterval
-
The interval, in milliseconds, between events indicating that no messages have recently been received. Use an
ApplicationListener<ListenerContainerIdleEvent>
to receive these events. See Example: Pausing and Resuming the Consumer for a usage example.Default:
30000
Kafka Producer Properties
The following properties are available for Kafka producers only and
must be prefixed with spring.cloud.stream.kafka.bindings.<channelName>.producer.
.
- admin.configuration
-
A
Map
of Kafka topic properties used when provisioning new topics — for example,spring.cloud.stream.kafka.bindings.input.consumer.admin.configuration.message.format.version=0.9.0.0
Default: none.
- admin.replicas-assignment
-
A Map<Integer, List<Integer>> of replica assignments, with the key being the partition and the value being the assignments. Used when provisioning new topics. See
NewTopic
javadocs in thekafka-clients
jar.Default: none.
- admin.replication-factor
-
The replication factor to use when provisioning new topics. Overrides the binder-wide setting. Ignored if
replicas-assignments
is present.Default: none (the binder-wide default of 1 is used).
- bufferSize
-
Upper limit, in bytes, of how much data the Kafka producer attempts to batch before sending.
Default:
16384
. - sync
-
Whether the producer is synchronous.
Default:
false
. - batchTimeout
-
How long the producer waits to allow more messages to accumulate in the same batch before sending the messages. (Normally, the producer does not wait at all and simply sends all the messages that accumulated while the previous send was in progress.) A non-zero value may increase throughput at the expense of latency.
Default:
0
. - messageKeyExpression
-
A SpEL expression evaluated against the outgoing message used to populate the key of the produced Kafka message — for example,
headers['myKey']
. The payload cannot be used because, by the time this expression is evaluated, the payload is already in the form of abyte[]
.Default:
none
. - headerPatterns
-
A comma-delimited list of simple patterns to match Spring messaging headers to be mapped to the Kafka
Headers
in theProducerRecord
. Patterns can begin or end with the wildcard character (asterisk). Patterns can be negated by prefixing with!
. Matching stops after the first match (positive or negative). For example!ask,as*
will passash
but notask
.id
andtimestamp
are never mapped.Default:
*
(all headers - except theid
andtimestamp
) - configuration
-
Map with a key/value pair containing generic Kafka producer properties.
Default: Empty map.
The Kafka binder uses the partitionCount setting of the producer as a hint to create a topic with the given partition count (in conjunction with the minPartitionCount , the maximum of the two being the value being used).
Exercise caution when configuring both minPartitionCount for a binder and partitionCount for an application, as the larger value is used.
If a topic already exists with a smaller partition count and autoAddPartitions is disabled (the default), the binder fails to start.
If a topic already exists with a smaller partition count and autoAddPartitions is enabled, new partitions are added.
If a topic already exists with a larger number of partitions than the maximum of (minPartitionCount or partitionCount ), the existing partition count is used.
|
Usage examples
In this section, we show the use of the preceding properties for specific scenarios.
Example: Setting autoCommitOffset
to false
and Relying on Manual Acking
This example illustrates how one may manually acknowledge offsets in a consumer application.
This example requires that spring.cloud.stream.kafka.bindings.input.consumer.autoCommitOffset
be set to false
.
Use the corresponding input channel name for your example.
@SpringBootApplication
@EnableBinding(Sink.class)
public class ManuallyAcknowdledgingConsumer {
public static void main(String[] args) {
SpringApplication.run(ManuallyAcknowdledgingConsumer.class, args);
}
@StreamListener(Sink.INPUT)
public void process(Message<?> message) {
Acknowledgment acknowledgment = message.getHeaders().get(KafkaHeaders.ACKNOWLEDGMENT, Acknowledgment.class);
if (acknowledgment != null) {
System.out.println("Acknowledgment provided");
acknowledgment.acknowledge();
}
}
}
Example: Security Configuration
Apache Kafka 0.9 supports secure connections between client and brokers.
To take advantage of this feature, follow the guidelines in the Apache Kafka Documentation as well as the Kafka 0.9 security guidelines from the Confluent documentation.
Use the spring.cloud.stream.kafka.binder.configuration
option to set security properties for all clients created by the binder.
For example, to set security.protocol
to SASL_SSL
, set the following property:
spring.cloud.stream.kafka.binder.configuration.security.protocol=SASL_SSL
All the other security properties can be set in a similar manner.
When using Kerberos, follow the instructions in the reference documentation for creating and referencing the JAAS configuration.
Spring Cloud Stream supports passing JAAS configuration information to the application by using a JAAS configuration file and using Spring Boot properties.
The JAAS and (optionally) krb5 file locations can be set for Spring Cloud Stream applications by using system properties. The following example shows how to launch a Spring Cloud Stream application with SASL and Kerberos by using a JAAS configuration file:
java -Djava.security.auth.login.config=/path.to/kafka_client_jaas.conf -jar log.jar \
--spring.cloud.stream.kafka.binder.brokers=secure.server:9092 \
--spring.cloud.stream.bindings.input.destination=stream.ticktock \
--spring.cloud.stream.kafka.binder.configuration.security.protocol=SASL_PLAINTEXT
As an alternative to having a JAAS configuration file, Spring Cloud Stream provides a mechanism for setting up the JAAS configuration for Spring Cloud Stream applications by using Spring Boot properties.
The following properties can be used to configure the login context of the Kafka client:
- spring.cloud.stream.kafka.binder.jaas.loginModule
-
The login module name. Not necessary to be set in normal cases.
Default:
com.sun.security.auth.module.Krb5LoginModule
. - spring.cloud.stream.kafka.binder.jaas.controlFlag
-
The control flag of the login module.
Default:
required
. - spring.cloud.stream.kafka.binder.jaas.options
-
Map with a key/value pair containing the login module options.
Default: Empty map.
The following example shows how to launch a Spring Cloud Stream application with SASL and Kerberos by using Spring Boot configuration properties:
java --spring.cloud.stream.kafka.binder.brokers=secure.server:9092 \
--spring.cloud.stream.bindings.input.destination=stream.ticktock \
--spring.cloud.stream.kafka.binder.autoCreateTopics=false \
--spring.cloud.stream.kafka.binder.configuration.security.protocol=SASL_PLAINTEXT \
--spring.cloud.stream.kafka.binder.jaas.options.useKeyTab=true \
--spring.cloud.stream.kafka.binder.jaas.options.storeKey=true \
--spring.cloud.stream.kafka.binder.jaas.options.keyTab=/etc/security/keytabs/kafka_client.keytab \
--spring.cloud.stream.kafka.binder.jaas.options.principal=kafka-client-1@EXAMPLE.COM
The preceding example represents the equivalent of the following JAAS file:
KafkaClient {
com.sun.security.auth.module.Krb5LoginModule required
useKeyTab=true
storeKey=true
keyTab="/etc/security/keytabs/kafka_client.keytab"
principal="[email protected]";
};
If the topics required already exist on the broker or will be created by an administrator, autocreation can be turned off and only client JAAS properties need to be sent.
Do not mix JAAS configuration files and Spring Boot properties in the same application.
If the -Djava.security.auth.login.config system property is already present, Spring Cloud Stream ignores the Spring Boot properties.
|
Be careful when using the autoCreateTopics and autoAddPartitions with Kerberos.
Usually, applications may use principals that do not have administrative rights in Kafka and Zookeeper.
Consequently, relying on Spring Cloud Stream to create/modify topics may fail.
In secure environments, we strongly recommend creating topics and managing ACLs administratively by using Kafka tooling.
|
Example: Pausing and Resuming the Consumer
If you wish to suspend consumption but not cause a partition rebalance, you can pause and resume the consumer.
This is facilitated by adding the Consumer
as a parameter to your @StreamListener
.
To resume, you need an ApplicationListener
for ListenerContainerIdleEvent
instances.
The frequency at which events are published is controlled by the idleEventInterval
property.
Since the consumer is not thread-safe, you must call these methods on the calling thread.
The following simple application shows how to pause and resume:
@SpringBootApplication
@EnableBinding(Sink.class)
public class Application {
public static void main(String[] args) {
SpringApplication.run(Application.class, args);
}
@StreamListener(Sink.INPUT)
public void in(String in, @Header(KafkaHeaders.CONSUMER) Consumer<?, ?> consumer) {
System.out.println(in);
consumer.pause(Collections.singleton(new TopicPartition("myTopic", 0)));
}
@Bean
public ApplicationListener<ListenerContainerIdleEvent> idleListener() {
return event -> {
System.out.println(event);
if (event.getConsumer().paused().size() > 0) {
event.getConsumer().resume(event.getConsumer().paused());
}
};
}
}
15.4. Error Channels
Starting with version 1.3, the binder unconditionally sends exceptions to an error channel for each consumer destination and can also be configured to send async producer send failures to an error channel. See Error Handling for more information.
The payload of the ErrorMessage
for a send failure is a KafkaSendFailureException
with properties:
-
failedMessage
: The Spring MessagingMessage<?>
that failed to be sent. -
record
: The rawProducerRecord
that was created from thefailedMessage
There is no automatic handling of producer exceptions (such as sending to a Dead-Letter queue). You can consume these exceptions with your own Spring Integration flow.
15.5. Kafka Metrics
Kafka binder module exposes the following metrics:
spring.cloud.stream.binder.kafka.offset
: This metric indicates how many messages have not been yet consumed from a given binder’s topic by a given consumer group.
The metrics provided are based on the Mircometer metrics library. The metric contains the consumer group information, topic and the actual lag in committed offset from the latest offset on the topic.
This metric is particularly useful for providing auto-scaling feedback to a PaaS platform.
15.6. Dead-Letter Topic Processing
Because you cannot anticipate how users would want to dispose of dead-lettered messages, the framework does not provide any standard mechanism to handle them. If the reason for the dead-lettering is transient, you may wish to route the messages back to the original topic. However, if the problem is a permanent issue, that could cause an infinite loop. The sample Spring Boot application within this topic is an example of how to route those messages back to the original topic, but it moves them to a “parking lot” topic after three attempts. The application is another spring-cloud-stream application that reads from the dead-letter topic. It terminates when no messages are received for 5 seconds.
The examples assume the original destination is so8400out
and the consumer group is so8400
.
There are a couple of strategies to consider:
-
Consider running the rerouting only when the main application is not running. Otherwise, the retries for transient errors are used up very quickly.
-
Alternatively, use a two-stage approach: Use this application to route to a third topic and another to route from there back to the main topic.
The following code listings show the sample application:
spring.cloud.stream.bindings.input.group=so8400replay
spring.cloud.stream.bindings.input.destination=error.so8400out.so8400
spring.cloud.stream.bindings.output.destination=so8400out
spring.cloud.stream.bindings.output.producer.partitioned=true
spring.cloud.stream.bindings.parkingLot.destination=so8400in.parkingLot
spring.cloud.stream.bindings.parkingLot.producer.partitioned=true
spring.cloud.stream.kafka.binder.configuration.auto.offset.reset=earliest
spring.cloud.stream.kafka.binder.headers=x-retries
@SpringBootApplication
@EnableBinding(TwoOutputProcessor.class)
public class ReRouteDlqKApplication implements CommandLineRunner {
private static final String X_RETRIES_HEADER = "x-retries";
public static void main(String[] args) {
SpringApplication.run(ReRouteDlqKApplication.class, args).close();
}
private final AtomicInteger processed = new AtomicInteger();
@Autowired
private MessageChannel parkingLot;
@StreamListener(Processor.INPUT)
@SendTo(Processor.OUTPUT)
public Message<?> reRoute(Message<?> failed) {
processed.incrementAndGet();
Integer retries = failed.getHeaders().get(X_RETRIES_HEADER, Integer.class);
if (retries == null) {
System.out.println("First retry for " + failed);
return MessageBuilder.fromMessage(failed)
.setHeader(X_RETRIES_HEADER, new Integer(1))
.setHeader(BinderHeaders.PARTITION_OVERRIDE,
failed.getHeaders().get(KafkaHeaders.RECEIVED_PARTITION_ID))
.build();
}
else if (retries.intValue() < 3) {
System.out.println("Another retry for " + failed);
return MessageBuilder.fromMessage(failed)
.setHeader(X_RETRIES_HEADER, new Integer(retries.intValue() + 1))
.setHeader(BinderHeaders.PARTITION_OVERRIDE,
failed.getHeaders().get(KafkaHeaders.RECEIVED_PARTITION_ID))
.build();
}
else {
System.out.println("Retries exhausted for " + failed);
parkingLot.send(MessageBuilder.fromMessage(failed)
.setHeader(BinderHeaders.PARTITION_OVERRIDE,
failed.getHeaders().get(KafkaHeaders.RECEIVED_PARTITION_ID))
.build());
}
return null;
}
@Override
public void run(String... args) throws Exception {
while (true) {
int count = this.processed.get();
Thread.sleep(5000);
if (count == this.processed.get()) {
System.out.println("Idle, terminating");
return;
}
}
}
public interface TwoOutputProcessor extends Processor {
@Output("parkingLot")
MessageChannel parkingLot();
}
}
15.7. Partitioning with the Kafka Binder
Apache Kafka supports topic partitioning natively.
Sometimes it is advantageous to send data to specific partitions — for example, when you want to strictly order message processing (all messages for a particular customer should go to the same partition).
The following example shows how to configure the producer and consumer side:
@SpringBootApplication
@EnableBinding(Source.class)
public class KafkaPartitionProducerApplication {
private static final Random RANDOM = new Random(System.currentTimeMillis());
private static final String[] data = new String[] {
"foo1", "bar1", "qux1",
"foo2", "bar2", "qux2",
"foo3", "bar3", "qux3",
"foo4", "bar4", "qux4",
};
public static void main(String[] args) {
new SpringApplicationBuilder(KafkaPartitionProducerApplication.class)
.web(false)
.run(args);
}
@InboundChannelAdapter(channel = Source.OUTPUT, poller = @Poller(fixedRate = "5000"))
public Message<?> generate() {
String value = data[RANDOM.nextInt(data.length)];
System.out.println("Sending: " + value);
return MessageBuilder.withPayload(value)
.setHeader("partitionKey", value)
.build();
}
}
spring:
cloud:
stream:
bindings:
output:
destination: partitioned.topic
producer:
partitioned: true
partition-key-expression: headers['partitionKey']
partition-count: 12
The topic must be provisioned to have enough partitions to achieve the desired concurrency for all consumer groups.
The above configuration supports up to 12 consumer instances (6 if their concurrency is 2, 4 if their concurrency is 3, and so on).
It is generally best to “over-provision” the partitions to allow for future increases in consumers or concurrency.
|
The preceding configuration uses the default partitioning (key.hashCode() % partitionCount ).
This may or may not provide a suitably balanced algorithm, depending on the key values.
You can override this default by using the partitionSelectorExpression or partitionSelectorClass properties.
|
Since partitions are natively handled by Kafka, no special configuration is needed on the consumer side. Kafka allocates partitions across the instances.
The following Spring Boot application listens to a Kafka stream and prints (to the console) the partition ID to which each message goes:
@SpringBootApplication
@EnableBinding(Sink.class)
public class KafkaPartitionConsumerApplication {
public static void main(String[] args) {
new SpringApplicationBuilder(KafkaPartitionConsumerApplication.class)
.web(false)
.run(args);
}
@StreamListener(Sink.INPUT)
public void listen(@Payload String in, @Header(KafkaHeaders.RECEIVED_PARTITION_ID) int partition) {
System.out.println(in + " received from partition " + partition);
}
}
spring:
cloud:
stream:
bindings:
input:
destination: partitioned.topic
group: myGroup
You can add instances as needed.
Kafka rebalances the partition allocations.
If the instance count (or instance count * concurrency
) exceeds the number of partitions, some consumers are idle.
16. Apache Kafka Streams Binder
16.1. Usage
For using the Kafka Streams binder, you just need to add it to your Spring Cloud Stream application, using the following Maven coordinates:
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-stream-binder-kafka-streams</artifactId>
</dependency>
16.2. Kafka Streams Binder Overview
Spring Cloud Stream’s Apache Kafka support also includes a binder implementation designed explicitly for Apache Kafka Streams binding. With this native integration, a Spring Cloud Stream "processor" application can directly use the Apache Kafka Streams APIs in the core business logic.
Kafka Streams binder implementation builds on the foundation provided by the Kafka Streams in Spring Kafka project.
As part of this native integration, the high-level Streams DSL provided by the Kafka Streams API is available for use in the business logic, too.
An early version of the Processor API support is available as well.
As noted early-on, Kafka Streams support in Spring Cloud Stream strictly only available for use in the Processor model. A model in which the messages read from an inbound topic, business processing can be applied, and the transformed messages can be written to an outbound topic. It can also be used in Processor applications with a no-outbound destination.
16.2.1. Streams DSL
This application consumes data from a Kafka topic (e.g., words
), computes word count for each unique word in a 5 seconds
time window, and the computed results are sent to a downstream topic (e.g., counts
) for further processing.
@SpringBootApplication
@EnableBinding(KStreamProcessor.class)
public class WordCountProcessorApplication {
@StreamListener("input")
@SendTo("output")
public KStream<?, WordCount> process(KStream<?, String> input) {
return input
.flatMapValues(value -> Arrays.asList(value.toLowerCase().split("\\W+")))
.groupBy((key, value) -> value)
.windowedBy(TimeWindows.of(5000))
.count(Materialized.as("WordCounts-multi"))
.toStream()
.map((key, value) -> new KeyValue<>(null, new WordCount(key.key(), value, new Date(key.window().start()), new Date(key.window().end()))));
}
public static void main(String[] args) {
SpringApplication.run(WordCountProcessorApplication.class, args);
}
Once built as a uber-jar (e.g., wordcount-processor.jar
), you can run the above example like the following.
java -jar wordcount-processor.jar --spring.cloud.stream.bindings.input.destination=words --spring.cloud.stream.bindings.output.destination=counts
This application will consume messages from the Kafka topic words
and the computed results are published to an output
topic counts
.
Spring Cloud Stream will ensure that the messages from both the incoming and outgoing topics are automatically bound as KStream objects. As a developer, you can exclusively focus on the business aspects of the code, i.e. writing the logic required in the processor. Setting up the Streams DSL specific configuration required by the Kafka Streams infrastructure is automatically handled by the framework.
16.3. Configuration Options
This section contains the configuration options used by the Kafka Streams binder.
For common configuration options and properties pertaining to binder, refer to the core documentation.
16.3.1. Kafka Streams Properties
The following properties are available at the binder level and must be prefixed with spring.cloud.stream.kafka.streams.binder.
literal.
- configuration
-
Map with a key/value pair containing properties pertaining to Apache Kafka Streams API. This property must be prefixed with
spring.cloud.stream.kafka.streams.binder.
. Following are some examples of using this property.
spring.cloud.stream.kafka.streams.binder.configuration.default.key.serde=org.apache.kafka.common.serialization.Serdes$StringSerde
spring.cloud.stream.kafka.streams.binder.configuration.default.value.serde=org.apache.kafka.common.serialization.Serdes$StringSerde
spring.cloud.stream.kafka.streams.binder.configuration.commit.interval.ms=1000
For more information about all the properties that may go into streams configuration, see StreamsConfig JavaDocs in Apache Kafka Streams docs.
- brokers
-
Broker URL
Default:
localhost
- zkNodes
-
Zookeeper URL
Default:
localhost
- serdeError
-
Deserialization error handler type. Possible values are -
logAndContinue
,logAndFail
orsendToDlq
Default:
logAndFail
- applicationId
-
Application ID for all the stream configurations in the current application context. You can override the application id for an individual
StreamListener
method using thegroup
property on the binding. You have to ensure that you are using the same group name for all input bindings in the case of multiple inputs on the same methods.Default:
default
The following properties are only available for Kafka Streams producers and must be prefixed with spring.cloud.stream.kafka.streams.bindings.<binding name>.producer.
literal.
- keySerde
-
key serde to use
Default:
none
. - valueSerde
-
value serde to use
Default:
none
. - useNativeEncoding
-
flag to enable native encoding
Default:
false
.
The following properties are only available for Kafka Streams consumers and must be prefixed with spring.cloud.stream.kafka.streams.bindings.<binding name>.consumer.
literal.
- keySerde
-
key serde to use
Default:
none
. - valueSerde
-
value serde to use
Default:
none
. - materializedAs
-
state store to materialize when using incoming KTable types
Default:
none
. - useNativeDecoding
-
flag to enable native decoding
Default:
false
. - dlqName
-
DLQ topic name.
Default:
none
.
16.3.2. TimeWindow properties:
Windowing is an important concept in stream processing applications. Following properties are available to configure time-window computations.
- spring.cloud.stream.kafka.streams.timeWindow.length
-
When this property is given, you can autowire a
TimeWindows
bean into the application. The value is expressed in milliseconds.Default:
none
. - spring.cloud.stream.kafka.streams.timeWindow.advanceBy
-
Value is given in milliseconds.
Default:
none
.
16.4. Multiple Input Bindings
For use cases that requires multiple incoming KStream objects or a combination of KStream and KTable objects, the Kafka Streams binder provides multiple bindings support.
Let’s see it in action.
16.4.1. Multiple Input Bindings as a Sink
@EnableBinding(KStreamKTableBinding.class)
.....
.....
@StreamListener
public void process(@Input("inputStream") KStream<String, PlayEvent> playEvents,
@Input("inputTable") KTable<Long, Song> songTable) {
....
....
}
interface KStreamKTableBinding {
@Input("inputStream")
KStream<?, ?> inputStream();
@Input("inputTable")
KTable<?, ?> inputTable();
}
In the above example, the application is written as a sink, i.e. there are no output bindings and the application has to decide concerning downstream processing. When you write applications in this style, you might want to send the information downstream or store them in a state store (See below for Queryable State Stores).
In the case of incoming KTable, if you want to materialize the computations to a state store, you have to express it through the following property.
spring.cloud.stream.kafka.streams.bindings.inputTable.consumer.materializedAs: all-songs
16.4.2. Multiple Input Bindings as a Processor
@EnableBinding(KStreamKTableBinding.class)
....
....
@StreamListener
@SendTo("output")
public KStream<String, Long> process(@Input("input") KStream<String, Long> userClicksStream,
@Input("inputTable") KTable<String, String> userRegionsTable) {
....
....
}
interface KStreamKTableBinding extends KafkaStreamsProcessor {
@Input("inputX")
KTable<?, ?> inputTable();
}
16.5. Multiple Output Bindings (aka Branching)
Kafka Streams allow outbound data to be split into multiple topics based on some predicates. The Kafka Streams binder provides
support for this feature without compromising the programming model exposed through StreamListener
in the end user application.
You can write the application in the usual way as demonstrated above in the word count example. However, when using the
branching feature, you are required to do a few things. First, you need to make sure that your return type is KStream[]
instead of a regular KStream
. Second, you need to use the SendTo
annotation containing the output bindings in the order
(see example below). For each of these output bindings, you need to configure destination, content-type etc., complying with
the standard Spring Cloud Stream expectations.
Here is an example:
@EnableBinding(KStreamProcessorWithBranches.class)
@EnableAutoConfiguration
public static class WordCountProcessorApplication {
@Autowired
private TimeWindows timeWindows;
@StreamListener("input")
@SendTo({"output1","output2","output3})
public KStream<?, WordCount>[] process(KStream<Object, String> input) {
Predicate<Object, WordCount> isEnglish = (k, v) -> v.word.equals("english");
Predicate<Object, WordCount> isFrench = (k, v) -> v.word.equals("french");
Predicate<Object, WordCount> isSpanish = (k, v) -> v.word.equals("spanish");
return input
.flatMapValues(value -> Arrays.asList(value.toLowerCase().split("\\W+")))
.groupBy((key, value) -> value)
.windowedBy(timeWindows)
.count(Materialized.as("WordCounts-1"))
.toStream()
.map((key, value) -> new KeyValue<>(null, new WordCount(key.key(), value, new Date(key.window().start()), new Date(key.window().end()))))
.branch(isEnglish, isFrench, isSpanish);
}
interface KStreamProcessorWithBranches {
@Input("input")
KStream<?, ?> input();
@Output("output1")
KStream<?, ?> output1();
@Output("output2")
KStream<?, ?> output2();
@Output("output3")
KStream<?, ?> output3();
}
}
Properties:
spring.cloud.stream.bindings.output1.contentType: application/json
spring.cloud.stream.bindings.output2.contentType: application/json
spring.cloud.stream.bindings.output3.contentType: application/json
spring.cloud.stream.kafka.streams.binder.configuration.commit.interval.ms: 1000
spring.cloud.stream.kafka.streams.binder.configuration:
default.key.serde: org.apache.kafka.common.serialization.Serdes$StringSerde
default.value.serde: org.apache.kafka.common.serialization.Serdes$StringSerde
spring.cloud.stream.bindings.output1:
destination: foo
producer:
headerMode: raw
spring.cloud.stream.bindings.output2:
destination: bar
producer:
headerMode: raw
spring.cloud.stream.bindings.output3:
destination: fox
producer:
headerMode: raw
spring.cloud.stream.bindings.input:
destination: words
consumer:
headerMode: raw
16.6. Message Conversion
Similar to message-channel based binder applications, the Kafka Streams binder adapts to the out-of-the-box content-type conversions without any compromise.
It is typical for Kafka Streams operations to know the type of SerDe’s used to transform the key and value correctly. Therefore, it may be more natural to rely on the SerDe facilities provided by the Apache Kafka Streams library itself at the inbound and outbound conversions rather than using the content-type conversions offered by the framework. On the other hand, you might be already familiar with the content-type conversion patterns provided by the framework, and that, you’d like to continue using for inbound and outbound conversions.
Both the options are supported in the Kafka Streams binder implementation.
Outbound serialization
If native encoding is disabled (which is the default), then the framework will convert the message using the contentType
set by the user (otherwise, the default application/json
will be applied). It will ignore any SerDe set on the outbound
in this case for outbound serialization.
Here is the property to set the contentType on the outbound.
spring.cloud.stream.bindings.output.contentType: application/json
Here is the property to enable native encoding.
spring.cloud.stream.bindings.output.nativeEncoding: true
If native encoding is enabled on the output binding (user has to enable it as above explicitly), then the framework will
skip any form of automatic message conversion on the outbound. In that case, it will switch to the Serde set by the user.
The valueSerde
property set on the actual output binding will be used. Here is an example.
spring.cloud.stream.kafka.streams.bindings.output.producer.valueSerde: org.apache.kafka.common.serialization.Serdes$StringSerde
If this property is not set, then it will use the "default" SerDe: spring.cloud.stream.kafka.streams.binder.configuration.default.value.serde
.
It is worth to mention that Kafka Streams binder does not serialize the keys on outbound - it simply relies on Kafka itself.
Therefore, you either have to specify the keySerde
property on the binding or it will default to the application-wide common
keySerde
.
Binding level key serde:
spring.cloud.stream.kafka.streams.bindings.output.producer.keySerde
Common Key serde:
spring.cloud.stream.kafka.streams.binder.configuration.default.key.serde
If branching is used, then you need to use multiple output bindings. For example,
interface KStreamProcessorWithBranches {
@Input("input")
KStream<?, ?> input();
@Output("output1")
KStream<?, ?> output1();
@Output("output2")
KStream<?, ?> output2();
@Output("output3")
KStream<?, ?> output3();
}
If nativeEncoding
is set, then you can set different SerDe’s on individual output bindings as below.
spring.cloud.stream.kafka.streams.bindings.output1.producer.valueSerde=IntegerSerde
spring.cloud.stream.kafka.streams.bindings.output2.producer.valueSerde=StringSerde
spring.cloud.stream.kafka.streams.bindings.output3.producer.valueSerde=JsonSerde
Then if you have SendTo
like this, @SendTo({"output1", "output2", "output3"}), the KStream[]
from the branches are
applied with proper SerDe objects as defined above. If you are not enabling nativeEncoding
, you can then set different
contentType values on the output bindings as below. In that case, the framework will use the appropriate message converter
to convert the messages before sending to Kafka.
spring.cloud.stream.bindings.output1.contentType: application/json
spring.cloud.stream.bindings.output2.contentType: application/java-serialzied-object
spring.cloud.stream.bindings.output3.contentType: application/octet-stream
Inbound Deserialization
Similar rules apply to data deserialization on the inbound.
If native decoding is disabled (which is the default), then the framework will convert the message using the contentType
set by the user (otherwise, the default application/json
will be applied). It will ignore any SerDe set on the inbound
in this case for inbound deserialization.
Here is the property to set the contentType on the inbound.
spring.cloud.stream.bindings.input.contentType: application/json
Here is the property to enable native decoding.
spring.cloud.stream.bindings.input.nativeDecoding: true
If native decoding is enabled on the input binding (user has to enable it as above explicitly), then the framework will
skip doing any message conversion on the inbound. In that case, it will switch to the SerDe set by the user. The valueSerde
property set on the actual output binding will be used. Here is an example.
spring.cloud.stream.kafka.streams.bindings.input.consumer.valueSerde: org.apache.kafka.common.serialization.Serdes$StringSerde
If this property is not set, it will use the default SerDe: spring.cloud.stream.kafka.streams.binder.configuration.default.value.serde
.
It is worth to mention that Kafka Streams binder does not deserialize the keys on inbound - it simply relies on Kafka itself.
Therefore, you either have to specify the keySerde
property on the binding or it will default to the application-wide common
keySerde
.
Binding level key serde:
spring.cloud.stream.kafka.streams.bindings.input.consumer.keySerde
Common Key serde:
spring.cloud.stream.kafka.streams.binder.configuration.default.key.serde
As in the case of KStream branching on the outbound, the benefit of setting value SerDe per binding is that if you have multiple input bindings (multiple KStreams object) and they all require separate value SerDe’s, then you can configure them individually. If you use the common configuration approach, then this feature won’t be applicable.
16.7. Error Handling
Apache Kafka Streams provide the capability for natively handling exceptions from deserialization errors.
For details on this support, please see this
Out of the box, Apache Kafka Streams provide two kinds of deserialization exception handlers - logAndContinue
and logAndFail
.
As the name indicates, the former will log the error and continue processing the next records and the latter will log the
error and fail. LogAndFail
is the default deserialization exception handler.
16.7.1. Handling Deserialization Exceptions
Kafka Streams binder supports a selection of exception handlers through the following properties.
spring.cloud.stream.kafka.streams.binder.serdeError: logAndContinue
In addition to the above two deserialization exception handlers, the binder also provides a third one for sending the erroneous records (poison pills) to a DLQ topic. Here is how you enable this DLQ exception handler.
spring.cloud.stream.kafka.streams.binder.serdeError: sendToDlq
When the above property is set, all the deserialization error records are automatically sent to the DLQ topic.
spring.cloud.stream.kafka.streams.bindings.input.consumer.dlqName: foo-dlq
If this is set, then the error records are sent to the topic foo-dlq
. If this is not set, then it will create a DLQ
topic with the name error.<input-topic-name>.<group-name>
.
A couple of things to keep in mind when using the exception handling feature in Kafka Streams binder.
-
The property
spring.cloud.stream.kafka.streams.binder.serdeError
is applicable for the entire application. This implies that if there are multipleStreamListener
methods in the same application, this property is applied to all of them. -
The exception handling for deserialization works consistently with native deserialization and framework provided message conversion.
16.7.2. Handling Non-Deserialization Exceptions
For general error handling in Kafka Streams binder, it is up to the end user applications to handle application level errors. As a side effect of providing a DLQ for deserialization exception handlers, Kafka Streams binder provides a way to get access to the DLQ sending bean directly from your application. Once you get access to that bean, you can programmatically send any exception records from your application to the DLQ.
It continues to remain hard to robust error handling using the high-level DSL; Kafka Streams doesn’t natively support error handling yet.
However, when you use the low-level Processor API in your application, there are options to control this behavior. See below.
@Autowired
private SendToDlqAndContinue dlqHandler;
@StreamListener("input")
@SendTo("output")
public KStream<?, WordCount> process(KStream<Object, String> input) {
input.process(() -> new Processor() {
ProcessorContext context;
@Override
public void init(ProcessorContext context) {
this.context = context;
}
@Override
public void process(Object o, Object o2) {
try {
.....
.....
}
catch(Exception e) {
//explicitly provide the kafka topic corresponding to the input binding as the first argument.
//DLQ handler will correctly map to the dlq topic from the actual incoming destination.
dlqHandler.sendToDlq("topic-name", (byte[]) o1, (byte[]) o2, context.partition());
}
}
.....
.....
});
}
16.8. Interactive Queries
As part of the public Kafka Streams binder API, we expose a class called QueryableStoreRegistry
. You can access this
as a Spring bean in your application. An easy way to get access to this bean from your application is to "autowire" the bean
in your application.
@Autowired
private QueryableStoreRegistry queryableStoreRegistry;
Once you gain access to this bean, then you can query for the particular state-store that you are interested. See below.
ReadOnlyKeyValueStore<Object, Object> keyValueStore =
queryableStoreRegistry.getQueryableStoreType("my-store", QueryableStoreTypes.keyValueStore());
16.9. Accessing the underlying KafkaStreams object
StreamBuilderFactoryBean
from spring-kafka that is responsible for constructing the KafkaStreams
object can be accessed programmatically.
Each StreamBuilderFactoryBean
is registered as stream-builder
and appended with the StreamListener
method name.
If your StreamListener
method is named as process
for example, the stream builder bean is named as stream-builder-process
.
Since this is a factory bean, it should be accessed by prepending an ampersand (&
) when accessing it programmatically.
Following is an example and it assumes the StreamListener
method is named as process
StreamsBuilderFactoryBean streamsBuilderFactoryBean = context.getBean("&stream-builder-process", StreamsBuilderFactoryBean.class);
KafkaStreams kafkaStreams = streamsBuilderFactoryBean.getKafkaStreams();
17. RabbitMQ Binder
17.1. Usage
To use the RabbitMQ binder, you can add it to your Spring Cloud Stream application, by using the following Maven coordinates:
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-stream-binder-rabbit</artifactId>
</dependency>
Alternatively, you can use the Spring Cloud Stream RabbitMQ Starter, as follows:
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-stream-rabbit</artifactId>
</dependency>
17.2. RabbitMQ Binder Overview
The following simplified diagram shows how the RabbitMQ binder operates:
By default, the RabbitMQ Binder implementation maps each destination to a TopicExchange
.
For each consumer group, a Queue
is bound to that TopicExchange
.
Each consumer instance has a corresponding RabbitMQ Consumer
instance for its group’s Queue
.
For partitioned producers and consumers, the queues are suffixed with the partition index and use the partition index as the routing key.
For anonymous consumers (those with no group
property), an auto-delete queue (with a randomized unique name) is used.
By using the optional autoBindDlq
option, you can configure the binder to create and configure dead-letter queues (DLQs) (and a dead-letter exchange DLX
, as well as routing infrastructure).
By default, the dead letter queue has the name of the destination, appended with .dlq
.
If retry is enabled (maxAttempts > 1
), failed messages are delivered to the DLQ after retries are exhausted.
If retry is disabled (maxAttempts = 1
), you should set requeueRejected
to false
(the default) so that failed messages are routed to the DLQ, instead of being re-queued.
In addition, republishToDlq
causes the binder to publish a failed message to the DLQ (instead of rejecting it).
This feature lets additional information (such as the stack trace in the x-exception-stacktrace
header) be added to the message in headers.
This option does not need retry enabled.
You can republish a failed message after just one attempt.
Starting with version 1.2, you can configure the delivery mode of republished messages.
See the republishDeliveryMode
property.
Setting requeueRejected to true (with republishToDlq=false ) causes the message to be re-queued and redelivered continually, which is likely not what you want unless the reason for the failure is transient.
In general, you should enable retry within the binder by setting maxAttempts to greater than one or by setting republishToDlq to true .
|
See RabbitMQ Binder Properties for more information about these properties.
The framework does not provide any standard mechanism to consume dead-letter messages (or to re-route them back to the primary queue). Some options are described in Dead-Letter Queue Processing.
When multiple RabbitMQ binders are used in a Spring Cloud Stream application, it is important to disable 'RabbitAutoConfiguration' to avoid the same configuration from RabbitAutoConfiguration being applied to the two binders.
You can exclude the class by using the @SpringBootApplication annotation.
|
Starting with version 2.0, the RabbitMessageChannelBinder
sets the RabbitTemplate.userPublisherConnection
property to true
so that the non-transactional producers avoid deadlocks on consumers, which can happen if cached connections are blocked because of a memory alarm on the broker.
17.3. Configuration Options
This section contains settings specific to the RabbitMQ Binder and bound channels.
For general binding configuration options and properties, see the Spring Cloud Stream core documentation.
RabbitMQ Binder Properties
By default, the RabbitMQ binder uses Spring Boot’s ConnectionFactory
.
Conseuqently, it supports all Spring Boot configuration options for RabbitMQ.
(For reference, see the Spring Boot documentation).
RabbitMQ configuration options use the spring.rabbitmq
prefix.
In addition to Spring Boot options, the RabbitMQ binder supports the following properties:
- spring.cloud.stream.rabbit.binder.adminAddresses
-
A comma-separated list of RabbitMQ management plugin URLs. Only used when
nodes
contains more than one entry. Each entry in this list must have a corresponding entry inspring.rabbitmq.addresses
. Only needed if you use a RabbitMQ cluster and wish to consume from the node that hosts the queue. See Queue Affinity and the LocalizedQueueConnectionFactory for more information.Default: empty.
- spring.cloud.stream.rabbit.binder.nodes
-
A comma-separated list of RabbitMQ node names. When more than one entry, used to locate the server address where a queue is located. Each entry in this list must have a corresponding entry in
spring.rabbitmq.addresses
. Only needed if you use a RabbitMQ cluster and wish to consume from the node that hosts the queue. See Queue Affinity and the LocalizedQueueConnectionFactory for more information.Default: empty.
- spring.cloud.stream.rabbit.binder.compressionLevel
-
The compression level for compressed bindings. See
java.util.zip.Deflater
.Default:
1
(BEST_LEVEL). - spring.cloud.stream.binder.connection-name-prefix
-
A connection name prefix used to name the connection(s) created by this binder. The name is this prefix followed by
#n
, wheren
increments each time a new connection is opened.Default: none (Spring AMQP default).
RabbitMQ Consumer Properties
The following properties are available for Rabbit consumers only and must be prefixed with spring.cloud.stream.rabbit.bindings.<channelName>.consumer.
.
- acknowledgeMode
-
The acknowledge mode.
Default:
AUTO
. - autoBindDlq
-
Whether to automatically declare the DLQ and bind it to the binder DLX.
Default:
false
. - bindingRoutingKey
-
The routing key with which to bind the queue to the exchange (if
bindQueue
istrue
). For partitioned destinations,-<instanceIndex>
is appended.Default:
#
. - bindQueue
-
Whether to bind the queue to the destination exchange. Set it to
false
if you have set up your own infrastructure and have previously created and bound the queue.Default:
true
. - deadLetterQueueName
-
The name of the DLQ
Default:
prefix+destination.dlq
- deadLetterExchange
-
A DLX to assign to the queue. Relevant only if
autoBindDlq
istrue
.Default: 'prefix+DLX'
- deadLetterRoutingKey
-
A dead letter routing key to assign to the queue. Relevant only if
autoBindDlq
istrue
.Default:
destination
- declareExchange
-
Whether to declare the exchange for the destination.
Default:
true
. - delayedExchange
-
Whether to declare the exchange as a
Delayed Message Exchange
. Requires the delayed message exchange plugin on the broker. Thex-delayed-type
argument is set to theexchangeType
.Default:
false
. - dlqDeadLetterExchange
-
If a DLQ is declared, a DLX to assign to that queue.
Default:
none
- dlqDeadLetterRoutingKey
-
If a DLQ is declared, a dead letter routing key to assign to that queue.
Default:
none
- dlqExpires
-
How long before an unused dead letter queue is deleted (in milliseconds).
Default:
no expiration
- dlqLazy
-
Declare the dead letter queue with the
x-queue-mode=lazy
argument. See “Lazy Queues”. Consider using a policy instead of this setting, because using a policy allows changing the setting without deleting the queue.Default:
false
. - dlqMaxLength
-
Maximum number of messages in the dead letter queue.
Default:
no limit
- dlqMaxLengthBytes
-
Maximum number of total bytes in the dead letter queue from all messages.
Default:
no limit
- dlqMaxPriority
-
Maximum priority of messages in the dead letter queue (0-255).
Default:
none
- dlqTtl
-
Default time to live to apply to the dead letter queue when declared (in milliseconds).
Default:
no limit
- durableSubscription
-
Whether the subscription should be durable. Only effective if
group
is also set.Default:
true
. - exchangeAutoDelete
-
If
declareExchange
is true, whether the exchange should be auto-deleted (that is, removed after the last queue is removed).Default:
true
. - exchangeDurable
-
If
declareExchange
is true, whether the exchange should be durable (that is, it survives broker restart).Default:
true
. - exchangeType
-
The exchange type:
direct
,fanout
ortopic
for non-partitioned destinations anddirect
ortopic
for partitioned destinations.Default:
topic
. - exclusive
-
Whether to create an exclusive consumer. Concurrency should be 1 when this is
true
. Often used when strict ordering is required but enabling a hot standby instance to take over after a failure. SeerecoveryInterval
, which controls how often a standby instance attempts to consume.Default:
false
. - expires
-
How long before an unused queue is deleted (in milliseconds).
Default:
no expiration
- failedDeclarationRetryInterval
-
The interval (in milliseconds) between attempts to consume from a queue if it is missing.
Default: 5000
- headerPatterns
-
Patterns for headers to be mapped from inbound messages.
Default:
['*']
(all headers). - lazy
-
Declare the queue with the
x-queue-mode=lazy
argument. See “Lazy Queues”. Consider using a policy instead of this setting, because using a policy allows changing the setting without deleting the queue.Default:
false
. - maxConcurrency
-
The maximum number of consumers.
Default:
1
. - maxLength
-
The maximum number of messages in the queue.
Default:
no limit
- maxLengthBytes
-
The maximum number of total bytes in the queue from all messages.
Default:
no limit
- maxPriority
-
The maximum priority of messages in the queue (0-255).
Default:
none
- missingQueuesFatal
-
When the queue cannot be found, whether to treat the condition as fatal and stop the listener container. Defaults to
false
so that the container keeps trying to consume from the queue — for example, when using a cluster and the node hosting a non-HA queue is down.Default:
false
- prefetch
-
Prefetch count.
Default:
1
. - prefix
-
A prefix to be added to the name of the
destination
and queues.Default: "".
- queueDeclarationRetries
-
The number of times to retry consuming from a queue if it is missing. Relevant only when
missingQueuesFatal
istrue
. Otherwise, the container keeps retrying indefinitely.Default:
3
- queueNameGroupOnly
-
When true, consume from a queue with a name equal to the
group
. Otherwise the queue name isdestination.group
. This is useful, for example, when using Spring Cloud Stream to consume from an existing RabbitMQ queue.Default: false.
- recoveryInterval
-
The interval between connection recovery attempts, in milliseconds.
Default:
5000
. - requeueRejected
-
Whether delivery failures should be re-queued when retry is disabled or
republishToDlq
isfalse
.Default:
false
.
- republishDeliveryMode
-
When
republishToDlq
istrue
, specifies the delivery mode of the republished message.Default:
DeliveryMode.PERSISTENT
- republishToDlq
-
By default, messages that fail after retries are exhausted are rejected. If a dead-letter queue (DLQ) is configured, RabbitMQ routes the failed message (unchanged) to the DLQ. If set to
true
, the binder republishs failed messages to the DLQ with additional headers, including the exception message and stack trace from the cause of the final failure.Default: false
- transacted
-
Whether to use transacted channels.
Default:
false
. - ttl
-
Default time to live to apply to the queue when declared (in milliseconds).
Default:
no limit
- txSize
-
The number of deliveries between acks.
Default:
1
.
Rabbit Producer Properties
The following properties are available for Rabbit producers only and
must be prefixed with spring.cloud.stream.rabbit.bindings.<channelName>.producer.
.
- autoBindDlq
-
Whether to automatically declare the DLQ and bind it to the binder DLX.
Default:
false
. - batchingEnabled
-
Whether to enable message batching by producers. Messages are batched into one message according to the following properties (described in the next three entries in this list): 'batchSize',
batchBufferLimit
, andbatchTimeout
. See Batching for more information.Default:
false
. - batchSize
-
The number of messages to buffer when batching is enabled.
Default:
100
. - batchBufferLimit
-
The maximum buffer size when batching is enabled.
Default:
10000
. - batchTimeout
-
The batch timeout when batching is enabled.
Default:
5000
. - bindingRoutingKey
-
The routing key with which to bind the queue to the exchange (if
bindQueue
istrue
). Only applies to non-partitioned destinations. Only applies ifrequiredGroups
are provided and then only to those groups.Default:
#
. - bindQueue
-
Whether to bind the queue to the destination exchange. Set it to
false
if you have set up your own infrastructure and have previously created and bound the queue. Only applies ifrequiredGroups
are provided and then only to those groups.Default:
true
. - compress
-
Whether data should be compressed when sent.
Default:
false
. - deadLetterQueueName
-
The name of the DLQ Only applies if
requiredGroups
are provided and then only to those groups.Default:
prefix+destination.dlq
- deadLetterExchange
-
A DLX to assign to the queue. Relevant only when
autoBindDlq
istrue
. Applies only whenrequiredGroups
are provided and then only to those groups.Default: 'prefix+DLX'
- deadLetterRoutingKey
-
A dead letter routing key to assign to the queue. Relevant only when
autoBindDlq
istrue
. Applies only whenrequiredGroups
are provided and then only to those groups.Default:
destination
- declareExchange
-
Whether to declare the exchange for the destination.
Default:
true
. - delayExpression
-
A SpEL expression to evaluate the delay to apply to the message (
x-delay
header). It has no effect if the exchange is not a delayed message exchange.Default: No
x-delay
header is set. - delayedExchange
-
Whether to declare the exchange as a
Delayed Message Exchange
. Requires the delayed message exchange plugin on the broker. Thex-delayed-type
argument is set to theexchangeType
.Default:
false
. - deliveryMode
-
The delivery mode.
Default:
PERSISTENT
. - dlqDeadLetterExchange
-
When a DLQ is declared, a DLX to assign to that queue. Applies only if
requiredGroups
are provided and then only to those groups.Default:
none
- dlqDeadLetterRoutingKey
-
When a DLQ is declared, a dead letter routing key to assign to that queue. Applies only when
requiredGroups
are provided and then only to those groups.Default:
none
- dlqExpires
-
How long (in milliseconds) before an unused dead letter queue is deleted. Applies only when
requiredGroups
are provided and then only to those groups.Default:
no expiration
- dlqLazy
-
Declare the dead letter queue with the
x-queue-mode=lazy
argument. See “Lazy Queues”. Consider using a policy instead of this setting, because using a policy allows changing the setting without deleting the queue. Applies only whenrequiredGroups
are provided and then only to those groups. - dlqMaxLength
-
Maximum number of messages in the dead letter queue. Applies only if
requiredGroups
are provided and then only to those groups.Default:
no limit
- dlqMaxLengthBytes
-
Maximum number of total bytes in the dead letter queue from all messages. Applies only when
requiredGroups
are provided and then only to those groups.Default:
no limit
- dlqMaxPriority
-
Maximum priority of messages in the dead letter queue (0-255) Applies only when
requiredGroups
are provided and then only to those groups.Default:
none
- dlqTtl
-
Default time (in milliseconds) to live to apply to the dead letter queue when declared. Applies only when
requiredGroups
are provided and then only to those groups.Default:
no limit
- exchangeAutoDelete
-
If
declareExchange
istrue
, whether the exchange should be auto-delete (it is removed after the last queue is removed).Default:
true
. - exchangeDurable
-
If
declareExchange
istrue
, whether the exchange should be durable (survives broker restart).Default:
true
. - exchangeType
-
The exchange type:
direct
,fanout
ortopic
for non-partitioned destinations anddirect
ortopic
for partitioned destinations.Default:
topic
. - expires
-
How long (in milliseconds) before an unused queue is deleted. Applies only when
requiredGroups
are provided and then only to those groups.Default:
no expiration
- headerPatterns
-
Patterns for headers to be mapped to outbound messages.
Default:
['*']
(all headers). - lazy
-
Declare the queue with the
x-queue-mode=lazy
argument. See “Lazy Queues”. Consider using a policy instead of this setting, because using a policy allows changing the setting without deleting the queue. Applies only whenrequiredGroups
are provided and then only to those groups.Default:
false
. - maxLength
-
Maximum number of messages in the queue. Applies only when
requiredGroups
are provided and then only to those groups.Default:
no limit
- maxLengthBytes
-
Maximum number of total bytes in the queue from all messages. Only applies if
requiredGroups
are provided and then only to those groups.Default:
no limit
- maxPriority
-
Maximum priority of messages in the queue (0-255). Only applies if
requiredGroups
are provided and then only to those groups.Default:
none
- prefix
-
A prefix to be added to the name of the
destination
exchange.Default: "".
- queueNameGroupOnly
-
When
true
, consume from a queue with a name equal to thegroup
. Otherwise the queue name isdestination.group
. This is useful, for example, when using Spring Cloud Stream to consume from an existing RabbitMQ queue. Applies only whenrequiredGroups
are provided and then only to those groups.Default: false.
- routingKeyExpression
-
A SpEL expression to determine the routing key to use when publishing messages. For a fixed routing key, use a literal expression, such as
routingKeyExpression='my.routingKey'
in a properties file orroutingKeyExpression: '''my.routingKey'''
in a YAML file.Default:
destination
ordestination-<partition>
for partitioned destinations. - transacted
-
Whether to use transacted channels.
Default:
false
. - ttl
-
Default time (in milliseconds) to live to apply to the queue when declared. Applies only when
requiredGroups
are provided and then only to those groups.Default:
no limit
In the case of RabbitMQ, content type headers can be set by external applications. Spring Cloud Stream supports them as part of an extended internal protocol used for any type of transport — including transports, such as Kafka (prior to 0.11), that do not natively support headers. |
17.4. Retry With the RabbitMQ Binder
When retry is enabled within the binder, the listener container thread is suspended for any back off periods that are configured. This might be important when strict ordering is required with a single consumer. However, for other use cases, it prevents other messages from being processed on that thread. An alternative to using binder retry is to set up dead lettering with time to live on the dead-letter queue (DLQ) as well as dead-letter configuration on the DLQ itself. See “RabbitMQ Binder Properties” for more information about the properties discussed here. You can use the following example configuration to enable this feature:
-
Set
autoBindDlq
totrue
. The binder create a DLQ. Optionally, you can specify a name indeadLetterQueueName
. -
Set
dlqTtl
to the back off time you want to wait between redeliveries. -
Set the
dlqDeadLetterExchange
to the default exchange. Expired messages from the DLQ are routed to the original queue, because the defaultdeadLetterRoutingKey
is the queue name (destination.group
). Setting to the default exchange is achieved by setting the property with no value, as shown in the next example.
To force a message to be dead-lettered, either throw an AmqpRejectAndDontRequeueException
or set requeueRejected
to true
(the default) and throw any exception.
The loop continue without end, which is fine for transient problems, but you may want to give up after some number of attempts.
Fortunately, RabbitMQ provides the x-death
header, which lets you determine how many cycles have occurred.
To acknowledge a message after giving up, throw an ImmediateAcknowledgeAmqpException
.
Putting it All Together
The following configuration creates an exchange myDestination
with queue myDestination.consumerGroup
bound to a topic exchange with a wildcard routing key #
:
---
spring.cloud.stream.bindings.input.destination=myDestination
spring.cloud.stream.bindings.input.group=consumerGroup
#disable binder retries
spring.cloud.stream.bindings.input.consumer.max-attempts=1
#dlx/dlq setup
spring.cloud.stream.rabbit.bindings.input.consumer.auto-bind-dlq=true
spring.cloud.stream.rabbit.bindings.input.consumer.dlq-ttl=5000
spring.cloud.stream.rabbit.bindings.input.consumer.dlq-dead-letter-exchange=
---
This configuration creates a DLQ bound to a direct exchange (DLX
) with a routing key of myDestination.consumerGroup
.
When messages are rejected, they are routed to the DLQ.
After 5 seconds, the message expires and is routed to the original queue by using the queue name as the routing key, as shown in the following example:
@SpringBootApplication
@EnableBinding(Sink.class)
public class XDeathApplication {
public static void main(String[] args) {
SpringApplication.run(XDeathApplication.class, args);
}
@StreamListener(Sink.INPUT)
public void listen(String in, @Header(name = "x-death", required = false) Map<?,?> death) {
if (death != null && death.get("count").equals(3L)) {
// giving up - don't send to DLX
throw new ImmediateAcknowledgeAmqpException("Failed after 4 attempts");
}
throw new AmqpRejectAndDontRequeueException("failed");
}
}
Notice that the count property in the x-death
header is a Long
.
17.5. Error Channels
Starting with version 1.3, the binder unconditionally sends exceptions to an error channel for each consumer destination and can also be configured to send async producer send failures to an error channel. See “[binder-error-channels]” for more information.
RabbitMQ has two types of send failures:
-
Returned messages,
-
Negatively acknowledged Publisher Confirms.
The latter is rare. According to the RabbitMQ documentation "[A nack] will only be delivered if an internal error occurs in the Erlang process responsible for a queue.".
As well as enabling producer error channels (as described in “[binder-error-channels]”), the RabbitMQ binder only sends messages to the channels if the connection factory is appropriately configured, as follows:
-
ccf.setPublisherConfirms(true);
-
ccf.setPublisherReturns(true);
When using Spring Boot configuration for the connection factory, set the following properties:
-
spring.rabbitmq.publisher-confirms
-
spring.rabbitmq.publisher-returns
The payload of the ErrorMessage
for a returned message is a ReturnedAmqpMessageException
with the following properties:
-
failedMessage
: The spring-messagingMessage<?>
that failed to be sent. -
amqpMessage
: The raw spring-amqpMessage
. -
replyCode
: An integer value indicating the reason for the failure (for example, 312 - No route). -
replyText
: A text value indicating the reason for the failure (for example,NO_ROUTE
). -
exchange
: The exchange to which the message was published. -
routingKey
: The routing key used when the message was published.
For negatively acknowledged confirmations, the payload is a NackedAmqpMessageException
with the following properties:
-
failedMessage
: The spring-messagingMessage<?>
that failed to be sent. -
nackReason
: A reason (if available — you may need to examine the broker logs for more information).
There is no automatic handling of these exceptions (such as sending to a dead-letter queue). You can consume these exceptions with your own Spring Integration flow.
17.6. Dead-Letter Queue Processing
Because you cannot anticipate how users would want to dispose of dead-lettered messages, the framework does not provide any standard mechanism to handle them.
If the reason for the dead-lettering is transient, you may wish to route the messages back to the original queue.
However, if the problem is a permanent issue, that could cause an infinite loop.
The following Spring Boot application shows an example of how to route those messages back to the original queue but moves them to a third “parking lot” queue after three attempts.
The second example uses the RabbitMQ Delayed Message Exchange to introduce a delay to the re-queued message.
In this example, the delay increases for each attempt.
These examples use a @RabbitListener
to receive messages from the DLQ.
You could also use RabbitTemplate.receive()
in a batch process.
The examples assume the original destination is so8400in
and the consumer group is so8400
.
Non-Partitioned Destinations
The first two examples are for when the destination is not partitioned:
@SpringBootApplication
public class ReRouteDlqApplication {
private static final String ORIGINAL_QUEUE = "so8400in.so8400";
private static final String DLQ = ORIGINAL_QUEUE + ".dlq";
private static final String PARKING_LOT = ORIGINAL_QUEUE + ".parkingLot";
private static final String X_RETRIES_HEADER = "x-retries";
public static void main(String[] args) throws Exception {
ConfigurableApplicationContext context = SpringApplication.run(ReRouteDlqApplication.class, args);
System.out.println("Hit enter to terminate");
System.in.read();
context.close();
}
@Autowired
private RabbitTemplate rabbitTemplate;
@RabbitListener(queues = DLQ)
public void rePublish(Message failedMessage) {
Integer retriesHeader = (Integer) failedMessage.getMessageProperties().getHeaders().get(X_RETRIES_HEADER);
if (retriesHeader == null) {
retriesHeader = Integer.valueOf(0);
}
if (retriesHeader < 3) {
failedMessage.getMessageProperties().getHeaders().put(X_RETRIES_HEADER, retriesHeader + 1);
this.rabbitTemplate.send(ORIGINAL_QUEUE, failedMessage);
}
else {
this.rabbitTemplate.send(PARKING_LOT, failedMessage);
}
}
@Bean
public Queue parkingLot() {
return new Queue(PARKING_LOT);
}
}
@SpringBootApplication
public class ReRouteDlqApplication {
private static final String ORIGINAL_QUEUE = "so8400in.so8400";
private static final String DLQ = ORIGINAL_QUEUE + ".dlq";
private static final String PARKING_LOT = ORIGINAL_QUEUE + ".parkingLot";
private static final String X_RETRIES_HEADER = "x-retries";
private static final String DELAY_EXCHANGE = "dlqReRouter";
public static void main(String[] args) throws Exception {
ConfigurableApplicationContext context = SpringApplication.run(ReRouteDlqApplication.class, args);
System.out.println("Hit enter to terminate");
System.in.read();
context.close();
}
@Autowired
private RabbitTemplate rabbitTemplate;
@RabbitListener(queues = DLQ)
public void rePublish(Message failedMessage) {
Map<String, Object> headers = failedMessage.getMessageProperties().getHeaders();
Integer retriesHeader = (Integer) headers.get(X_RETRIES_HEADER);
if (retriesHeader == null) {
retriesHeader = Integer.valueOf(0);
}
if (retriesHeader < 3) {
headers.put(X_RETRIES_HEADER, retriesHeader + 1);
headers.put("x-delay", 5000 * retriesHeader);
this.rabbitTemplate.send(DELAY_EXCHANGE, ORIGINAL_QUEUE, failedMessage);
}
else {
this.rabbitTemplate.send(PARKING_LOT, failedMessage);
}
}
@Bean
public DirectExchange delayExchange() {
DirectExchange exchange = new DirectExchange(DELAY_EXCHANGE);
exchange.setDelayed(true);
return exchange;
}
@Bean
public Binding bindOriginalToDelay() {
return BindingBuilder.bind(new Queue(ORIGINAL_QUEUE)).to(delayExchange()).with(ORIGINAL_QUEUE);
}
@Bean
public Queue parkingLot() {
return new Queue(PARKING_LOT);
}
}
Partitioned Destinations
With partitioned destinations, there is one DLQ for all partitions. We determine the original queue from the headers.
republishToDlq=false
When republishToDlq
is false
, RabbitMQ publishes the message to the DLX/DLQ with an x-death
header containing information about the original destination, as shown in the following example:
@SpringBootApplication
public class ReRouteDlqApplication {
private static final String ORIGINAL_QUEUE = "so8400in.so8400";
private static final String DLQ = ORIGINAL_QUEUE + ".dlq";
private static final String PARKING_LOT = ORIGINAL_QUEUE + ".parkingLot";
private static final String X_DEATH_HEADER = "x-death";
private static final String X_RETRIES_HEADER = "x-retries";
public static void main(String[] args) throws Exception {
ConfigurableApplicationContext context = SpringApplication.run(ReRouteDlqApplication.class, args);
System.out.println("Hit enter to terminate");
System.in.read();
context.close();
}
@Autowired
private RabbitTemplate rabbitTemplate;
@SuppressWarnings("unchecked")
@RabbitListener(queues = DLQ)
public void rePublish(Message failedMessage) {
Map<String, Object> headers = failedMessage.getMessageProperties().getHeaders();
Integer retriesHeader = (Integer) headers.get(X_RETRIES_HEADER);
if (retriesHeader == null) {
retriesHeader = Integer.valueOf(0);
}
if (retriesHeader < 3) {
headers.put(X_RETRIES_HEADER, retriesHeader + 1);
List<Map<String, ?>> xDeath = (List<Map<String, ?>>) headers.get(X_DEATH_HEADER);
String exchange = (String) xDeath.get(0).get("exchange");
List<String> routingKeys = (List<String>) xDeath.get(0).get("routing-keys");
this.rabbitTemplate.send(exchange, routingKeys.get(0), failedMessage);
}
else {
this.rabbitTemplate.send(PARKING_LOT, failedMessage);
}
}
@Bean
public Queue parkingLot() {
return new Queue(PARKING_LOT);
}
}
republishToDlq=true
When republishToDlq
is true
, the republishing recoverer adds the original exchange and routing key to headers, as shown in the following example:
@SpringBootApplication
public class ReRouteDlqApplication {
private static final String ORIGINAL_QUEUE = "so8400in.so8400";
private static final String DLQ = ORIGINAL_QUEUE + ".dlq";
private static final String PARKING_LOT = ORIGINAL_QUEUE + ".parkingLot";
private static final String X_RETRIES_HEADER = "x-retries";
private static final String X_ORIGINAL_EXCHANGE_HEADER = RepublishMessageRecoverer.X_ORIGINAL_EXCHANGE;
private static final String X_ORIGINAL_ROUTING_KEY_HEADER = RepublishMessageRecoverer.X_ORIGINAL_ROUTING_KEY;
public static void main(String[] args) throws Exception {
ConfigurableApplicationContext context = SpringApplication.run(ReRouteDlqApplication.class, args);
System.out.println("Hit enter to terminate");
System.in.read();
context.close();
}
@Autowired
private RabbitTemplate rabbitTemplate;
@RabbitListener(queues = DLQ)
public void rePublish(Message failedMessage) {
Map<String, Object> headers = failedMessage.getMessageProperties().getHeaders();
Integer retriesHeader = (Integer) headers.get(X_RETRIES_HEADER);
if (retriesHeader == null) {
retriesHeader = Integer.valueOf(0);
}
if (retriesHeader < 3) {
headers.put(X_RETRIES_HEADER, retriesHeader + 1);
String exchange = (String) headers.get(X_ORIGINAL_EXCHANGE_HEADER);
String originalRoutingKey = (String) headers.get(X_ORIGINAL_ROUTING_KEY_HEADER);
this.rabbitTemplate.send(exchange, originalRoutingKey, failedMessage);
}
else {
this.rabbitTemplate.send(PARKING_LOT, failedMessage);
}
}
@Bean
public Queue parkingLot() {
return new Queue(PARKING_LOT);
}
}
17.7. Partitioning with the RabbitMQ Binder
RabbitMQ does not support partitioning natively.
Sometimes, it is advantageous to send data to specific partitions — for example, when you want to strictly order message processing, all messages for a particular customer should go to the same partition.
The RabbitMessageChannelBinder
provides partitioning by binding a queue for each partition to the destination exchange.
The following Java and YAML examples show how to configure the producer:
@SpringBootApplication
@EnableBinding(Source.class)
public class RabbitPartitionProducerApplication {
private static final Random RANDOM = new Random(System.currentTimeMillis());
private static final String[] data = new String[] {
"abc1", "def1", "qux1",
"abc2", "def2", "qux2",
"abc3", "def3", "qux3",
"abc4", "def4", "qux4",
};
public static void main(String[] args) {
new SpringApplicationBuilder(RabbitPartitionProducerApplication.class)
.web(false)
.run(args);
}
@InboundChannelAdapter(channel = Source.OUTPUT, poller = @Poller(fixedRate = "5000"))
public Message<?> generate() {
String value = data[RANDOM.nextInt(data.length)];
System.out.println("Sending: " + value);
return MessageBuilder.withPayload(value)
.setHeader("partitionKey", value)
.build();
}
}
spring:
cloud:
stream:
bindings:
output:
destination: partitioned.destination
producer:
partitioned: true
partition-key-expression: headers['partitionKey']
partition-count: 2
required-groups:
- myGroup
The configuration in the prececing example uses the default partitioning ( The |
The following configuration provisions a topic exchange:
The following queues are bound to that exchange:
The following bindings associate the queues to the exchange:
The following Java and YAML examples continue the previous examples and show how to configure the consumer:
@SpringBootApplication
@EnableBinding(Sink.class)
public class RabbitPartitionConsumerApplication {
public static void main(String[] args) {
new SpringApplicationBuilder(RabbitPartitionConsumerApplication.class)
.web(false)
.run(args);
}
@StreamListener(Sink.INPUT)
public void listen(@Payload String in, @Header(AmqpHeaders.CONSUMER_QUEUE) String queue) {
System.out.println(in + " received from queue " + queue);
}
}
spring:
cloud:
stream:
bindings:
input:
destination: partitioned.destination
group: myGroup
consumer:
partitioned: true
instance-index: 0
The RabbitMessageChannelBinder does not support dynamic scaling.
There must be at least one consumer per partition.
The consumer’s instanceIndex is used to indicate which partition is consumed.
Platforms such as Cloud Foundry can have only one instance with an instanceIndex .
|
Appendices
Appendix A: Building
A.1. Basic Compile and Test
To build the source you will need to install JDK 1.7.
The build uses the Maven wrapper so you don’t have to install a specific version of Maven. To enable the tests for Redis, Rabbit, and Kafka bindings you should have those servers running before building. See below for more information on running the servers.
The main build command is
$ ./mvnw clean install
You can also add '-DskipTests' if you like, to avoid running the tests.
You can also install Maven (>=3.3.3) yourself and run the mvn command
in place of ./mvnw in the examples below. If you do that you also
might need to add -P spring if your local Maven settings do not
contain repository declarations for spring pre-release artifacts.
|
Be aware that you might need to increase the amount of memory
available to Maven by setting a MAVEN_OPTS environment variable with
a value like -Xmx512m -XX:MaxPermSize=128m . We try to cover this in
the .mvn configuration, so if you find you have to do it to make a
build succeed, please raise a ticket to get the settings added to
source control.
|
The projects that require middleware generally include a
docker-compose.yml
, so consider using
Docker Compose to run the middeware servers
in Docker containers. See the README in the
scripts demo
repository for specific instructions about the common cases of mongo,
rabbit and redis.
A.2. Documentation
There is a "full" profile that will generate documentation.
A.3. Working with the code
If you don’t have an IDE preference we would recommend that you use Spring Tools Suite or Eclipse when working with the code. We use the m2eclipe eclipse plugin for maven support. Other IDEs and tools should also work without issue.
A.3.1. Importing into eclipse with m2eclipse
We recommend the m2eclipe eclipse plugin when working with eclipse. If you don’t already have m2eclipse installed it is available from the "eclipse marketplace".
Unfortunately m2e does not yet support Maven 3.3, so once the projects
are imported into Eclipse you will also need to tell m2eclipse to use
the .settings.xml
file for the projects. If you do not do this you
may see many different errors related to the POMs in the
projects. Open your Eclipse preferences, expand the Maven
preferences, and select User Settings. In the User Settings field
click Browse and navigate to the Spring Cloud project you imported
selecting the .settings.xml
file in that project. Click Apply and
then OK to save the preference changes.
Alternatively you can copy the repository settings from .settings.xml into your own ~/.m2/settings.xml .
|
A.3.2. Importing into eclipse without m2eclipse
If you prefer not to use m2eclipse you can generate eclipse project metadata using the following command:
$ ./mvnw eclipse:eclipse
The generated eclipse projects can be imported by selecting import existing projects
from the file
menu.
[[contributing]
== Contributing
Spring Cloud is released under the non-restrictive Apache 2.0 license, and follows a very standard Github development process, using Github tracker for issues and merging pull requests into master. If you want to contribute even something trivial please do not hesitate, but follow the guidelines below.
A.4. Sign the Contributor License Agreement
Before we accept a non-trivial patch or pull request we will need you to sign the contributor’s agreement. Signing the contributor’s agreement does not grant anyone commit rights to the main repository, but it does mean that we can accept your contributions, and you will get an author credit if we do. Active contributors might be asked to join the core team, and given the ability to merge pull requests.
A.5. Code Conventions and Housekeeping
None of these is essential for a pull request, but they will all help. They can also be added after the original pull request but before a merge.
-
Use the Spring Framework code format conventions. If you use Eclipse you can import formatter settings using the
eclipse-code-formatter.xml
file from the Spring Cloud Build project. If using IntelliJ, you can use the Eclipse Code Formatter Plugin to import the same file. -
Make sure all new
.java
files to have a simple Javadoc class comment with at least an@author
tag identifying you, and preferably at least a paragraph on what the class is for. -
Add the ASF license header comment to all new
.java
files (copy from existing files in the project) -
Add yourself as an
@author
to the .java files that you modify substantially (more than cosmetic changes). -
Add some Javadocs and, if you change the namespace, some XSD doc elements.
-
A few unit tests would help a lot as well — someone has to do it.
-
If no-one else is using your branch, please rebase it against the current master (or other target branch in the main project).
-
When writing a commit message please follow these conventions, if you are fixing an existing issue please add
Fixes gh-XXXX
at the end of the commit message (where XXXX is the issue number).