From the vertical perspective, a layered architecture facilitates separation of concerns, and interface-based contracts between layers promote loose coupling. Spring-based applications are typically designed this way, and the Spring framework and portfolio provide a strong foundation for following this best practice for the full-stack of an enterprise application. Message-driven architectures add a horizontal perspective, yet these same goals are still relevant. Just as "layered architecture" is an extremely generic and abstract paradigm, messaging systems typically follow the similarly abstract "pipes-and-filters" model. The "filters" represent any component that is capable of producing and/or consuming messages, and the "pipes" transport the messages between filters so that the components themselves remain loosely-coupled. It is important to note that these two high-level paradigms are not mutually exclusive. The underlying messaging infrastructure that supports the "pipes" should still be encapsulated in a layer whose contracts are defined as interfaces. Likewise, the "filters" themselves would typically be managed within a layer that is logically above the application's service layer, interacting with those services through interfaces much in the same way that a web-tier would.
In Spring Integration, a Message is a generic wrapper for any Java object combined with metadata used by the framework while handling that object. It consists of a payload and headers. The payload can be of any type and the headers hold commonly required information such as id, timestamp, expiration, and return address. Developers can also store any arbitrary key-value pair in the headers.
Since a Spring Integration Message is a generic wrapper for any Object, there is no limit to the number of potential sources for such messages. In fact, a source implementation can act as an adapter that converts Objects from any other system into Spring Integration Messages.
There are two types of source: those which require polling and those which send Messages directly. Therefore,
Spring Integration provides two main interfaces that extend the MessageSource
interface: PollableSource and SubscribableSource.
While it is relatively easy to implement these interfaces directly, an adapter is also available for invoking
arbitrary methods on plain Objects. Also, several MessageSource implementations
are already available within the Spring Integration Adapters module. For a detailed discussion of the various
adapters, see Chapter 3, Adapters.
Just as the MessageSource implementations enable Message reception, a
MessageTarget handles the responsibility of sending Messages. As with the
MessageSource, a MessageTarget can act as an
adapter that converts Messages into the Objects expected by some other system.
The MessageTarget interface may be implemented directly, but an adapter is also available for invoking arbitrary
methods on plain Objects (delegating to a MessageMapper strategy in the process).
As with MessageSources, several MessageTarget implementations are already available within the Spring Integration
Adapters module as discussed in Chapter 3, Adapters.
As described above, the MessageSource and MessageTarget components support conversion between Objects and
Messages so that application code and/or external systems can be connected to a Spring Integration application
rather easily. However, both MessageSource and MessageTarget are unidirectional while the application code or
external system to be invoked may provide a return value. The MessageHandler
interface supports these request-reply scenarios.
As with the MessageSource and MessageTarget, Spring Integration also provides an adapter that itself implements
the MessageHandler interface while supporting the invocation of arbitrary methods
on plain Objects. For more information about the Message Handler, see Section 2.6, “MessageHandler”.
A Message Channel represents the "pipe" of a pipes-and-filters architecture. Producers send Messages to a channel, and consumers receive Messages from a channel. By providing both send and receive operations, a Message Channel basically combines the roles of MessageSource and MessageTarget.
Every channel is also a MessageTarget, so Messages can be sent to a channel.
Likewise, every channel is a MessageSource, but as discussed above, Spring
Integration defines two types of source: pollable and subscribable. Subscribable channels include
PublishSubscribeChannel and DirectChannel. Pollable channels include QueueChannel, PriorityChannel,
RendezvousChannel, and ThreadLocalChannel. These are described in detail in
Section 2.4, “MessageChannel”.
Thus far, the component diagrams show consumers, producers, and requesters invoking the MessageSource, MessageTarget, and MessageHandlers respectively. However, one of the primary goals of Spring Integration is to simplify the development of enterprise integration solutions through inversion of control. This means that you should not have to implement such consumers, producers, and requesters directly. Instead, you should be able to focus on your domain logic with an implementation based on plain Objects. Then, by providing declarative configuration, you can "connect" your application code to the messaging infrastructure provided by Spring Integration. The components responsible for these connections are Message Endpoints.
A Message Endpoint represents the "filter" of a pipes-and-filters architecture. As mentioned above, the endpoint's primary role is to connect application code to the messaging framework and to do so in a non-invasive manner. In other words, the application code should have no awareness of the Message objects or the Message Channels. This is similar to the role of a Controller in the MVC paradigm. Just as a Controller handles HTTP requests, the Message Endpoint handles Messages. Just as Controllers are mapped to URL patterns, Message Endpoints are mapped to Message Channels. The goal is the same in both cases: isolate application code from the infrastructure. Spring Integration provides Message Endpoints for connecting each of the component types described above. The description of each of the main types of endpoint follows.
A Channel Adapter is an endpoint that connects either a MessageSource or a MessageTarget to a MessageChannel. If a MessageSource is being adapted, then the adapter is responsible for receiving Messages from the MessageSource and sending them to the MessageChannel. If a Message Target is being adapted, then the adapter is responsible for receiving Messages from the MessageChannel and sending them to the MessageTarget.
When a Channel Adapter is used to connect a PollableSource implementation to a Message Channel, the invocation of the MessageSource's receive operation may be controlled by scheduling information provided within the Channel Adapter's configuration. Any time the receive operation returns a non-null Message, it is sent to the MessageChannel.
If the source being connected by the Channel Adapter is a SubscribableSource, then no scheduling information should be provided. Instead the source will send the Message directly, and the Channel Adapter will pass it along to its Message Channel.
When a Channel Adapter is used to connect a MessageTarget implementation to a Pollable Message Channel, the invocation of the MessageChannel's receive operation may be controlled by scheduling information provided within the Channel Adapter's configuration. Any time a non-null Message is received from the MessageChannel, it is sent to the MessageTarget.
If the MessageChannel being connected is not Pollable but Subscribable (e.g. Direct Channel or Publish Subscribe Channel), then no scheduling information should be provided. Instead the channel will send Messages directly, and the Channel Adapter will pass them along to the MessageTarget.
When the Object to be invoked is capable of returning a value, another type of endpoint is needed to accommodate the additional responsibilities of the request/reply interaction. The general behavior is similar to a Channel Adapter, but this type of endpoint - the Service Activator - must make a distinction between the "input-channel" and the "output-channel". Also, the Service Activator invokes an operation on some Message Handler to process the request Message. Whenever the Message-handling Object returns a reply Message, that Message is sent to the output channel. If no output channel has been configured, then the reply will be sent to the channel specified in the MessageHeader's "return address" if available.
A Message Router is a particular type of Message Endpoint that is capable of receiving a Message from a MessageChannel and then deciding what channel or channels should receive the Message next (if any). Typically the decision is based upon the Message's content and/or metadata available in the MessageHeader. A Message Router is often used as a dynamic alternative to a statically configured output channel on a Service Activator or other Message-handling endpoint.
A Splitter is another type of Message Endpoint whose responsibility is to accept a Message from its input channel, split that Message into multiple Messages, and then send each of those to its output channel. This is typically used for dividing a "composite" payload object into a group of Messages containing the sub-divided payloads.
Basically a mirror-image of the Splitter, the Aggregator is a type of Message Endpoint that receives multiple
Messages and combines them into a single Message. In fact, Aggregators are often downstream consumers in a
pipeline that includes a Splitter. Technically, the Aggregator is more complex than a Splitter, because it
is required to maintain state (the Messages to-be-aggregated), to decide when the complete group of Messages
is available, and to timeout if necessary. Furthermore, in case of a timeout, the Aggregator needs to know
whether to send the partial results or to discard them to a separate channel. Spring Integration provides
a CompletionStrategy as well as configurable settings for timeout, whether
to send partial results, and the discard channel.
The Message Bus acts as a registry for Message Channels and Message Endpoints. It also encapsulates the complexity of message retrieval and dispatching. Essentially, the Message Bus forms a logical extension of the Spring application context into the messaging domain. For example, it will automatically detect Message Channel and Message Endpoint components from within the application context. It handles the scheduling of pollers, the creation of thread pools, and the lifecycle management of all messaging components that can be initialized, started, and stopped. The Message Bus is the primary example of inversion of control within Spring Integration.
