Cloud Native Applications

A Spring Cloud application operates by creating a “bootstrap” context, which is a parent context for the main application. This context is responsible for loading configuration properties from the external sources and for decrypting properties in the local external configuration files. The two contexts share an Environment, which is the source of external properties for any Spring application. By default, bootstrap properties (not bootstrap.properties but properties that are loaded during the bootstrap phase) are added with high precedence, so they cannot be overridden by local configuration.

The bootstrap context uses a different convention for locating external configuration than the main application context. Instead of application.yml (or .properties), you can use bootstrap.yml, keeping the external configuration for bootstrap and main context nicely separate. The following listing shows an example:

If your application needs any application-specific configuration from the server, it is a good idea to set the spring.application.name (in bootstrap.yml or application.yml). For the property spring.application.name to be used as the application’s context ID, you must set it in bootstrap.[properties | yml].

If you want to retrieve specific profile configuration, you should also set spring.profiles.active in bootstrap.[properties | yml].

You can disable the bootstrap process completely by setting spring.cloud.bootstrap.enabled=false (for example, in system properties).

If you build an application context from SpringApplication or SpringApplicationBuilder, the Bootstrap context is added as a parent to that context. It is a feature of Spring that child contexts inherit property sources and profiles from their parent, so the “main” application context contains additional property sources, compared to building the same context without Spring Cloud Config. The additional property sources are:

Because of the ordering rules of property sources, the “bootstrap” entries take precedence. However, note that these do not contain any data from bootstrap.yml, which has very low precedence but can be used to set defaults.

You can extend the context hierarchy by setting the parent context of any ApplicationContext you create — for example, by using its own interface or with the SpringApplicationBuilder convenience methods (parent(), child() and sibling()). The bootstrap context is the parent of the most senior ancestor that you create yourself. Every context in the hierarchy has its own “bootstrap” (possibly empty) property source to avoid promoting values inadvertently from parents down to their descendants. If there is a config server, every context in the hierarchy can also (in principle) have a different spring.application.name and, hence, a different remote property source. Normal Spring application context behavior rules apply to property resolution: properties from a child context override those in the parent, by name and also by property source name. (If the child has a property source with the same name as the parent, the value from the parent is not included in the child).

Note that the SpringApplicationBuilder lets you share an Environment amongst the whole hierarchy, but that is not the default. Thus, sibling contexts (in particular) do not need to have the same profiles or property sources, even though they may share common values with their parent.

The bootstrap.yml (or .properties) location can be specified by setting spring.cloud.bootstrap.name (default: bootstrap), spring.cloud.bootstrap.location (default: empty) or spring.cloud.bootstrap.additional-location (default: empty) — for example, in System properties.

Those properties behave like the spring.config.* variants with the same name. With spring.cloud.bootstrap.location the default locations are replaced and only the specified ones are used. To add locations to the list of default ones, spring.cloud.bootstrap.additional-location can be used. In fact, they are used to set up the bootstrap ApplicationContext by setting those properties in its Environment. If there is an active profile (from spring.profiles.active or through the Environment API in the context you are building), properties in that profile get loaded as well, the same as in a regular Spring Boot app — for example, from bootstrap-development.properties for a development profile.

The property sources that are added to your application by the bootstrap context are often “remote” (from example, from Spring Cloud Config Server). By default, they cannot be overridden locally. If you want to let your applications override the remote properties with their own system properties or config files, the remote property source has to grant it permission by setting spring.cloud.config.allowOverride=true (it does not work to set this locally). Once that flag is set, two finer-grained settings control the location of the remote properties in relation to system properties and the application’s local configuration:

The bootstrap context can be set to do anything you like by adding entries to /META-INF/spring.factories under a key named org.springframework.cloud.bootstrap.BootstrapConfiguration. This holds a comma-separated list of Spring @Configuration classes that are used to create the context. Any beans that you want to be available to the main application context for autowiring can be created here. There is a special contract for @Beans of type ApplicationContextInitializer. If you want to control the startup sequence, you can mark classes with the @Order annotation (the default order is last).

The bootstrap process ends by injecting initializers into the main SpringApplication instance (which is the normal Spring Boot startup sequence, whether it runs as a standalone application or is deployed in an application server). First, a bootstrap context is created from the classes found in spring.factories. Then, all @Beans of type ApplicationContextInitializer are added to the main SpringApplication before it is started.

The default property source for external configuration added by the bootstrap process is the Spring Cloud Config Server, but you can add additional sources by adding beans of type PropertySourceLocator to the bootstrap context (through spring.factories). For instance, you can insert additional properties from a different server or from a database.

As an example, consider the following custom locator:

The Environment that is passed in is the one for the ApplicationContext about to be created — in other words, the one for which we supply additional property sources. It already has its normal Spring Boot-provided property sources, so you can use those to locate a property source specific to this Environment (for example, by keying it on spring.application.name, as is done in the default Spring Cloud Config Server property source locator).

If you create a jar with this class in it and then add a META-INF/spring.factories containing the following setting, the customProperty PropertySource appears in any application that includes that jar on its classpath:

As of Spring Cloud 2021.0.8, Spring Cloud will now call PropertySourceLocators twice. The first fetch will retrieve any property sources without any profiles. These property sources will have the opportunity to activate profiles using spring.profiles.active. After the main application context starts PropertySourceLocators will be called a second time, this time with any active profiles allowing PropertySourceLocators to locate any additional PropertySources with profiles.

If you use Spring Boot to configure log settings, you should place this configuration in bootstrap.[yml | properties] if you would like it to apply to all events.

The application listens for an EnvironmentChangeEvent and reacts to the change in a couple of standard ways (additional ApplicationListeners can be added as @Beans in the normal way). When an EnvironmentChangeEvent is observed, it has a list of key values that have changed, and the application uses those to:

Note that the Spring Cloud Config Client does not, by default, poll for changes in the Environment. Generally, we would not recommend that approach for detecting changes (although you can set it up with a @Scheduled annotation). If you have a scaled-out client application, it is better to broadcast the EnvironmentChangeEvent to all the instances instead of having them polling for changes (for example, by using the Spring Cloud Bus).

The EnvironmentChangeEvent covers a large class of refresh use cases, as long as you can actually make a change to the Environment and publish the event. Note that those APIs are public and part of core Spring). You can verify that the changes are bound to @ConfigurationProperties beans by visiting the /configprops endpoint (a standard Spring Boot Actuator feature). For instance, a DataSource can have its maxPoolSize changed at runtime (the default DataSource created by Spring Boot is a @ConfigurationProperties bean) and grow capacity dynamically. Re-binding @ConfigurationProperties does not cover another large class of use cases, where you need more control over the refresh and where you need a change to be atomic over the whole ApplicationContext. To address those concerns, we have @RefreshScope.

When there is a configuration change, a Spring @Bean that is marked as @RefreshScope gets special treatment. This feature addresses the problem of stateful beans that get their configuration injected only when they are initialized. For instance, if a DataSource has open connections when the database URL is changed through the Environment, you probably want the holders of those connections to be able to complete what they are doing. Then, the next time something borrows a connection from the pool, it gets one with the new URL.

Sometimes, it might even be mandatory to apply the @RefreshScope annotation on some beans that can be only initialized once. If a bean is “immutable”, you have to either annotate the bean with @RefreshScope or specify the classname under the property key: spring.cloud.refresh.extra-refreshable.

Refresh scope beans are lazy proxies that initialize when they are used (that is, when a method is called), and the scope acts as a cache of initialized values. To force a bean to re-initialize on the next method call, you must invalidate its cache entry.

The RefreshScope is a bean in the context and has a public refreshAll() method to refresh all beans in the scope by clearing the target cache. The /refresh endpoint exposes this functionality (over HTTP or JMX). To refresh an individual bean by name, there is also a refresh(String) method.

To expose the /refresh endpoint, you need to add following configuration to your application:

Spring Cloud has an Environment pre-processor for decrypting property values locally. It follows the same rules as the Spring Cloud Config Server and has the same external configuration through encrypt.*. Thus, you can use encrypted values in the form of {cipher}*, and, as long as there is a valid key, they are decrypted before the main application context gets the Environment settings. To use the encryption features in an application, you need to include Spring Security RSA in your classpath (Maven co-ordinates: org.springframework.security:spring-security-rsa), and you also need the full strength JCE extensions in your JVM.

If you get an exception due to "Illegal key size" and you use Sun’s JDK, you need to install the Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files. See the following links for more information:

Extract the files into the JDK/jre/lib/security folder for whichever version of JRE/JDK x64/x86 you use.

For a Spring Boot Actuator application, some additional management endpoints are available. You can use:

Spring Cloud Commons provides the @EnableDiscoveryClient annotation. This looks for implementations of the DiscoveryClient and ReactiveDiscoveryClient interfaces with META-INF/spring.factories. Implementations of the discovery client add a configuration class to spring.factories under the org.springframework.cloud.client.discovery.EnableDiscoveryClient key. Examples of DiscoveryClient implementations include Spring Cloud Netflix Eureka, Spring Cloud Consul Discovery, and Spring Cloud Zookeeper Discovery.

Spring Cloud will provide both the blocking and reactive service discovery clients by default. You can disable the blocking and/or reactive clients easily by setting spring.cloud.discovery.blocking.enabled=false or spring.cloud.discovery.reactive.enabled=false. To completely disable service discovery you just need to set spring.cloud.discovery.enabled=false.

By default, implementations of DiscoveryClient auto-register the local Spring Boot server with the remote discovery server. This behavior can be disabled by setting autoRegister=false in @EnableDiscoveryClient.

Commons now provides a ServiceRegistry interface that provides methods such as register(Registration) and deregister(Registration), which let you provide custom registered services. Registration is a marker interface.

The following example shows the ServiceRegistry in use:

Each ServiceRegistry implementation has its own Registry implementation.

If you are using the ServiceRegistry interface, you are going to need to pass the correct Registry implementation for the ServiceRegistry implementation you are using.

You can configure a RestTemplate to use a Load-balancer client. To create a load-balanced RestTemplate, create a RestTemplate @Bean and use the @LoadBalanced qualifier, as the following example shows:

The URI needs to use a virtual host name (that is, a service name, not a host name). The BlockingLoadBalancerClient is used to create a full physical address.

You can configure WebClient to automatically use a load-balancer client. To create a load-balanced WebClient, create a WebClient.Builder @Bean and use the @LoadBalanced qualifier, as follows:

The URI needs to use a virtual host name (that is, a service name, not a host name). The Spring Cloud LoadBalancer is used to create a full physical address.

If you want a RestTemplate that is not load-balanced, create a RestTemplate bean and inject it. To access the load-balanced RestTemplate, use the @LoadBalanced qualifier when you create your @Bean, as the following example shows:

If you want a WebClient that is not load-balanced, create a WebClient bean and inject it. To access the load-balanced WebClient, use the @LoadBalanced qualifier when you create your @Bean, as the following example shows:

The Spring WebFlux can work with both reactive and non-reactive WebClient configurations, as the topics describe:

Sometimes, it is useful to ignore certain named network interfaces so that they can be excluded from Service Discovery registration (for example, when running in a Docker container). A list of regular expressions can be set to cause the desired network interfaces to be ignored. The following configuration ignores the docker0 interface and all interfaces that start with veth:

You can also force the use of only specified network addresses by using a list of regular expressions, as the following example shows:

You can also force the use of only site-local addresses, as the following example shows:

See Inet4Address.html.isSiteLocalAddress() for more details about what constitutes a site-local address.

Spring Cloud Commons provides beans for creating both Apache HTTP clients (ApacheHttpClientFactory) and OK HTTP clients (OkHttpClientFactory). The OkHttpClientFactory bean is created only if the OK HTTP jar is on the classpath. In addition, Spring Cloud Commons provides beans for creating the connection managers used by both clients: ApacheHttpClientConnectionManagerFactory for the Apache HTTP client and OkHttpClientConnectionPoolFactory for the OK HTTP client. If you would like to customize how the HTTP clients are created in downstream projects, you can provide your own implementation of these beans. In addition, if you provide a bean of type HttpClientBuilder or OkHttpClient.Builder, the default factories use these builders as the basis for the builders returned to downstream projects. You can also disable the creation of these beans by setting spring.cloud.httpclientfactories.apache.enabled or spring.cloud.httpclientfactories.ok.enabled to false.

Spring Cloud Commons provides a /features actuator endpoint. This endpoint returns features available on the classpath and whether they are enabled. The information returned includes the feature type, name, version, and vendor.

Due to the fact that some users have problem with setting up Spring Cloud application, we’ve decided to add a compatibility verification mechanism. It will break if your current setup is not compatible with Spring Cloud requirements, together with a report, showing what exactly went wrong.

At the moment we verify which version of Spring Boot is added to your classpath.

Example of a report

In order to disable this feature, set spring.cloud.compatibility-verifier.enabled to false. If you want to override the compatible Spring Boot versions, just set the spring.cloud.compatibility-verifier.compatible-boot-versions property with a comma separated list of compatible Spring Boot versions.

The ReactiveLoadBalancer implementation that is used by default is RoundRobinLoadBalancer. To switch to a different implementation, either for selected services or all of them, you can use the custom LoadBalancer configurations mechanism.

For example, the following configuration can be passed via @LoadBalancerClient annotation to switch to using the RandomLoadBalancer:

In order to make it easy to use Spring Cloud LoadBalancer, we provide ReactorLoadBalancerExchangeFilterFunction that can be used with WebClient and BlockingLoadBalancerClient that works with RestTemplate. You can see more information and examples of usage in the following sections:

Apart from the basic ServiceInstanceListSupplier implementation that retrieves instances via DiscoveryClient each time it has to choose an instance, we provide two caching implementations.

To enable zone-based load-balancing, we provide the ZonePreferenceServiceInstanceListSupplier. We use DiscoveryClient-specific zone configuration (for example, eureka.instance.metadata-map.zone) to pick the zone that the client tries to filter available service instances for.

The ZonePreferenceServiceInstanceListSupplier filters retrieved instances and only returns the ones within the same zone. If the zone is null or there are no instances within the same zone, it returns all the retrieved instances.

In order to use the zone-based load-balancing approach, you will have to instantiate a ZonePreferenceServiceInstanceListSupplier bean in a custom configuration.

We use delegates to work with ServiceInstanceListSupplier beans. We suggest using a DiscoveryClientServiceInstanceListSupplier delegate, wrapping it with a CachingServiceInstanceListSupplier to leverage LoadBalancer caching mechanism, and then passing the resulting bean in the constructor of ZonePreferenceServiceInstanceListSupplier.

You can use this sample configuration to set it up:

It is possible to enable a scheduled HealthCheck for the LoadBalancer. The HealthCheckServiceInstanceListSupplier is provided for that. It regularly verifies if the instances provided by a delegate ServiceInstanceListSupplier are still alive and only returns the healthy instances, unless there are none - then it returns all the retrieved instances.

HealthCheckServiceInstanceListSupplier uses properties prefixed with spring.cloud.loadbalancer.health-check. You can set the initialDelay and interval for the scheduler. You can set the default path for the healthcheck URL by setting the value of the spring.cloud.loadbalancer.health-check.path.default property. You can also set a specific value for any given service by setting the value of the spring.cloud.loadbalancer.health-check.path.[SERVICE_ID] property, substituting [SERVICE_ID] with the correct ID of your service. If the [SERVICE_ID] is not specified, /actuator/health is used by default. If the [SERVICE_ID] is set to null or empty as a value, then the health check will not be executed. You can also set a custom port for health-check requests by setting the value of spring.cloud.loadbalancer.health-check.port. If none is set, the port under which the requested service is available at the service instance.

In order to use the health-check scheduler approach, you will have to instantiate a HealthCheckServiceInstanceListSupplier bean in a custom configuration.

We use delegates to work with ServiceInstanceListSupplier beans. We suggest passing a DiscoveryClientServiceInstanceListSupplier delegate in the constructor of HealthCheckServiceInstanceListSupplier.

You can use this sample configuration to set it up:

You can set up the LoadBalancer in such a way that it prefers the instance that was previously selected, if that instance is available.

For that, you need to use SameInstancePreferenceServiceInstanceListSupplier. You can configure it either by setting the value of spring.cloud.loadbalancer.configurations to same-instance-preference or by providing your own ServiceInstanceListSupplier bean — for example:

You can set up the LoadBalancer in such a way that it prefers the instance with instanceId provided in a request cookie. We currently support this if the request is being passed to the LoadBalancer through either ClientRequestContext or ServerHttpRequestContext, which are used by the SC LoadBalancer exchange filter functions and filters.

For that, you need to use the RequestBasedStickySessionServiceInstanceListSupplier. You can configure it either by setting the value of spring.cloud.loadbalancer.configurations to request-based-sticky-session or by providing your own ServiceInstanceListSupplier bean — for example:

For that functionality, it is useful to have the selected service instance (which can be different from the one in the original request cookie if that one is not available) to be updated before sending the request forward. To do that, set the value of spring.cloud.loadbalancer.sticky-session.add-service-instance-cookie to true.

By default, the name of the cookie is sc-lb-instance-id. You can modify it by changing the value of the spring.cloud.loadbalancer.instance-id-cookie-name property.

Spring Cloud LoadBalancer lets you set String hints that are passed to the LoadBalancer within the Request object and that can later be used in ReactiveLoadBalancer implementations that can handle them.

You can set a default hint for all services by setting the value of the spring.cloud.loadbalancer.hint.default property. You can also set a specific value for any given service by setting the value of the spring.cloud.loadbalancer.hint.[SERVICE_ID] property, substituting [SERVICE_ID] with the correct ID of your service. If the hint is not set by the user, default is used.

We also provide a HintBasedServiceInstanceListSupplier, which is a ServiceInstanceListSupplier implementation for hint-based instance selection.

HintBasedServiceInstanceListSupplier checks for a hint request header (the default header-name is X-SC-LB-Hint, but you can modify it by changing the value of the spring.cloud.loadbalancer.hint-header-name property) and, if it finds a hint request header, uses the hint value passed in the header to filter service instances.

If no hint header has been added, HintBasedServiceInstanceListSupplier uses hint values from properties to filter service instances.

If no hint is set, either by the header or by properties, all service instances provided by the delegate are returned.

While filtering, HintBasedServiceInstanceListSupplier looks for service instances that have a matching value set under the hint key in their metadataMap. If no matching instances are found, all instances provided by the delegate are returned.

You can use the following sample configuration to set it up:

You can use the selected ServiceInstance to transform the load-balanced HTTP Request.

For RestTemplate, you need to implement and define LoadBalancerRequestTransformer as follows:

For WebClient, you need to implement and define LoadBalancerClientRequestTransformer as follows:

If multiple transformers are defined, they are applied in the order in which Beans are defined. Alternatively, you can use LoadBalancerRequestTransformer.DEFAULT_ORDER or LoadBalancerClientRequestTransformer.DEFAULT_ORDER to specify the order.

We also provide a starter that allows you to easily add Spring Cloud LoadBalancer in a Spring Boot app. In order to use it, just add org.springframework.cloud:spring-cloud-starter-loadbalancer to your Spring Cloud dependencies in your build file.

You can also use the @LoadBalancerClient annotation to pass your own load-balancer client configuration, passing the name of the load-balancer client and the configuration class, as follows:

You can use this feature to instantiate different implementations of ServiceInstanceListSupplier or ReactorLoadBalancer, either written by you, or provided by us as alternatives (for example ZonePreferenceServiceInstanceListSupplier) to override the default setup.

You can see an example of a custom configuration here.

You can also pass multiple configurations (for more than one load-balancer client) through the @LoadBalancerClients annotation, as the following example shows:

One type of bean that it may be useful to register using Custom LoadBalancer configuration is LoadBalancerLifecycle.

The LoadBalancerLifecycle beans provide callback methods, named onStart(Request<RC> request), onStartRequest(Request<RC> request, Response<T> lbResponse) and onComplete(CompletionContext<RES, T, RC> completionContext), that you should implement to specify what actions should take place before and after load-balancing.

onStart(Request<RC> request) takes a Request object as a parameter. It contains data that is used to select an appropriate instance, including the downstream client request and hint. onStartRequest also takes the Request object and, additionally, the Response<T> object as parameters. On the other hand, a CompletionContext object is provided to the onComplete(CompletionContext<RES, T, RC> completionContext) method. It contains the LoadBalancer Response, including the selected service instance, the Status of the request executed against that service instance and (if available) the response returned to the downstream client, and (if an exception has occurred) the corresponding Throwable.

The supports(Class requestContextClass, Class responseClass, Class serverTypeClass) method can be used to determine whether the processor in question handles objects of provided types. If not overridden by the user, it returns true.

We provide a LoadBalancerLifecycle bean called MicrometerStatsLoadBalancerLifecycle, which uses Micrometer to provide statistics for load-balanced calls.

In order to get this bean added to your application context, set the value of the spring.cloud.loadbalancer.stats.micrometer.enabled to true and have a MeterRegistry available (for example, by adding Spring Boot Actuator to your project).

MicrometerStatsLoadBalancerLifecycle registers the following meters in MeterRegistry:

Additional information regarding the service instances, request data, and response data is added to metrics via tags whenever available.

Individual Loadbalancer clients may be configured individually with a different prefix spring.cloud.loadbalancer.clients.<clientId>. where clientId is the name of the loadbalancer. Default configuration values may be set in the spring.cloud.loadbalancer. namespace and will be merged with the client specific values taking precedence

The above example will result in a merged health-check @ConfigurationProperties object with initial-delay=1s and interval=30s.

The per-client configuration properties work for most of the properties, apart from the following global ones:

Spring Cloud Circuit breaker provides an abstraction across different circuit breaker implementations. It provides a consistent API to use in your applications, letting you, the developer, choose the circuit breaker implementation that best fits your needs for your application.

To create a circuit breaker in your code, you can use the CircuitBreakerFactory API. When you include a Spring Cloud Circuit Breaker starter on your classpath, a bean that implements this API is automatically created for you. The following example shows a simple example of how to use this API:

The CircuitBreakerFactory.create API creates an instance of a class called CircuitBreaker. The run method takes a Supplier and a Function. The Supplier is the code that you are going to wrap in a circuit breaker. The Function is the fallback that is run if the circuit breaker is tripped. The function is passed the Throwable that caused the fallback to be triggered. You can optionally exclude the fallback if you do not want to provide one.

You can configure your circuit breakers by creating beans of type Customizer. The Customizer interface has a single method (called customize) that takes the Object to customize.

For detailed information on how to customize a given implementation see the following documentation:

Some CircuitBreaker implementations such as Resilience4JCircuitBreaker call customize method every time CircuitBreaker#run is called. It can be inefficient. In that case, you can use CircuitBreaker#once method. It is useful where calling customize many times doesn’t make sense, for example, in case of consuming Resilience4j’s events.

The following example shows the way for each io.github.resilience4j.circuitbreaker.CircuitBreaker to consume events.