2018-08-25 Cloud Native is a style of application development that encourages easy adoption of best practices in the areas of continuous delivery and value-driven development. A related discipline is that of building 12-factor Apps in which development practices are aligned with delivery and operations goals, for instance by using declarative programming and management and monitoring. Spring Cloud facilitates these styles of development in a number of specific ways and the starting point is a set of features that all components in a distributed system either need or need easy access to when required. Many of those features are covered by Spring Boot, which we build on in Spring Cloud. Some more are delivered by Spring Cloud as two libraries: Spring Cloud Context and Spring Cloud Commons. Spring Cloud Context provides utilities and special services for the ApplicationContext of a Spring Cloud application (bootstrap context, encryption, refresh scope and environment endpoints). Spring Cloud Commons is a set of abstractions and common classes used in different Spring Cloud implementations (eg. Spring Cloud Netflix vs. Spring Cloud Consul). If you are getting an exception due to "Illegal key size" and you are using Sun’s JDK, you need to install the Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files. See the following links for more information: Java 6 JCE Java 7 JCE Java 8 JCE Extract files into JDK/jre/lib/security folder (whichever version of JRE/JDK x64/x86 you are using). Spring Cloud is released under the non-restrictive Apache 2.0 license. If you would like to contribute to this section of the documentation or if you find an error, please find the source code and issue trackers in the project at github. Spring Boot has an opinionated view of how to build an application with Spring: for instance it has conventional locations for common configuration file, and endpoints for common management and monitoring tasks. Spring Cloud builds on top of that and adds a few features that probably all components in a system would use or occasionally need.

A Spring Cloud application operates by creating a "bootstrap" context, which is a parent context for the main application. Out of the box it is responsible for loading configuration properties from the external sources, and also 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. Bootstrap properties are added with high precedence, so they cannot be overridden by local configuration, by default. The bootstrap context uses a different convention for locating external configuration than the main application context, so instead of application.yml (or .properties) you use bootstrap.yml, keeping the external configuration for bootstrap and main context nicely separate. Example: spring: application: name: foo cloud: config: uri: ${SPRING_CONFIG_URI:http://localhost:8888} It is a good idea to set the spring.application.name (in bootstrap.yml or application.yml) if your application needs any application-specific configuration from the server. You can disable the bootstrap process completely by setting spring.cloud.bootstrap.enabled=false (e.g. in System properties).

If you build an application context from SpringApplication or SpringApplicationBuilder, then 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 will contain additional property sources, compared to building the same context without Spring Cloud Config. The additional property sources are: "bootstrap": an optional CompositePropertySource appears with high priority if any PropertySourceLocators are found in the Bootstrap context, and they have non-empty properties. An example would be properties from the Spring Cloud Config Server. See below for instructions on how to customize the contents of this property source. "applicationConfig: [classpath:bootstrap.yml]" (and friends if Spring profiles are active). If you have a bootstrap.yml (or properties) then those properties are used to configure the Bootstrap context, and then they get added to the child context when its parent is set. They have lower precedence than the application.yml (or properties) and any other property sources that are added to the child as a normal part of the process of creating a Spring Boot application. See below for instructions on how to customize the contents of these property sources. Because of the ordering rules of property sources the "bootstrap" entries take precedence, but 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 simply setting the parent context of any ApplicationContext you create, e.g. using its own interface, or with the SpringApplicationBuilder convenience methods (parent(), child() and sibling()). The bootstrap context will be the parent of the most senior ancestor that you create yourself. Every context in the hierarchy will have its own "bootstrap" property source (possibly empty) to avoid promoting values inadvertently from parents down to their descendants. Every context in the hierarchy can also (in principle) have a different spring.application.name and hence a different remote property source if there is a Config Server. Normal Spring application context behaviour 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 one from the parent is not included in the child). Note that the SpringApplicationBuilder allows you to 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 will share common things with their parent.

The bootstrap.yml (or .properties) location can be specified using spring.cloud.bootstrap.name (default "bootstrap") or spring.cloud.bootstrap.location (default empty), e.g. in System properties. Those properties behave like the spring.config.* variants with the same name, 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) then properties in that profile will be loaded as well, just like in a regular Spring Boot app, e.g. from bootstrap-development.properties for a "development" profile.

The property sources that are added to you application by the bootstrap context are often "remote" (e.g. from a Config Server), and by default they cannot be overridden locally, except on the command line. If you want to allow your applications to 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 doesn’t work to set this locally). Once that flag is set there are some finer grained settings to control the location of the remote properties in relation to System properties and the application’s local configuration: spring.cloud.config.overrideNone=true to override with any local property source, and spring.cloud.config.overrideSystemProperties=false if only System properties and env vars should override the remote settings, but not the local config files.

The bootstrap context can be trained to do anything you like by adding entries to /META-INF/spring.factories under the key org.springframework.cloud.bootstrap.BootstrapConfiguration. This is a comma-separated list of Spring @Configuration classes which will be used to create the context. Any beans that you want to be available to the main application context for autowiring can be created here, and also there is a special contract for @Beans of type ApplicationContextInitializer. Classes can be marked with an @Order if you want to control the startup sequence (the default order is "last"). Be careful when adding custom BootstrapConfiguration that the classes you add are not @ComponentScanned by mistake into your "main" application context, where they might not be needed. Use a separate package name for boot configuration classes that is not already covered by your @ComponentScan or @SpringBootApplication annotated configuration classes. The bootstrap process ends by injecting initializers into the main SpringApplication instance (i.e. the normal Spring Boot startup sequence, whether it is running as a standalone app or deployed in an application server). First a bootstrap context is created from the classes found in spring.factories and 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 Config Server, but you can add additional sources by adding beans of type PropertySourceLocator to the bootstrap context (via spring.factories). You could use this to insert additional properties from a different server, or from a database, for instance. As an example, consider the following trivial custom locator: @Configuration public class CustomPropertySourceLocator implements PropertySourceLocator { @Override public PropertySource<?> locate(Environment environment) { return new MapPropertySource("customProperty", Collections.<String, Object>singletonMap("property.from.sample.custom.source", "worked as intended")); } } The Environment that is passed in is the one for the ApplicationContext about to be created, i.e. the one that we are supplying additional property sources for. It will already have its normal Spring Boot-provided property sources, so you can use those to locate a property source specific to this Environment (e.g. by keying it on the spring.application.name, as is done in the default Config Server property source locator). If you create a jar with this class in it and then add a META-INF/spring.factories containing: org.springframework.cloud.bootstrap.BootstrapConfiguration=sample.custom.CustomPropertySourceLocator then the "customProperty" PropertySource will show up in any application that includes that jar on its classpath.

The application will listen for an EnvironmentChangeEvent and react to the change in a couple of standard ways (additional ApplicationListeners can be added as @Beans by the user in the normal way). When an EnvironmentChangeEvent is observed it will have a list of key values that have changed, and the application will use those to: Re-bind any @ConfigurationProperties beans in the context Set the logger levels for any properties in logging.level.* Note that the Config Client does not by default poll for changes in the Environment, and generally we would not recommend that approach for detecting changes (although you could set it up with a @Scheduled annotation). If you have a scaled-out client application then it is better to broadcast the EnvironmentChangeEvent to all the instances instead of having them polling for changes (e.g. 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 (those APIs are public and part of core Spring). You can verify the changes are bound to @ConfigurationProperties beans by visiting the /configprops endpoint (normal Spring Boot Actuator feature). For instance a DataSource can have its maxPoolSize changed at runtime (the default DataSource created by Spring Boot is an @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.

A Spring @Bean that is marked as @RefreshScope will get special treatment when there is a configuration change. This addresses the problem of stateful beans that only get their configuration injected when they are initialized. For instance if a DataSource has open connections when the database URL is changed via the Environment, we probably want the holders of those connections to be able to complete what they are doing. Then the next time someone borrows a connection from the pool he gets one with the new URL. Refresh scope beans are lazy proxies that initialize when they are used (i.e. 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 just need to invalidate its cache entry. The RefreshScope is a bean in the context and it has a public method refreshAll() to refresh all beans in the scope by clearing the target cache. There is also a refresh(String) method to refresh an individual bean by name. This functionality is exposed in the /refresh endpoint (over HTTP or JMX). @RefreshScope works (technically) on an @Configuration class, but it might lead to surprising behaviour: e.g. it does not mean that all the @Beans defined in that class are themselves @RefreshScope. Specifically, anything that depends on those beans cannot rely on them being updated when a refresh is initiated, unless it is itself in @RefreshScope (in which it will be rebuilt on a refresh and its dependencies re-injected, at which point they will be re-initialized from the refreshed @Configuration).

Spring Cloud has an Environment pre-processor for decrypting property values locally. It follows the same rules as the Config Server, and has the same external configuration via encrypt.*. Thus you can use encrypted values in the form {cipher}* and as long as there is a valid key then they will be decrypted before the main application context gets the Environment. 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 are getting an exception due to "Illegal key size" and you are using Sun’s JDK, you need to install the Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files. See the following links for more information: Java 6 JCE Java 7 JCE Java 8 JCE Extract files into JDK/jre/lib/security folder (whichever version of JRE/JDK x64/x86 you are using).

For a Spring Boot Actuator application there are some additional management endpoints: POST to /env to update the Environment and rebind @ConfigurationProperties and log levels /refresh for re-loading the boot strap context and refreshing the @RefreshScope beans /restart for closing the ApplicationContext and restarting it (disabled by default) /pause and /resume for calling the Lifecycle methods (stop() and start() on the ApplicationContext)

Patterns such as service discovery, load balancing and circuit breakers lend themselves to a common abstraction layer that can be consumed by all Spring Cloud clients, independent of the implementation (e.g. discovery via Eureka or Consul).

Commons provides the @EnableDiscoveryClient annotation. This looks for implementations of the DiscoveryClient interface via META-INF/spring.factories. Implementations of Discovery Client will add a configuration class to spring.factories under the org.springframework.cloud.client.discovery.EnableDiscoveryClient key. Examples of DiscoveryClient implementations: are Spring Cloud Netflix Eureka, Spring Cloud Consul Discovery and Spring Cloud Zookeeper Discovery. By default, implementations of DiscoveryClient will auto-register the local Spring Boot server with the remote discovery server. This can be disabled by setting autoRegister=false in @EnableDiscoveryClient.

Commons now provides a ServiceRegistry interface which provides methods like register(Registration) and deregister(Registration) which allow you to provide custom registered services. Registration is a marker interface. @Configuration @EnableDiscoveryClient(autoRegister=false) public class MyConfiguration { private ServiceRegistry registry; public MyConfiguration(ServiceRegistry registry) { this.registry = registry; } // called via some external process, such as an event or a custom actuator endpoint public void register() { Registration registration = constructRegistration(); this.registry.register(registration); } } Each ServiceRegistry implementation has its own Registry implementation.

By default, the ServiceRegistry implementation will auto-register the running service. To disable that behavior, there are two methods. You can set @EnableDiscoveryClient(autoRegister=false) to permanently disable auto-registration. You can also set spring.cloud.service-registry.auto-registration.enabled=false to disable the behavior via configuration.

A /service-registry actuator endpoint is provided by Commons. This endpoint relys on a Registration bean in the Spring Application Context. Calling /service-registry/instance-status via a GET will return the status of the Registration. A POST to the same endpoint with a String body will change the status of the current Registration to the new value. Please see the documentation of the ServiceRegistry implementation you are using for the allowed values for updating the status and the values retured for the status.

RestTemplate can be automatically configured to use ribbon. To create a load balanced RestTemplate create a RestTemplate @Bean and use the @LoadBalanced qualifier. A RestTemplate bean is no longer created via auto configuration. It must be created by individual applications. @Configuration public class MyConfiguration { @LoadBalanced @Bean RestTemplate restTemplate() { return new RestTemplate(); } } public class MyClass { @Autowired private RestTemplate restTemplate; public String doOtherStuff() { String results = restTemplate.getForObject("http://stores/stores", String.class); return results; } } The URI needs to use a virtual host name (ie. service name, not a host name). The Ribbon client is used to create a full physical address. See RibbonAutoConfiguration for details of how the RestTemplate is set up.

A load balanced RestTemplate can be configured to retry failed requests. By default this logic is disabled, you can enable it by adding Spring Retry to your application’s classpath. The load balanced RestTemplate will honor some of the Ribbon configuration values related to retrying failed requests. If you would like to disable the retry logic with Spring Retry on the classpath you can set spring.cloud.loadbalancer.retry.enabled=false. The properties you can use are client.ribbon.MaxAutoRetries, client.ribbon.MaxAutoRetriesNextServer, and client.ribbon.OkToRetryOnAllOperations. See the Ribbon documentation for a description of what there properties do. client in the above examples should be replaced with your Ribbon client’s name.

If you want a RestTemplate that is not load balanced, create a RestTemplate bean and inject it as normal. To access the load balanced RestTemplate use the @LoadBalanced qualifier when you create your @Bean. Notice the @Primary annotation on the plain RestTemplate declaration in the example below, to disambiguate the unqualified @Autowired injection. @Configuration public class MyConfiguration { @LoadBalanced @Bean RestTemplate loadBalanced() { return new RestTemplate(); } @Primary @Bean RestTemplate restTemplate() { return new RestTemplate(); } } public class MyClass { @Autowired private RestTemplate restTemplate; @Autowired @LoadBalanced private RestTemplate loadBalanced; public String doOtherStuff() { return loadBalanced.getForObject("http://stores/stores", String.class); } public String doStuff() { return restTemplate.getForObject("http://example.com", String.class); } } If you see errors like java.lang.IllegalArgumentException: Can not set org.springframework.web.client.RestTemplate field com.my.app.Foo.restTemplate to com.sun.proxy.$Proxy89 try injecting RestOperations instead or setting spring.aop.proxyTargetClass=true.

Sometimes it is useful to ignore certain named network interfaces so they can be excluded from Service Discovery registration (eg. running in a Docker container). A list of regular expressions can be set that will cause the desired network interfaces to be ignored. The following configuration will ignore the "docker0" interface and all interfaces that start with "veth". spring: cloud: inetutils: ignoredInterfaces: - docker0 - veth.* You can also force to use only specified network addresses using list of regular expressions: spring: cloud: inetutils: preferredNetworks: - 192.168 - 10.0 You can also force to use only site local addresses. See Inet4Address.html.isSiteLocalAddress() for more details what is site local address. spring: cloud: inetutils: useOnlySiteLocalInterfaces: true