Spring Cloud Stream

This project allows a user to develop and run messaging microservices using Spring Integration and run them locally, or in the cloud, or even on Spring XD. Just add @EnableBinding and run your app as a Spring Boot app (single application context). You just need to connect to the physical broker for the bindings, which is automatic if the relevant binder implementation is available on the classpath. The sample uses Redis.

Here’s a sample source module (output channel only):

@SpringBootApplication
public class ModuleApplication {

  public static void main(String[] args) {
    SpringApplication.run(ModuleApplication.class, args);
  }

}

@EnableBinding(Source.class)
public class TimerSource {

  @Value("${format}")
  private String format;

  @Bean
  @InboundChannelAdapter(value = Source.OUTPUT, poller = @Poller(fixedDelay = "${fixedDelay}", maxMessagesPerPoll = "1"))
  public MessageSource<String> timerMessageSource() {
    return () -> new GenericMessage<>(new SimpleDateFormat(format).format(new Date()));
  }

}

@EnableBinding is parameterized by an interface (in this case Source) which declares input and output channels. Source, Sink and Processor are provided off the shelf, but you can define others. Here’s the definition of Source:

public interface Source {
  @Output("output")
  MessageChannel output();
}

The @Output annotation is used to identify output channels (messages leaving the module) and @Input is used to identify input channels (messages entering the module). It is optionally parameterized by a channel name - if the name is not provided the method name is used instead. An implementation of the interface is created for you and can be used in the application context by autowiring it, e.g. into a test case:

@RunWith(SpringJUnit4ClassRunner.class)
@SpringApplicationConfiguration(classes = ModuleApplication.class)
@WebAppConfiguration
@DirtiesContext
public class ModuleApplicationTests {

	@Autowired
	private Source source;

	@Test
	public void contextLoads() {
		assertNotNull(this.source.output());
	}

}

Note

In this case there is only one Source in the application context so there is no need to qualify it when it is autowired. If there is ambiguity, e.g. if you are composing one module from some others, you can use @Bindings qualifier to inject a specific channel set. The @Bindings qualifier takes a parameter which is the class that carries the @EnableBinding annotation (in this case the TimerSource).

Multiple Input or Output Channels

A module can have multiple input or output channels all defined either as @Input and @Output methods in an interface (preferrable) or as bean definitions. Instead of just one channel named "input" or "output" you can add multiple MessageChannel methods annotated @Input or @Output and the names are converted to external channel names on the broker. The external channel names can be specified as properties that consist of the channel names prefixed with spring.cloud.stream.bindings (e.g. spring.cloud.stream.bindings.input.destination or spring.cloud.stream.bindings.output.destination). External channel names can have a channel type as a colon-separated prefix, and the semantics of the external bus channel changes accordingly. For example, you can have two MessageChannels called "output" and "foo" in a module with spring.cloud.stream.bindings.output.destination=bar and spring.cloud.stream.bindings.foo.destination=topic:foo, and the result is 2 external channels called "bar" and "topic:foo".

Samples

There are several samples, all running on the redis transport (so you need redis running locally to test them).

Note	The main set of samples are "vanilla" in the sense that they are not deployable as XD modules by the current generation (1.x) of XD. You can still interact with an XD system using the appropriate naming convention for input and output channel names (<stream>.<index> format).

• source is a Java config version of the classic "timer" module from Spring XD. It has a "fixedDelay" option (in milliseconds) for the period between emitting messages.

• sink is a Java config version of the classic "log" module from Spring XD. It has no options (but some could easily be added), and just logs incoming messages at INFO level.

• transform is a simple pass through logging transformer (just logs the incoming message and passes it on).

• double is a combination of 2 modules defined locally (a source and a sink, so the whole app is self contained).

• extended is a multi-module mashup of source | transform | transform | sink, where the modules are defined in the other samples and referred to in this app just as dependencies.

• multibinder is a sample application that shows how an application could use multiple binders. In this case, the processor’s input/output channels connect to different brokers using their own binder configurations.

• multibinder-differentsystems shows how an application could use same binder implementation but different configurations for its channels. In this case, a processor’s input/output channels connect to same binder implementation but with two separate broker configurations.

If you run the source and the sink and point them at the same redis instance (e.g. do nothing to get the one on localhost, or the one they are both bound to as a service on Cloud Foundry) then they will form a "stream" and start talking to each other. All the samples have friendly JMX and Actuator endpoints for inspecting what is going on in the system.

Module or App

Code using this library can be deployed as a standalone app or as an XD module. In standalone mode your app will run happily as a service or in any PaaS (Cloud Foundry, Lattice, Heroku, Azure, etc.). Depending on whether your main aim is to develop an XD module and you just want to test it locally using the standalone mode, or if the ultimate goal is a standalone app, there are some things that you might do differently.

Fat JAR

You can run in standalone mode from your IDE for testing. To run in production you can create an executable (or "fat") JAR using the standard Spring Boot tooling.

Making Standalone Modules Talk to Each Other

The [input,output]ChannelName are used to create physical endpoints in the external broker (e.g. queue.<channelName> in Redis).

For an XD module the channel names are <group>.<index> and a source (output only) has index=0 (the default) and downstream modules have the same group but incremented index, with a sink module (input only) having the highest index. To listen to the output from a running XD module, just use the same "group" name and an index 1 larger than the app before it in the chain.

Note: since the same naming conventions are used in XD, you can steal messages from or send messages to an existing XD stream by copying the stream name (to spring.cloud.stream.<channelName>.group) and knowing the index of the XD module you want to interact with.

Contributing

We love contributions. Follow this link for more information on how to contribute.