Simple message-driven microservice in JAVA

RabbitMQ

• Open source written in Earlang
• Maintained since 2013 by Pivotal (Spring)
• Broker which implements AMQP protocol
  • standard way of messages exchanges between heterogeneous applications
  • not an API
  • independant of technology (Python, Java, PHP...)

RabbitMQ core concepts

Exchange - where the messages are published
Queue - where the messages are consumed
Binding - distribution rules from exchanges to queues

RabbitMQ clients

There are a lot of client implementations : Java, .Net, PHP, Go, Ruby, Python, Scala, Perl...

In Java, several possibilities :

• RabbitMQ Java client
• Apache Camel - RabbitMQ component
• Spring AMQP
• Spring Cloud Stream

RabbitMQ for developers

Official docker image available on Docker Hub

docker run --rm -d --hostname my-rabbit --name rabbit -p 5672:5672 -p 15672:15672 rabbitmq:3-management

Management console : http://localhost:15672

Spring Cloud Stream

Built on top of Spring Boot and Spring Integration, it is used to build easily message/event-driven microservices.

It supports bindings with RabbitMQ and Apache Kafka brokers.

It allows minimal configuration and connexion code, thanks to annotations, to focus on business code.

Annotation style programming

@EnableBinding on class declaration
@StreamListener on method declaration to consume messages
@SendTo on method declaration (+ return type) to publish messages

Sample

@EnableBinding(Sink.class)
public class MessageHandler {

	@StreamListener(Sink.INPUT)
	public void handle(MyEvent myEvent) {
	    // do what you have to do with the message
		...
	}
}
					

Let's go

- Start RabbitMQ ! Play with the UI localhost:15672

- Create the 1st project ShippingService which will consume some order events. (Can use Spring Initializr through https://start.spring.io or directly in IDE)

Try it

• RabbitMq management UI => see the created exchange and queue
• Publish a message with the UI and log it
• Manage name and durability of created objects ; try properties
  • spring.cloud.stream.bindings.input.destination
  • spring.cloud.stream.bindings.input.group

Build order-service

This one is publisher and consumer : a processor (source + sink)

It should connect to same exchange than shipping-service

Add an output

You can extend Processor interface...

Error handling

• Try to throw an Exception and see what happens
• Manage DLQ with properties
  • spring.cloud.stream.rabbit.bindings.input.consumer.auto-bind-dlq
  • spring.cloud.stream.rabbit.bindings.input.consumer.republish-to-dlq
  • (You will have to delete the queue before)

Dynamic publishing

It needs to code the message sending

@Autowired
private BinderAwareChannelResolver resolver;
...
resolver
    .resolveDestination("nameofchannel")
    .send(MessageBuilder.withPayload(messagePayload).build());
					

Handling in function of header

@StreamListener(target=Processor.INPUT, condition = "headers['bu-id']=='1'")
public void receiveForBu1() {
	// ...
}
@StreamListener(target=Processor.INPUT, condition = "headers['bu-id']!='1'")
public void receiveForBuNot1() {
	// ...
}
    				

This is not compatible with @SendTo annotation ☹