Exercise_21_Receive_MQ_Messages.md

[Optional] Exercise 21: Receive MQ Messages

Learning Goal

The task of this exercise is to consume Advanced Message Queueing Protocol (AMQP) messages from the RabbitMQ message queue that are sent out by the Statistics service periodically.

Prerequisite

Continue with your solution of the last exercise. If this does not work, you can checkout the branch origin/solution-20-Use-Message-Queues.

Ensure that the bulletinboard-statistics application is deployed in your CF space and that you have created a rabbitmq service with name mq-bulletinboard.

Step 1: Define a Message Listener

The Statistics service periodically sends out messages containing its statistics for the advertisement with ID="1".

In this step you want to implement a listener that handles the messages from the Statistics service.

Create a StatisticsListener class as part of the com.sap.bulletinboard.ads.services package that implements the MessageListener interface and annotate the class with @Component and @Profile("cloud").

Implement the onMessage(message message) method, that the received message gets logged:

logger.info("got statistics: {}", new String(message.getBody(), Charset.forName("UTF-8")));

Step 2: Register the Message Listener for the Queue

First of all let's make sure that the queue, we would like to listen to, does exist. Therefore in the CloudRabbitConfig class as part of the amqpAdmin Bean definition we declare another Queue with statistics.periodicalStatistics as routing key.

In order to register the StatisticsListener as listener for the queue you can enhance the CloudRabbitConfig with the following Bean definition:

@Bean
public SimpleMessageListenerContainer pushMessageContainer(
        @Qualifier("statisticsListener") final MessageListener messageListener,
        ConnectionFactory connectionFactory) {

    SimpleMessageListenerContainer container = new SimpleMessageListenerContainer(connectionFactory);

    container.setQueueNames();
    container.setPrefetchCount(20);// limit number of unacknowledged messages for a particular channel (creates backpressure to RabbitMQ)
    container.setDefaultRequeueRejected(false); // to prevent requeuing in case of exception
    container.setMessageListener(messageListener); // registers StatisticsListener
    container.setAutoStartup(true);

    return container;
}

Step 3: Deploy and Test

Deploy your microservice and check if messages similar to got statistics: Statistics [id=1, viewCount=<ViewCount>] are logged. The Statistics service sends out messages every five seconds.

Used Frameworks and Tools

• RabbitMQ
• AMQP - Advanced Message Queuing Protocol
• Spring AMQP

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