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Exercise_20_Use_Message_Queues.md

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[Optional] Exercise 20: Add Message into Message Queue

Learning Goal

Asynchronous communication by publish-subscribe / messaging systems is a very important element of high performance and resilient cloud applications. Instead of calling a client service and waiting for a response, you just send a message and can immediately continue. There are different levels of loose coupling. In the extreme case the sender of an event does not know or care how many other services (if any) subscribed to their event.

The goal of this exercise is that you learn how to send a message to inform other services about a specific event. Specifically, the task is to send an AMQP message to the RabbitMQ Message Queue Service (part of the standard CF backing services) whenever your Advertisement Service receives a request for a specific advertisement. This event (AMQP message) will then be picked up by a Statistics Service that counts how many times an advertisement was viewed.

Note: In this exercise we will only send messages. Receiving and processing is handled in the next excercise.

Prerequisite

Continue with your solution of the last exercise. If this does not work, you can checkout the branch origin/solution-19-Transfer-CorrelationID.

Step 1: Add Maven Dependency

Add the spring-rabbit dependency to your pom.xml:

<!-- AMQP / RabbitMQ messaging -->
<dependency>
    <groupId>org.springframework.amqp</groupId>
    <artifactId>spring-rabbit</artifactId>
    <version>2.0.1.RELEASE</version>
    <exclusions>
        <exclusion>
            <!-- We need a more recent version of spring-context than the one included in spring-rabbit -->
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
        </exclusion>
    </exclusions>
</dependency>

Note: After you've changed the Maven settings, don't forget to update your Eclipse project (Alt+F5)!

Step 2: Add CloudRabbitConfig

Create a new class CloudRabbitConfig in package com.sap.bulletinboard.ads.config and copy the code from here.

Step 3: Implement StatisticsServiceClient

  • Create a new class StatisticsServiceClient in package com.sap.bulletinboard.ads.services and annotate it with @Component. Note: the classes we will be using in the following are from org.springframework.amqp.core, so when you're fixing the import errors pick classes from this package.
  • In the constructor of StatisticsServiceClient you expect that the RabbitTemplate gets injected.
  • Create a method advertisementIsShown(long id). The responsibility of this method is to send out messages to the queue named "statistics.adIsShown":
public void advertisementIsShown(long id) {
    rabbitTemplate.convertAndSend(null, "statistics.adIsShown", String.valueOf(id));
}

Note: In our setup, as we are not specifying any Exchange, we bind to the Direct Exchange so we bind to a Queue via a fixed routing key, which currently relates to the queue named "statistics.adIsShown".

Other implementation hints:

Step 4: Use StatisticsServiceClient in AdvertisementController

Inject the StatisticsServiceClient into your AdvertisementController class as part of the constructor and notify the Statistics Service whenever an individual advertisement is requested.

Step 5: Test Locally

The virtual machine also provides a RabbitMQ service that you can use for local testing.

Prepare VCAP_SERVICES

Based on the VCAP_SERVICES environment variable the spring-cloud connector instantiates as part of the CloudRabbitConfig class the AmqpAdmin and AmqpTemplate instance that are used to communicate with the bound RabbitMQ service. Locally this environment variable needs to be set:

  • In Eclipse, open the Tomcat server settings (by double-clicking on the server) and then open the launch configuration. In the Environment tab edit the VCAP_SERVICES variable. Replace the value with the following:
{"rabbitmq-lite":[{"credentials":{"hostname":"127.0.0.1","password":"guest","uri":"amqp://guest:guest@127.0.0.1:5672","username":"guest"},"name":"rabbitmq-lite","label":"rabbitmq-lite","tags":["rabbitmq33","rabbitmq","amqp"]}],"postgresql-9.3":[{"name":"postgresql-lite","label":"postgresql-9.3","credentials":{"dbname":"test","hostname":"127.0.0.1","password":"test123!","port":"5432","uri":"postgres://testuser:test123!@localhost:5432/test","username":"testuser"},"tags":["relational","postgresql"],"plan":"free"}]}
  • If you want to view / edit VCAP_SERVICESor JSON structures in general, you can e.g. use the Chrome JSON Editor plugin.
  • If you run the application from the command line, update your localEnvironmentSetup script: localEnvironmentSetup.sh(localEnvironmentSetup.bat) with the new value of VCAP_SERVICES.

Run and test the application

You can run the application using Eclipse or on the command line:

$ source localEnvironmentSetup.sh
$ mvn tomcat7:run
  • Send some GET-By-Id requests to the Advertisement Service like localhost:8080/api/v1/ads/1.
  • Then, in the browser, visit the RabbitMQ Admin UI at http://localhost:15672/ (login with guest:guest). Observe the information shown in the Totals view, and find out how many messages your application has sent.

Step 6: Fix Tests

In the current state the tests fail, as the statisticsServiceClient bean can't be created, because the beans like the AMQPTemplate to be injected are undefined. In previous exercises when we needed to connect to a (PostgreSQL) database, we used an embedded database (H2) in the tests. In the case of RabbitMQ a similar approach is not easily possible. Instead, we just provide mock instances of every queue-related bean that is defined in the CloudRabbitConfig class.

Create MockRabbitConfig

As part of the src/test/java source folder create a new class MockRabbitConfig in the package com.sap.bulletinboard.ads.config and copy the code from here. With that there is no need to mock the StatisticsServiceClient.

After that the tests should run again.

Step 7: Create and Bind CF RabbitMQ Service

A Message Queue is provided as backing service. There must be a RabbitMQ service instance on Cloud Foundry the application can connect to:

Creating the service

In the terminal, run the following command:

cf create-service rabbitmq v3.6-dev mq-bulletinboard

Note you can get the exact names of the available services and its plans with cf marketplace.

Bind service

In the manifest.yml file, make sure the mq-bulletinboard service is bound to your application:

services:
 - mq-bulletinboard

With this the application should start when deployed in the cloud. Deploy your Advertisement Service and check in Kibana (via log messages) if the expected notifications are sent.

Step 8: Deploy Statistics Service as Message Consumer

In order to test whether the messages in the queue can be processed by a consumer, you can deploy the Statistics Service as consuming application into your CF space. It is bound to the mq-bulletinboard service and listens to it:

  • Checkout the branch Statistics Service branch
  • In order to deploy the statistics application enter on the command line:
# Ensure that you are in the project root e.g. ~/git/cloud-bulletinboard-ads
$ mvn clean verify
$ cf push -n bulletinboard-statistics-<<your user id>>
  • Whenever you request an advertisment the Statistics Service should increase the counter for the same. To test this interaction you can call in the browser for example [bulletinboard-statistics-<<<your user id>>>.cfapps.<<region>>.hana.ondemand.com/api/v1/statistics/1](https://bulletinboard-statistics-<<your user id>>.cfapps.<<region>>.hana.ondemand.com/api/v1/statistics/1) - where "1" is the advertisment ID.

[Optional] Step 9: Use Hystrix

For HTTP/REST communication we used Hystrix so that failures of external dependencies can be mitigated. For message queue systems like RabbitMQ we do not notice failures of applications receiving the messages. However, the queue itself might fail for various reasons: e.g. the queue accepts no more messages.

Adapt your configuration so that the amqpTemplate.send method is wrapped in a HystrixCommand, similar as you've done in Exercise 17. Make sure to store the Correlation ID in a field, as the run method will be called in a different thread, and use this field to initialize the log context using LogContext.initializeContext.

Have a look at our sample branch exercise 20.

Used Frameworks and Tools

© 2018 SAP SE
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