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In the final article, we move on to AWS Messaging Support.

1. AWS Messaging Support

1.1. SQS (Simple Queue Service)

We can send messages to an SQS queue using the QueueMessagingTemplate.

To create this bean, we can use an AmazonSQSAsync client which is available by default in the application context when using Spring Boot starters:

@Bean
public QueueMessagingTemplate queueMessagingTemplate(
  AmazonSQSAsync amazonSQSAsync) {
    return new QueueMessagingTemplate(amazonSQSAsync);
}

Then, we can send the messages using the convertAndSend() method:

@Autowired
QueueMessagingTemplate messagingTemplate;
 
public void send(String topicName, Object message) {
    messagingTemplate.convertAndSend(topicName, message);
}

Since Amazon SQS only accepts String payloads, Java objects are automatically serialized to JSON.

We can also configure listeners using @SqsListener:

@SqsListener("spring-cloud-test-queue")
public void receiveMessage(String message, 
  @Header("SenderId") String senderId) {
    // ...
}

This method will receive messages from spring-cloud-test-queue and then process them. We can also retrieve message headers using the @Header annotation on method parameters.

If the first parameter is a custom Java object instead of String, Spring will convert the message to that type using JSON conversion.

1.2. SNS (Simple Notification Service)

Similar to SQS, we can use NotificationMessagingTemplate to publish messages to a topic.

To create it, we need an AmazonSNS client:

@Bean
public NotificationMessagingTemplate notificationMessagingTemplate(
  AmazonSNS amazonSNS) {
    return new NotificationMessagingTemplate(amazonSNS);
}

Then, we can send notifications to the topic:

@Autowired
NotificationMessagingTemplate messagingTemplate;

public void send(String Object message, String subject) {
    messagingTemplate
      .sendNotification("spring-cloud-test-topic", message, subject);
}

Out of the multiple SNS endpoints supported by AWS – SQS, HTTP(S), email and SMS, the project only supports HTTP(S).

We can configure the endpoints in an MVC controller:

@Controller
@RequestMapping("/topic-subscriber")
public class SNSEndpointController {

    @NotificationSubscriptionMapping
    public void confirmUnsubscribeMessage(
      NotificationStatus notificationStatus) {
        notificationStatus.confirmSubscription();
    }
 
    @NotificationMessageMapping
    public void receiveNotification(@NotificationMessage String message, 
      @NotificationSubject String subject) {
        // handle message
    }

    @NotificationUnsubscribeConfirmationMapping
    public void confirmSubscriptionMessage(
      NotificationStatus notificationStatus) {
        notificationStatus.confirmSubscription();
    }
}

We need to add the topic name to the @RequestMapping annotation on the controller level. This controller enables an HTTP(s) endpoint – /topic-subscriber which be used by an SNS topic to create a subscription.

For example, we can subscribe to a topic by calling the URL:

https://host:port/topic-subscriber/

The header in the request determines which of the three methods is invoked.

The method with @NotificationSubscriptionMapping annotation is invoked when the header [x-amz-sns-message-type=SubscriptionConfirmation] is present and confirms a new subscription to a topic.

Once subscribed, the topic will send notifications to the endpoint with the header [x-amz-sns-message-type=Notification]. This will invoke the method annotated with @NotificationMessageMapping.

Finally, when the endpoint unsubscribes from the topic, a confirmation request is received with the header [x-amz-sns-message-type=UnsubscribeConfirmation].

This calls the method annotated with @NotificationUnsubscribeConfirmationMapping which confirms unsubscribe action.

Please note that the value in @RequestMapping has nothing to do with the topic name to which it’s subscribed.

2. Conclusion

In this final article, we explored Spring Cloud’s support for AWS Messaging – which concludes this quick series about Spring Cloud and AWS.

I just announced the new Spring 5 modules in REST With Spring:

>> CHECK OUT THE LESSONS