AWS SNS Topic subscriptions with SQS

I was looking for a lightweight replacement for JMS (Java Messaging Service) and wanted to avoid deploying ActiveMQ or another Java JMS service/product.

Amazon SQS is a good alternative for JMS Queues - and there is also an equivalet to JMS Topics and Subscriptions using AWS SNS.

Here is what you do:

1. Create a SNS Topic
2. Create a SQS Queue
3. Subscribe to the SNS Topic as "sqs"
4. Set the Permissions of the SQS Queue to receive SNS notifications
5. Publish to the SQS topic

Here the Java code snipets - other than #4 are pretty obvious.

(1) Create the SNS Topic

AmazonSNSAsyncClient m_sns = new AmazonSNSAsyncClient(credentials);
CreateTopicRequest createTopicRequest = new CreateTopicRequest("MyTopic");
CreateTopicResult createResult = m_sns.createTopic(createTopicRequest);
String m_snsName = createResult.getTopicArn();

(2) Create a SQS Queue

AmazonSQSAsyncClient m_sqs = new AmazonSQSAsyncClient(credentials);
CreateQueueRequest request = new CreateQueueRequest("MySubscription");
CreateQueueResult result = m_sqs.createQueue(request);
String m_sqsName = result.getQueueUrl();

(3) Subscribe to the SNS Topic

First, we need to get the SQS ARN (not url):

GetQueueAttributesRequest queueAttributesRequest = new GetQueueAttributesRequest(m_sqsName)
 .withAttributeNames("All");
GetQueueAttributesResult queueAttributesResult = m_sqs.getQueueAttributes(queueAttributesRequest);
Map<String, String> sqsAttributeMap = queueAttributesResult.getAttributes();
String sqsArn = sqsAttributeMap.get("QueueArn");

Then subscribe to the sqsARN

SubscribeRequest subscribeRequest = new SubscribeRequest(m_snsName, "sqs", sqsArn);
SubscribeResult subscribeResult = m_sns.subscribe(subscribeRequest);
String subscriptionArn = subscribeResult.getSubscriptionArn();

(4) Set the Permission

As SNS publishes under it's own account, you need to set a permission to receive anything in your queue. This is the "little trick" to make it work.

First we need to create a Policy:

Statement statement = new Statement(Effect.Allow)
 .withActions(SQSActions.SendMessage)
 .withPrincipals(new Principal("*"))
 .withConditions(ConditionFactory.newSourceArnCondition(m_snsName))
 .withResources(new Resource(sqsArn));
Policy policy = new Policy("SubscriptionPermission")
 .withStatements(statement);

An then set the queue attribute (the addPermission API is mainly for adding other AWS Account IDs)

HashMap<String, String> attributes = new HashMap<String, String>();
attributes.put("Policy", policy.toJson());
SetQueueAttributesRequest request = new SetQueueAttributesRequest(m_sqsName, attributes);
m_sqs.setQueueAttributes(request);

That's it. Works nicely.