AWS Compute Blog

Cross-Account Integration with Amazon SNS

Contributed by Zak Islam, Senior Manager, Software Development, AWS Messaging

 

Amazon Simple Notification Service (Amazon SNS) is a fully managed AWS service that makes it easy to decouple your application components and fan-out messages. SNS provides topics (similar to topics in message brokers such as RabbitMQ or ActiveMQ) that you can use to create 1:1, 1:N, or N:N producer/consumer design patterns. For more information about how to send messages from SNS to Amazon SQS, AWS Lambda, or HTTP(S) endpoints in the same account, see Sending Amazon SNS Messages to Amazon SQS Queues.

SNS can be used to send messages within a single account or to resources in different accounts to create administrative isolation. This enables administrators to grant only the minimum level of permissions required to process a workload (for example, limiting the scope of your application account to only send messages and to deny deletes). This approach is commonly known as the “principle of least privilege.” If you are interested, read more about AWS’s multi-account security strategy.

This is great from a security perspective, but why would you want to share messages between accounts? It may sound scary, but it’s a common practice to isolate application components (such as producer and consumer) to operate using different AWS accounts to lock down privileges in case credentials are exposed. In this post, I go slightly deeper and explore how to set up your SNS topic so that it can route messages to SQS queues that are owned by a separate AWS account.

Potential use cases

First, look at a common order processing design pattern:

This is a simple architecture. A web server submits an order directly to an SNS topic, which then fans out messages to two SQS queues. One SQS queue is used to track all incoming orders for audits (such as anti-entropy, comparing the data of all replicas and updating each replica to the newest version). The other is used to pass the request to the order processing systems.

Imagine now that a few years have passed, and your downstream processes no longer scale, so you are kicking around the idea of a re-architecture project. To thoroughly test your system, you need a way to replay your production messages in your development system. Sure, you can build a system to replicate and replay orders from your production environment in your development environment. Wouldn’t it be easier to subscribe your development queues to the production SNS topic so you can test your new system in real time? That’s exactly what you can do here.

Here’s another use case. As your business grows, you recognize the need for more metrics from your order processing pipeline. The analytics team at your company has built a metrics aggregation service and ingests data via a central SQS queue. Their architecture is as follows:

Again, it’s a fairly simple architecture. All data is ingested via SQS queues (master_ingest_queue, in this case). You subscribe the master_ingest_queue, running under the analytics team’s AWS account, to the topic that is in the order management team’s account.

Making it work

Now that you’ve seen a few scenarios, let’s dig into the details. There are a couple of ways to link an SQS queue to an SNS topic (subscribe a queue to a topic):

  1. The queue owner can create a subscription to the topic.
  2. The topic owner can subscribe a queue in another account to the topic.

Queue owner subscription

What happens when the queue owner subscribes to a topic? In this case, assume that the topic owner has given permission to the subscriber’s account to call the Subscribe API action using the topic ARN (Amazon Resource Name). For the examples below, also assume the following:

  •  Topic_Owner is the identifier for the account that owns the topic MainTopic
  • Queue_Owner is the identifier for the account that owns the queue subscribed to the main topic

To enable the subscriber to subscribe to a topic, the topic owner must add the sns:Subscribe and topic ARN to the topic policy via the AWS Management Console, as follows:

{
  "Version":"2012-10-17",
  "Id":"MyTopicSubscribePolicy",
  "Statement":[{
      "Sid":"Allow-other-account-to-subscribe-to-topic",
      "Effect":"Allow",
      "Principal":{
        "AWS":"Topic_Owner"
      },
      "Action":"sns:Subscribe",
      "Resource":"arn:aws:sns:us-east-1:Queue_Owner:MainTopic"
    }
  ]
}

After this has been set up, the subscriber (using account Queue_Owner) can call Subscribe to link the queue to the topic. After the queue has been successfully subscribed, SNS starts to publish notifications. In this case, neither the topic owner nor the subscriber have had to process any kind of confirmation message.

Topic owner subscription

The second way to subscribe an SQS queue to an SNS topic is to have the Topic_Owner account initiate the subscription for the queue from account Queue_Owner. In this case, SNS first sends a confirmation message to the queue. To confirm the subscription, a user who can read messages from the queue must visit the URL specified in the SubscribeURL value in the message. Until the subscription is confirmed, no notifications published to the topic are sent to the queue. To confirm a subscription, you can use the SQS console or the ReceiveMessage API action.

What’s next?

In this post, I covered a few simple use cases but the principles can be extended to complex systems as well. As you architect new systems and refactor existing ones, think about where you can leverage queues (SQS) and topics (SNS) to build a loosely coupled system that can be quickly and easily extended to meet your business need.

For step by step instructions, see Sending Amazon SNS messages to an Amazon SQS queue in a different account. You can also visit the following resources to get started working with message queues and topics: