Introducing Amazon SNS FIFO – First-In-First-Out Pub/Sub Messaging

When designing a distributed software architecture, it is important to define how services exchange information. For example, the use of asynchronous communication decouples components and simplifies scaling, reducing the impact of changes and making it easier to release new features.

The two most common forms of asynchronous service-to-service communication are message queues and publish/subscribe messaging:

• With message queues, messages are stored on the queue until they are processed and deleted by a consumer. On AWS, Amazon Simple Queue Service (Amazon SQS) provides a fully managed message queuing service with no administrative overhead.
  
• With pub/sub messaging, a message published to a topic is delivered to all subscribers to the topic. On AWS, Amazon Simple Notification Service (Amazon SNS) is a fully managed pub/sub messaging service that enables message delivery to a large number of subscribers. Each subscriber can also set a filter policy to receive only the messages that it cares about.
  

You can use topics when you want to fan out messages to multiple applications, and queues when you want to send messages to one application. Using topics and queues together, you can decouple microservices, distributed systems, and serverless applications.

With SQS, you can use FIFO (First-In-First-Out) queues to preserve the order in which messages are sent and received, and to avoid that a message is processed more than once.

Introducing SNS FIFO Topics
Today, we are adding similar capabilities for pub/sub messaging with the introduction of SNS FIFO topics, providing strict message ordering and deduplicated message delivery to one or more subscribers.

FIFO topics manage ordering and deduplication similar to FIFO queues:

Ordering – You configure a message group by including a message group ID when publishing a message to a FIFO topic. For each message group ID, all messages are sent and delivered in order of their arrival. For example, to ensure the delivery of messages related to the same customer in order, you can publish these messages to the topic using the customer’s account number as the message group ID. There is no limit in the number of message groups with FIFO topics and queues. You don’t need to declare in advance the message group ID, any value will work. If you don’t have a logical distinction between messages, you can simply use the same message group ID for all and have a single group of ordered messages. The message group ID is passed to any subscribed FIFO queue.

Deduplication – Distributed systems (like SNS) and client applications sometimes generate duplicate messages. You can avoid duplicated message deliveries from the topic in two ways: either by enabling content-based deduplication on the topic, or by adding a deduplication ID to the messages that you publish. With message content-based deduplication, SNS uses a SHA-256 hash to generate the message deduplication ID using the body of the message. After a message with a specific deduplication ID is published successfully, there is a 5-minute interval during which any message with the same deduplication ID is accepted but not delivered. If you subscribe a FIFO queue to a FIFO topic, the deduplication ID is passed to the queue and it is used by SQS to avoid duplicate messages being received.

You can use FIFO topics and queues together to simplify the implementation of applications where the order of operations and events is critical, or when you cannot tolerate duplicates. For example, to process financial operations and inventory updates, or to asynchronously apply commands that you receive from a client device. FIFO queues can use message filtering in FIFO topics to selectively receive only a subset of messages rather than every message published to the topic.

How to Use SNS FIFO Topics
A common scenario where FIFO topics can help is when you receive updates that need to be processed in order. For example, I can use a FIFO topic to receive updates from an application where my customers edit their account profiles. Then, I subscribe an SQS FIFO queue to the FIFO topic, and use the queue as trigger for a Lambda function that applies the account updates to an Amazon DynamoDB table used by my Customer management system that needs to be kept in sync.

The decoupling introduced by the FIFO topic makes it easier to add new functionality with minimal impact to existing applications. For example, to reward my loyal customers with additional promotions, I add a new Loyalty application that is storing information in a relational database managed by Amazon Aurora. To keep the customer’s information stored in the Loyalty database in sync with my other applications, I can subscribe a new FIFO queue to the same FIFO topic, and add a new Lambda function that receives customer updates in the same order as they are generated, and applies them to the Loyalty database. In this way, I don’t need to change code and configuration of other applications to integrate the new Loyalty app.

First, I create two FIFO queues in the SQS console, leaving all options to their defaults:

• The customer.fifo queue to process updates in my Customer management system.
• The loyalty.fifo queue to help me collect and store customer updates for the Loyalty application.

In the SNS console, I create the updates.fifo topic. I select FIFO as type, and enable Content-based message deduplication.

Then,  I subscribe the customer.fifo and loyalty.fifo queues to the topic.

To be able to receive messages, I add a statement to the access policy of both queues granting the updates.fifo topic permissions to send messages to the queues. For example, for the customer.fifo queue the statement is:

{
  "Effect": "Allow",
  "Principal": {
    "Service": "sns.amazonaws.com"
  },
  "Action": "SQS:SendMessage",
  "Resource": "arn:aws:sqs:us-east-2:123412341234:customer.fifo",
  "Condition": {
    "ArnLike": {
      "aws:SourceArn": "arn:aws:sns:us-east-2:123412341234:updates.fifo"
    }
  }
}

Now, I use the SNS console to publish 4 messages in sequence. For all messages, I use the same message group ID. In this way, they are all in the same message group. The only part that is different is the message body, where I use in order:

• Update One
• Update Two
• Update Three
• Update One

In the SQS console, I see that only 3 messages have been delivered to the FIFO queues:

Why is that? When I created the FIFO topics, I enabled content-based deduplication. The 4 messages were sent within the 5-minute deduplication window. The last message has been recognized as a duplicate of the first one and has not been delivered to the subscribed queues.

Let’s see the actual messages in the queues. I use the AWS Command Line Interface (AWS CLI) to receive the messages from SQS, and the jq command-line JSON processor to format the output and get only the Message in the Body.

Here are the messages in the customer.fifo queue:

$ aws sqs receive-message --queue-url https://sqs.us-east-2.amazonaws.com/123412341234/customer.fifo --max-number-of-messages 10 | jq '.Messages[].Body | fromjson | .Message'

"Update One"
"Update Two"
"Update Three"

And these are the messages in the loyalty.fifo queue:

$ aws sqs receive-message --queue-url https://sqs.us-east-2.amazonaws.com/123412341234/loyalty.fifo --max-number-of-messages 10 | jq '.Messages[].Body | fromjson | .Message'

"Update One"
"Update Two"
"Update Three"

As expected, the 3 messages with unique content have been delivered to both queues in the same order as they were sent.

Available Now
You can use SNS FIFO topics in all commercial regions. You can process up to 300 transactions per second (TPS) per FIFO topic or FIFO queue. With SNS, you pay only for what you use, you can find more information in the pricing page.

To learn more, please see the documentation.

— Danilo