First In First Out (FIFO) in Message Brokers

Message brokers are a crucial part of many asynchronous distributed systems. Just like blood vessels transport blood throughout the human body, message brokers transmit streams of countless messages between applications. Today, we're going to take a quick look (as always) at a very useful yet often forgotten feature of our beloved friends — message brokers — the First In First Out mechanic.

The Problem

Imagine we have a producer — a service that emits messages. Every message belongs to a certain group. There are a few consumers — services that process these messages. We want to secure the order of the messages related to the same group while allowing messages for different groups to be processed in a non-deterministic way.

At first glance, there seem to be no issues: the message broker fans out the messages to the consumers in the same order it received them from the producer, so we can expect them to be processed sequentially, right? Unfortunately, no. For instance, if consumer A is much slower than the others, while it processes message 1, consumer B cansage 2, then receive from the broker and process message 3. As a result, the messages will be processed in the following order: 2, 3, 1 instead of 1, 2, 3.

☠️ Approach 1: Keep Only One Consumer

Reduce the number of consumers to one, turn off parallel message processing to ensure instance doesn't process multiple messages at the same time, and enjoy the result (no, you won't enjoy it).

I've seen it done this way a few times. Don't do this, please. Just don't, okay?

✅ Approach 2: Use The Power Of Message Broker

Many messaging systems support the FIFO pattern, making it fairly easy to maintain the order of messages.

Azure Service Bus

In Azure Service Bus, it's called Sessions: a mechanism to ensure that messages with the same value of the session ID header will be processed by consumers in the same order as they were received by the Message Bus from publishers.

(picture from https://learn.microsoft.com/en-us/azure/service-bus-messaging/message-sessions)

Amazon SQS

AWS SQS supports FIFO queues, which ensure the order of messages. These queues maintain the order in which messages are sent and received and also ensure that a message is delivered only once. Similar to Azure Service Bus, FIFO queues from AWS support grouping messages (messages in a group will be processed in order while different groups are processed in parallel).

Apache Kafka

Kafka is designed with partitioning at its core, which inherently supports message ordering within each partition. By designating a key for messages, Kafka ensures that all messages with the same key go to the same partition, thus preserving the order.

Conclusion

Many popular message brokers provide this powerful and often exceptionally useful feature — message ordering. It's good to know about it so that when you need to deal with message ordering or synchronization of parallel events, you have this tool in your toolbox.

Thanks for reading, and see you next time ❤️