Azure Service Bus: Implementing the Priority Queue Pattern

Azure Service Bus is one of Azure messaging services. As one of its features, Azure Service Bus allows us to perform queuing of messages so that their processor and emitter are decoupled.

These messages are placed in the queue in the order that they arrive. Consequently, they are read and processed in that order.

But what if we have a top priority message? How can we force it so that it gets processed quickly (i.e., without waiting for all the messages placed in front of it to be processed)?

There's a pattern called the "Priority Queue Pattern," and we can implement it using Azure Service Bus.

First, it's important to note that Azure Service Bus doesn't natively implement the Priority Queue Pattern. However, there are various ways to implement that pattern using Azure Service Bus. Let's see three of them: two involving Azure Service Bus Queues and one involving Azure Service Bus Topics and Subscriptions.

1st approach: Dedicated queues with dedicated processors per priority

Challenges of this approach: (1) the emitter is responsible for pushing the messages into the appropriate queues based on their priorities; (2) you may want to have more capacitated processors for higher priority messages than for lower ones to ensure fast processing of these high priority messages.

2nd approach: Dedicated queues with no dedicated processors

Challenge of this approach: Since processors listen to every queue, they are responsible for ensuring that they process messages based on priorities, processing lower priority messages only if there is no higher priority message to process. However, this approach allows for having identical processors.

3rd approach: Single topic with dedicated subscriptions per priority

Challenge of this approach: Since priority messages have, well… priority, you may want to have a larger number of processors registered to this subscription than lower priority subscriptions to ensure these messages are processed quickly.

Hence, as you can see, there are many ways to implement the Priority Queue Pattern using Azure Service Bus. The approach you choose depends on your application's considerations and constraints.