What’s the difference between an Azure Service Bus queue and topic?

Starting out with an Azure Service Bus? It can be confusing trying to work out whether you should use a queue or a topic. In this post, we’ll try to break down the difference between the two and which one you should use when.

What is a service bus queue or topic?

When configuring a service bus, you have two options for configuring how messages are processed – a queue, or a topic.

Lets start with a queue. A queue has a one-to-one relationship between each message and its consumer, and is a way to ensure reliable first-in-first-out (FIFO) delivery to one processor from many sources. For example, you might have one WebJob that processes requests that get placed in the queue from many different sources. In most cases, the processor (or receiver) receives the messages in the same order that they were placed on the queue. The key to a queue is that each message from the queue is only ever processed by a single consumer.

In contrast, a topic follows the publish/subscribe pattern and can have many consumers who can each subscribe to receive notifications when a message is sent to the topic. In effect, this means you can have a one-to-many relationship between messages and consumers although this does depend on how you configure your filter rules (you can opt to have each message delivered to only 1 subscriber if you wish).

What’s the difference?

In effect, the difference between a queue and topic can be described as follows:

  • A queue can only be listened to by one consumer, whereas a topic can have multiple subscribers.
  • Topic subscriptions can enable powerful filtering capabilities, such that you can define certain parameters that messages must meet in order to be copied into a subscriptions virtual queue. This can be handy if you need to handle different types of messages, or messages with variable data structures in the same topic.
  • Topics can be more scalable than queues, as more than one consumer can listen for messages. If you need to scale a queue, you’re still limited to having the one consumer listening, so aside from horizontal scaling you’re out of luck.
  • Both queue and topic subscriptions support PeekLock and ReceiveAndDelete modes, so you can ensure a message is processed before being dismissed if required.

When should I use a topic or queue?

This is a trickier question to answer, and ultimately depends on how much you’re willing to spend on your service bus. If you’re using the basic tier, then you only have the one option – queues. Topics are only supported in the standard and premium tier.

By moving to the standard or premium tier, you incur an hourly base charge as well as the per million operations fee which is charged in the basic tier.

Pricing aside, it also depends on the type of data that you’re ingesting into your service bus. If its time-sensitive and high volume, topics would be the ideal approach as you can more easily scale your downstream consumer to handle the larger volume of messages.

If on the other hand you receive a relatively stable or low volume of messages which aren’t necessarily time-critical (ie. they may sit on a queue for some time until the processor reaches them if the load is higher than anticipated) then you can probably get away with using a queue.

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