durable subscriber pattern with azure service bus

Question

I'm looking to use Azure Service Bus with topics but need to handle the scenario where a subscriber might not be listening for a message it's interested in (e.g. server being rebooted etc.). This is the typical durable subscriber pattern as described here http://www.eaipatterns.com/DurableSubscription.html.

What I can't work out is how to apply this with Azure Service Bus and I can't seem to find any examples or discussion of this in the documentation. Is this something that Azure service bus provides or should I start looking at alternatives to Azure Service Bus?

Answer 1

This is built straight into Service Bus. As long as a subscription is created it is durable. You create a topic and then create one or more subscriptions. One or more consumers then listen to a subscription when they are active. If they go inactive, such as the server being rebooted, then the subscription stores the messages until a consumer comes back up and asks for one.

Service Bus would only be nondurable if you were creating and destroying subscriptions on the fly as each consumer becomes active or becomes inactive. If there are no subscriptions then messages sent to a topic are lost. Once you create a subscription, any messages sent to the topic (if they pass any filters applied) will be available on the subscription regardless if there are any active consumers using that subscription. Subscriptions exist until you remove them or, if you have the idle removal feature turned on, they surpass the idle deletion time.

You can verify this with a simple console application, or using LinqPad to set up code that does the following:

1. Create a topic.
2. Create a subscription on that topic (no filters)
3. Send a few messages to the topic.
4. In a different script or console app, create a MessageReceiver for that subscription and pull down the messages.

The messages within a subscription are durable for the life of that subscription, until they are processed (completed, etc.), they are forwarded somewhere else or they expire.

Answer 2

I am not sure where you looked for documentation, following are good to read:

1) http://azure.microsoft.com/en-us/documentation/articles/service-bus-dotnet-how-to-use-topics-subscriptions/

2) http://code.msdn.microsoft.com/windowsazure/Simple-Publish-Subscribe-d406eb03