What happens if multiple azure function apps bind to the same storage queue for input

Question

I have function apps running in two different regions for redundancy. i.e. there are two separate apps in azure portal (deployed from the same code). So both apps have the function that input binds to the same storage queue. Would all messages be delivered to both or would the messages get split between the two?

I am using C#, dotnet core, and Functions 2.0.

Answer 1

You do not have to worry about it. The function runtime will lock the messages using the default storage queue behavior.

From the docs:

The queue trigger automatically prevents a function from processing a queue message multiple times; functions do not have to be written to be idempotent.

Now I do know the docs are talking about one function that is scaling out but the same applies to two functions with the same qeueue binding.

So

Would all messages be delivered to both or would the messages get split between the two?

The latter, messages will split between the two.

Answer 2

Is anyone seeing anything different with this? I'm using the Azure Message Bus Queues, but it should be the same. I can see in our log where the queue starts processing the item at almost exactly the same time and it does it twice which matches the number of function apps pointing to the same queue. It doesn't do it every time, but often enough where I can say it's not locking the message from being picked up multiple times.

Answer 3

This might be the case for a single function app that has logic to mitigate processing the same message twice.

However: I Have seen a scaled function app using a queue trigger sometimes getting the same message on multiple instances.

You have to be prepared for that when scaling the function.