Azure functions with service bus: How to keep a message in the queue if something goes wrong with its processing?

Question

I'm new to service bus and not able to figure this out.

Basically i'm using Azure function app which is hooked onto the service bus queue. Let's say a trigger is fired from the service bus and I receive a message from the queue, and in the processing of that message something goes wrong in my code. In such cases how do I make sure to put that message back in the queue again? Currently its just disappearing into thin air and when I restart my function app on VS, the next message from the queue is taken.

Ideally only when all my data processing is done and when i hit myMsg.Success() do I want it to be removed from the queue.

public static async Task RunAsync([ServiceBusTrigger("xx", "yy", AccessRights.Manage)]BrokeredMessage mySbMsg, TraceWriter log)
{
      try{ // do something with mySbMsg }

      catch{ // put that mySbMsg back in the queue so it doesn't disappear. and throw exception}
}

I was reading up on mySbMsg.Abandon() but it looks like that puts the message in the dead letter queue and I am not sure how to access it? and if there is a better way to error handle?

Answer 1

Cloud queues are a bit different than in-memory queues because they need to be robust to the possibility of the client crashing after it received the queue message but before it finished processing the message.

When a queue message is received, the message becomes "invisible" so that other clients can't pick it up. This gives the client a chance to process it and the client must mark it as completed when it is done (Azure Functions will do this automatically when you return from the function). That way, if the client were to crash in the middle of processing the message (we're on the cloud, so be robust to random machine crashes due to powerloss, etc), the server will see the absence of the completed message, assume the client crashed, and eventually resend the message.

Practically, this means that if you receive a queue message, and throw an exception (and thus we don't mark the message as completed), it will be invisible for a few minutes, but then it will show up again after a few minutes and another client can attempt to handle it. Put another way, in Azure functions, queue messages are automatically retried after exceptions, but the message will be invisible for a few minutes inbetween retries.

Answer 2

If you want the message to remain on the queue to be retried, the function should not swallow exception and rather throw. That way Function will not auto-complete the message and retry it.

Keep in mind that this will cause message to be retried and eventually, if exception persists, to be moved into dead-letter queue.

Answer 3

As per my understanding, I think what you are for is if there is an error in processing the message it needs to retry the execution instead of swallowing it. If you are using Azure Functions V2.0 you define the message handler options in the host.json

 "extensions": {
        "serviceBus": {
            "prefetchCount": 100,
            "messageHandlerOptions": {
                "autoComplete": false,
                "maxConcurrentCalls": 1
            }
        }
    }

prefetchCount - Gets or sets the number of messages that the message receiver can simultaneously request.

autoComplete - Whether the trigger should automatically call complete after processing, or if the function code will manually call complete.

After retrying the message n(defaults to 10) number of times it will transfer the message to DLQ.