Azure service bus - lock duration and duplication, how are they related?

Question

I have an azure function monitoring an Azure service bus queue with default lock duration of 30 seconds. This azure function sends out email notifications based on the user information in the message coming from the queue.

I noticed duplicate emails going out and thus checked the trace logs to find out that azure function has been invoked twice for the same user. Log entries are as follows:

2018-08-09T14:38:05.1249371Z - Executing 'AzureFunction' (Reason='New ServiceBus message detected on 'servicebusqueue'.', Id=4657012a-94ac-4b22-a628-2e94285aeeb7)

2018-08-09T14:38:33.3335833Z - Executing 'AzureFunction' (Reason='New ServiceBus message detected on 'servicebusqueue'.', Id=3ff8eea3-9b9b-43ae-a797-5acf01c2ae6c)

The message was added only once to the queue and I am trying to understand what could generate the other one. Can it be because of the lock duration?

Answer 1

Yes it could be due to the lock duration. The message will be completed (received and deleted) from the Queue, only after the execution of the function is complete. If the execution time exceeds 30 seconds, the message will be unlocked, making it available for any other receivers.

In your case, the receiver would be the same Azure Function, which reads the message another time, that's why you are seeing duplicated processing of the message.

The maximum value of Lock Duration is 5 minutes. If the Azure Function just sends an email notification when the message is received, you can increase the lock duration to 5 minutes. The email transmission shouldn't take that long, so that the message will not be available for another receiver.

If you have plans to add something to the Azure Function in addition to sending the notification, you can set auto renewal for the lock in Azure Function, check here for more details. This will keep the message locked, leaving no option for duplication.

Answer 2

Messages added only once, but it's at-least-once delivery guarantee with PeekLock. If your message was not successfully completed within 30 seconds from receiving, it will be unlocked and recieved again. Increasing lock duration or reducing processing time should address it.