Exception in QueueTrigger Azure Function

Question

I have an azure function which is triggered by adding a new message into a queue. It should download a file from an FTP server and the name of the file is a part of the message that I push into the queue.

At some points, the server which is hosting files might become inaccessible and I will get exceptions, of course.

I would like to know how the queue behaves in these cases? does it pop the message and leave it? Or does it keep it and call the function again and again until the task gets completed without any exceptions?

Answer 1

From the docs :

The Functions runtime receives a message in PeekLock mode. It calls Complete on the message if the function finishes successfully, or calls Abandon if the function fails. If the function runs longer than the PeekLock timeout, the lock is automatically renewed as long as the function is running.

So, if the function fails it will be available for a next run, for a maximum of 10 retries. After 10 retries it goes to the DeadLetter Queue ( source ):

Service Bus Queues and Subscriptions each have a QueueDescription.MaxDeliveryCount and SubscriptionDescription.MaxDeliveryCount property respectively. the default value is 10. Whenever a message has been delivered under a lock (ReceiveMode.PeekLock), but has been either explicitly abandoned or the lock has expired, the message BrokeredMessage.DeliveryCount is incremented. When DeliveryCount exceeds MaxDeliveryCount, the message is moved to the DLQ, specifying the MaxDeliveryCountExceeded reason code.