Azure queue - can I verify a message will be read only once?

Question

I am using an Azure queue and have several different processes reading from the queue.
My system is built in a way that assumes each message is read only once.
This Microsoft article claims Azure queues have an at least once delivery guarantee which potentially means two processes can read the same message from the queue.
This StackOverflow thread claims that if I use GetMessage then the message becomes invisible to all other processes for the invisibility timeout.

Assuming I use GetMessage() and never exceed the message invisibility time before I DeleteMessage, can I assume I will get each message only once?

Answer 1

I think there is a property in queue message named DequeueCount, which is the number of times this message has been dequeued. And it's maintained by queue service. I think you can use this property to identify whether your message had been read before.

https://docs.microsoft.com/en-us/dotnet/api/azure.storage.queues.models.queuemessage.dequeuecount?view=azure-dotnet

Answer 2

No. The following can happen:

• GetMessage()
• Add some records in a database...
• Generate some files...
• DeleteMessage() -> Unexpected failure (process that crashes, instance that reboots, network connectivity issues, ...)

In this case your logic was executed without calling DeleteMessage. This means, once the invisibility timeout expires, the message will appear in the queue and be processed once again. You will need to make sure that your process is idempotent:

Idempotence is the property of certain operations in mathematics and computer science, that they can be applied multiple times without changing the result beyond the initial application.

An alternative solution would be to use Service Bus Queues with the ReceiveAndDelete mode (see this page under How to Receive Messages from a Queue). If you receive the message it will be marked as consumed and never appear again. This way you can be sure it is delivered At-Most-Once (see the comparison with Storage Queues here). But then again, if something happens while your are processing the message (ie: server crashes, ...), you could loose valuable information.

Update:

This will simulate an At-Most-Once in storage queues. The message can arrive multiple times via GetMessage, but will only be processed once by your business logic (with the risk that some of your business logic will never execute).

• GetMessage()
• DeleteMessage()
• AddRecordsToDatabase()
• GenerateFiles()