How does Azure Service Bus Queue guarantees at most once delivery?

Question

According to this doc service bus supports two modes Receive-and-Delete and Peek-Lock.

If using Peek-Lock Mode if the consumer crashes/hangs/do a very long GC right after processing the message, but before the messageId is "Completed" and visibility time expires there's a chance that same message is delivered twice.

Then how does Microsoft says that Service Bus supports at most once delivery mode. Is it because of the Receive-and-Delete mode which sends messages only once.But then again, if something happens while consumers are processing the message then that valuable info is lost.

If yes then what is the best way to ensure exact once delivery using Azure Services Bus as Queue and Azure Functions as Consumers.

P.S. The one approach I can think of is storing MessageID's in blob but since in my case number of MessageID's could be very large storing and loading all of them is not right approach.

Answer 1

Azure Functions will always consume Service Bus messages in Peek-Lock mode. Exactly Once delivery is basically not possible in general case: there's always a chance that consuming application will crash at wrong time just before completing the message, and then the message will be re-delivered.

You should strive to implement Effectively Once processing. This is usually achieved with idempotent message processor.

Storing MessageID's (consumer-side de-duplication) is one option. You could have a policy to clean up old Message IDs to keep the size of such storage manageable. To make this 100% reliable you would have to store Message ID in the same transaction as other modifications done by processor.

Other options really depend on your processing scenario. Find a way to make it idempotent - so that processing the same message multiple times is functionally same as processing it just once.