How to guarantee at least once delivery with Azure Function with Cosmos DB trigger

Question

I have a Cosmos DB trigger for an Azure function. I want to flatten and write some data from the incoming Document(s) to an (Azure) SQL Server.

What is a way to guarantee at least once delivery?

I looked at https://hackernoon.com/reliable-event-processing-in-azure-functions-37054dc2d0fc which gives some options in the case of an Azure Function triggered by an Event Hub event, but I am not sure if the same applies for the CosmosDB changefeed that causes the trigger to fire.

On the Cosmos DB Change Feed site https://learn.microsoft.com/en-us/azure/cosmos-db/change-feed it states:

Each change to a document appears exactly once in the change feed, and clients manage their checkpointing logic. The change feed processor library provides automatic checkpointing and "at least once" semantics.

Does that mean that it implements the same (or something similar to) the checkpoint system from Event Hub?

Does the circuit breaker pattern work the same way if applied to this flow of a CosmosDB trigger to an Azure Function as detailed at the end of https://hackernoon.com/reliable-event-processing-in-azure-functions-37054dc2d0fc ?

Friary

both azure storage queue and azure service bus queues gurantee at-least-once delivery, so post a message on any one of them and process it asynchronously using that – Definitive

With this option how would an unsuccessful delivery in an Azure Function be communicated back to Service Bus? – Friary

you have to mark message as success or failed for unsuccessful delivery to be communicated, but answer from Mikhail look better than this option – Definitive

Answer 1

Azure Functions Cosmos DB trigger is based on Change Feed processor library. You will get at-least-once out of the box.

Tannenberg

What happens if there is a major "non-hiccup" type of problem and in the middle of 200 messages on message 105 the function is stopped and then restarted once an issue is fixed, will it process the entire backlog in order starting at 105? – Friary

@Friary If those 200 were part of one batch, then checkpoint won't be saved, so next time it will get the whole batch of 200 again – Tannenberg

thank you! Is there some documentation I can refer to for this? The Change Feed processor library one I couldn't see what you just mentioned. – Friary

@Friary Azure Functions reliable event processing – Belshazzar

If the runtime fails, it will move forward. Sadly. learn.microsoft.com/en-us/azure/cosmos-db/nosql/… – Sulphide