Give the client a chance to save offset on graceful shutdown

[ppou-novibet]

I'm using a stream with IsSingleActiveConsumer=true and I am tracking the offset as messages are being consumed, periodically saving the offset by calling consumer.StoreOffset as described in the documentation.

However I haven't found a clean way of saving the last consumed offset just before shutting down my application.

One nice way of doing that is if I could provide a callback to Consumer.Create that would be called just before the consumer is closed. The callback would receive the RawConsumer and it would be able to call consumer.StoreOffset to save last offset to stream.

[Gsantomaggio]

Hi @ppou-novibet,
Thank you for using the client. I will investigate what the best way to do that would be.

[jonnepmyra]

Hi @ppou-novibet

We have the very same requirment and resolved it by registering a StoreOffset delegate in

IHostApplicationLifetime.ApplicationStopping.Register(() =>{
   consumer.StoreOffset(); //and continue closing our consumer
});

[ppou-novibet]

Thanks @jonnepmyra, we are doing something along those lines too - keeping a reference of the RawConsumer that we get from our MessageHandler as messages arrive, then using that reference to call StoreOffset() before shutting down.

If one is being really careful, one should create a critical section inside MessageHandler, guarded by a semaphore for example, to make sure that the handler will not process any messages after the call to StoreOffset() because of course there is a race condition there. That creates a little more complexity.

Moreover, I just don't like keeping references of things that the implementor has decided I only need inside a specific callback, in this case MessageHandler. Who knows if the implementor decides to invalidate the reference I kept and that last StoreOffset() fails?

So ideally this concern would be handled by the library when I call Consumer.Close(), by invoking a callback we provide and inside we can store the last offset without worrying about messages being consumed in parallel.

[Gsantomaggio]

@ppou-novibet have you tried StatusChanged event? https://github.com/rabbitmq/rabbitmq-stream-dotnet-client/blob/main/docs/ReliableClient/BestPracticesClient.cs#L265