Client-level Flow Control

[udnes99]

It would be amazing if it was possible to set flow control at the Pub/Sub client level.

We have a microservice architecture, where a service could subscribe to many topics A challenge with subscription-level flow control is that it it is not possible to regulate the throughput among the subscriptions.

Our current solution is to implement backpressure downstream and fail-fast by throwing an error which bubbles up leading to the onMessage callback, quickly nacking the message if we our resources have been exhausted.

I am not sure if this is even possible with the current underlying APIs for Pub/Sub.

What would you like to see in the library?

No response

Describe alternatives you've considered

Manually writing a wrapper for the underlying APIs.

Additional context/notes

No response

[feywind]

@udnes99 Can you describe more about what you mean with set flow control at the Pub/Sub client level? I don't think we have anything that would do flow control across multiple subscriptions, currently.

For a single subscription, you probably know; but it's possible to tell the server to send less, or to pause. The former you can do with this:

sub.setOptions({
  useLegacyFlowControl: false,
  maxMessages: <the value you want>,
  maxBytes: <the value you want>,
});

This won't change unless you do it before the first call to on() or after streams are re-created over time.

In theory, you could set maxMessages to zero, and that should also pause the stream.

...it could probably be more graceful. :)

But I'll ask around and see if anyone has ideas for the multi-subscription case.

[udnes99]

@feywind Thank you for the reply! You are right in your interpretation of what I meant by "client-level". I should have been more precise, though. It would be a good feature to have for the aforementioned use case.

[feywind]

The recommendation I got on this is that we probably aren't going to do it right now, but if you want to put in an issue in the public issue tracker, it'll be considered later. (And those actually are referenced when justifying new features, so... not a waste.)

[udnes99]

Thank you :) I posted it on the public issue tracker: https://issuetracker.google.com/issues/424716348