How To Rate-Limit Google Cloud Pub/Sub Queue

Question

I'm using Google's Pub/Sub queue to handle messages between services. Some of the subscribers connect to rate-limit APIs.

For example, I'm pushing street addresses onto a pub/sub topic. I have a Cloud function which subscribes (via push) to that topic, and calls out to an external rate-limited geocoding service. Ideally, my street addresses could be pushed onto the topic with no delay, and the topic would retain those messages - calling the subscriber in a rate-limited fashion.

Is there anyway to configure such a delay, or a message distribution rate limit? Increasing the Ack window doesn't really help: I've architected this system to prevent long-running functions.

Answer 1

An aproach to solve your problem is by using: async.queue

There you have a concurrency attribute wich you can manage the rate limit.

// create a queue object with concurrency 2
var q = async.queue(function(task, callback) {
    console.log('hello ' + task.name);
    callback();
}, 2);

// assign a callback
q.drain = function() {
    console.log('all items have been processed');
};

// add some items to the queue
q.push({name: 'foo'}, function(err) {
    console.log('finished processing foo');
});

// quoted from async documentation

Answer 2

Because there's no answer so far describing workarounds, I'm going to answer this now by stating that there is currently no way to do this. There are workarounds (see the comments on the question that explain how to create a queueing system using Cloud Scheduler), but there's no way to just set a setting on a pull subscription that creates a rate limit between it and its topic.

I opened a feature request for this though. Please speak up on the tracked issue if you'd like this feature.

https://issuetracker.google.com/issues/197906331