How do I make sure that I process one message at a time at most?

I am wondering how to process one message at a time using Googles pub/sub functionality in Go. I am using the official library for this, https://pkg.go.dev/cloud.google.com/go/pubsub#section-readme. The event is being consumed by a service that runs with multiple instances, so any in memory locking mechanism will not work.

I realise that it's an anti-pattern to do this, so let me explain my use-case. Using mongoDB I store an array of objects as an embedded document for each entity. The event being published is modifying parts of this array and saves it. If I receive more than one event at a time and they start processing exactly at the same time, one of the saves will override the other. So I was thinking a solution for this is to make sure that only one message will be processed at a time, and it would be nice to use any built-in functionality in cloud pub/sub to do so. Otherwise I was thinking of implementing some locking mechanism in the DB but i'd like to avoid that.

Any help would be appreciated.

Solution

You can imagine 2 things:

You can use ordering key in PubSub. Like that, all the message in relation with the same object will be delivered in order and one by one.
You can use a PUSH subscription to PubSub, to push to Cloud Run or Cloud Functions. With Cloud Run, set the concurrency to 1 (it's by default with Cloud Functions gen1), and set the max instance to 1 also. Like that you can process only one message at a time, all the other message will be rejected (429 HTTP error code) and will be requeued to PubSub. The problem is that you can not parallelize the processing as before with ordering key

A similar thing, and simpler to implement, is to use Cloud Tasks instead of PubSub. With Cloud Tasks you can set a rate limit on a queue, and set the maxConcurrentDispatches to 1 (and you haven't to do the same with Cloud Functions max instances or Cloud Run max instances and concurrency)