Seems to me a solution is to have a "generic" queue handler that will take any message, and then have tenant specific queue handlers.

Now, mind, I'm not thinking of a dedicated handler for each tenant, rather one that can round robin, or rotate through the tenants to pull items specific to that tenant off the queue. This can be the "fairness" arbiter.

With the Postgres SKIP LOCKED technique, you can be selective as to which messages you pull (you're not limited to the head of the queue, but can dig deep if you want).

So, you have one general purpose "head of the line" handler, and then one (or several) that acts like the fellow at the Post Office "Anyone here just picking up mail?", and pull them out of the queue.

Of course the generic version can just switch modes (every 1m, or 100 messages, go into "fair mode" for a short time), vs having different handlers.

I'm thinking about relatively simple systems and not "real" fairness. The case where I've seen this being a bit problematic is with such a queue in an application where jobs are sometimes inserted in large bursts. It doesn't actually matter much if they are truly fair, but in the cases where a large number of jobs is inserted at once the result is that for some tenants the queue doesn't seem to move at all.

It's not a huge problem in that case because the regular priority system still puts the urgent jobs at the front of the queue. But it does sometimes cause confusion for someone looking at this from a single-tenant perspective because the queue looks stuck. There are other ways to fix the confusion, but it would be nice to have a very rudimentary fairness in this simple queue that would ensure that the queue iterates a bit over the tenants.

> On the other hand, is that even necessary? At what scale is this important?

I can see it being helpful in front of a shared resource that doesn't have any way to control priority or fairness. My use case would be Elasticsearch. I don't want to block any searches from running if there's capacity, but also I don't want misbehaving clients to monopolise resources, and I want to make sure that all customers/users get to run their searches eventually, though I may prioritise some customers/users over others. And I may need my own internal jobs to run at a lower priority than a user search, but still within a guaranteed window (to avoid timeouts on the client side).

Honestly for my use case this could be useful with just a handful of users - less than 10, even.

> On the other hand, is that even necessary? At what scale is this important?

That's the critical question. To that I had that there might be a myriad of articles on how someone used postgres to build a message queue instead of using a dedicated message broker, but there isn't a single article showing any form of benchmarking, performance tests, load test results, and more importantly report the correlation between load and performance degradation and eventual overload and resulting failure modes. This information is of critical importance to be able to design systems and pick which tool is suited for the job.