While I appreciate the Postgres for everything point of view, and most of the times I use other things it could fit in Postgres, there are two areas that keep me using RabbitMQ, Redis, or a something like Elastic.

First, I frequently use Celery and Celery doesn't support using Postgres as a broker. It seems like it should, but I guess no one has stepped up to write that. So, when I use Celery, I end up also using Redis or RabbitMQ.

Second, if I need mqtt clients coming in from the internet at large, I don't feel comfortable exposing Postgres to that. Also, I'd rather use the mqtt ecosystem of libraries rather than having all of those devices talk Postgres directly.

Third, sometimes I want a size constrained memory only database or a database that automatically expires untouched records, and for either of those I usually use Redis. For these two tasks I use Redis. I imagine that it would be worth making a reusable set of stored procedures to accomplish the auto-expiring of unused records, but I haven't implemented it. I have no idea how to make Postgres be memory memory only with a constrained memory side.