[gen220]

I think your decisions were reasonable, as is the opinion of the person you're responding to.

To be fair, even in its current form, it should be possible to operate this system with sqlite (i.e. no db server) and in-process celery workers (i.e. no rabbit MQ) if configured correctly, assuming they're not using MySQL-specific features in the app.

Using a message bus, a persistent data store behind a SQL interface, and a caching layer are all good design choices. I think the OP's concern is less with your particular implementations, and more with the principle of preventing operators from bringing their own preferred implementation of those interfaces to the table.

They mentioned that it makes sense because you were a standalone product, so stack portability was less of a concern. But as FOSS, you're opening yourself up to different standards on portability.

It requires some work on the maintainer to make the application tolerant to different fulfillments of the same interfaces. But it's good work. It usually results in cleaner separation of concerns between application logic and caching/message bus/persistence logic, for one. It also allows your app to serve a wider audience: for example, those who are locked-in to using Postgres/Kafka/Memcached.