Managing Slick DB resources

[pjfanning]

Discussed in #168

^{Originally posted by pjfanning June 13, 2024}
When you call databaseConfig.db, you are expected to close that db instance.

pekko-projection-slick has never bothered to do this.

It appears that pekko-projection-slick users create their own databaseConfig and it is left to them (at the moment) to know about and implement the databaseConfig.db.close() call. Have a look at SlickProjectionSpec to see how this test spec creates a databaseConfig and then in afterAll it calls databaseConfig.db.close()

If we were to take responsibility for closing the databaseConfig.db instances, there are some problems:

• there is no lifecycle that we can rely on, we are probably left with adding a shutdown hook or something like that
• There are a lot of functions in SlickProjection that call createOffsetStore and this createOffsetStore calls databaseConfig.db - potentially create a new db instance that needs managing. If you call databaseConfig.db more than once on the same databaseConfig instance, you should get the same db instance (they are lazily created and reused).

This is my opinion - I think we should not change anything and just document that users need to look after their own databaseConfig lifecycle.