juju override defaults in service

Question

Working on the postgresql charm a bit, and I'm wondering if there is a way to supply overrides for default values in a "juju-way".

In my charm that will be consuming the services provided by postgresql, I was thinking I could override things like the database name, or the name of the super_user created. If I stick something like this in the relation-joined:

relation-set user=my_superuser

I'm having an issue where the postgres/db-admin-relation-changed can actually run before the consumer joins runs (or finishes running at least).

Eventually, the postgresql/db-admin-relation-changed will run and get my data, but if it runs too quickly, I could wind up with 2 superusers.

TL;DR: Is there a way to guarantee that default configuration is overridden in a service on the provides side of the relationship?

Answer 1

There is no way to guarantee which hook runs first in the hook execution scheme, but you can predict which hook series will execute first. When a relation is added the *-relation-joined hooks are fired first (on both services, with no clear winner of who goes first), followed by the *-relation-changed hooks for each service. Since you can't guarantee which of the who hooks in a given relation state will fire first you can trump the relation-changed hook on the Postgres charm by placing your relation-set commands in relation-joined hook in your charm.

This won't work for every charm/relation as it depends on where the heavy lifting is done for each relation. For example, the MySQL charm does all the database creation instructions during the relation-joined hook, so the above wouldn't actually help you achieve anything as you'd still have the race condition.

Ideally unless there's a really good reason using what the postgres charm provides is probably the best way to go.