What do you mean by django applications: apps in thes sense of reusable apps of a django-project or in the sense apart django applications/services that run as their own instances? If I understood correctly the latter one.

If all your apps run on one server but need access to different databases you can create a custom database router, see the django-docs on this topic: https://docs.djangoproject.com/en/5.2/topics/db/multi-db/ An authRouter is explicitly listed as example.

Your auth app could then use one database and the other apps could use another db or each their own database ... .

If, however, your apps run as separate Django-applications (e.g., on different servers), you have two options:

• The first Option would be, that each of your django-applications shares the same reusable auth-app and has a custom db-adapter, that will ensure that this app uses another databases than the other model of the project use. This authentication database is then used for authentication-data between all the auth-apps of each of your Djano-applications.

• The Second option would be to use SAML or better OpenId connect to have single-sign-on (SSO). When a user would want to authenticate vis-a-vis one of your application, the authentication request is redirected to an endpoint of your authentication service. There, the user is presented with a login form and authenticates using their credentials. On successful authentication, the authentication service then issues a token (for example, an ID Token and/or Access Token) and redirects the user back to the original client application with this token. The client application verifies the token (usually via the authentication service’s public keys or another endpoint of your auth-application and establishes a session for the user.