Imagine you run a company and provide multiple web applications for your customers. In the beginning, you probably created a user table and the authentication methods yourself, but from the second application onwards, you will think about using already made open source solutions.

That’s the situation I am in right now. I run multiple web applications, which each have their own auth mechanisms and user tables. This means, a user from App1 can’t use App2 unless he creates a new account.