IDEA: Centralized userId

Store where?

CI has always been pretty adamant about not having an authentication lib because they don't want to be that opinionated. It's pretty simple to store the user_id in the session, redis, database, or wherever you want. If you're looking for an overall authentication library that handles the current user state, I'd take a look at https://community-auth.com/ (Note, i've never used it but i know a lot of people have)

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