Function Plug vs Module Plug

Question

I wanted to write a plug for user authetication in my app. Was going through phoenix plug documentation and was a bit confused on which kind of plug to use, function or module.

Generally speaking also, of the two plugs which plug is preferred when?

Answer 1

There are two main uses I can think of when you would want a function plug:

1. You want a plug that is local to a file (controller, router, endpoint, etc.) A module plug is preferred if you want it to be available from multiple files.
2. You are writing a library that allows you to use the function plug in the module (Phoenix does this for put_layout and scrub_params)

Most of the time, my plugs start as functions while I am developing them, then I move them out to their own modules.

The module plug has the benefit of being able to perform some login in the init/1 function that will later be passed to the call/2 function.