Elixir Plugs - question about Plug Modules

Phoenix actually never performs the “call” itself as far as I can tell. Phoenix is nothing more than a set of plugs and supporting libraries.

You are right about the init function, it gets called only when app stars. The call function, on the other hand, will be called on each request.

When a HTTP request comes in, Cowboy server is the first that starts handling such request. plug library adds hooks, builds up the chain of plug middlewares, and calls it, I believe here: plug/lib/plug/adapters/cowboy/handler.ex at 2df667389dbaebb0e2102343c8d7a6ed4de6e280 · elixir-plug/plug · GitHub

So, calling “call” happens on each and every request. Moreover, those calls get nested so a pipeline of plugs get processed by some macro (plug/lib/plug/builder.ex at main · elixir-plug/plug · GitHub) and when one call finishes, another call is executed, until some plug returns a valid response, in such case it is halted, response returned to the browser by cowboy. You don’t have to manually call upstream plugs because of smart macros that do it for us.

Basically, a request comes in → cowboy handler executed → plug1.call() → plug2.call() → plug3.call() etc.

Actually closer to this (still not perfectly accurate, but keeps the init in this idea):

conn # the initial conn handled by cowboy, passed on to Phoenix
|> accepts(["html"]))
|> fetch_session()
|> fetch_flash()
|> protect_from_forgery()
|> put_secure_browser_headers()
|> Rumbl.Auth.call(Rumbl.Auth.init(repo: Rumbl.Repo))

But yeah, it is the ‘pipeline’ call that does the plugs there. In something like Endpoint the plugs are run via the plug builder that builds the ‘state’.

You can think of this:

pipeline :browser do
  plug :accepts, ["html"]
  plug :fetch_session
  plug :fetch_flash
  plug :protect_from_forgery
  plug :put_secure_browser_headers
  plug Rumbl.Auth, repo: Rumbl.Repo
end

will generate this:

conn # the initial conn handled by cowboy, passed on to Phoenix
|> accepts(["html"])
|> fetch_session
|> fetch_flash
|> protect_from_forgery
|> put_secure_browser_headers
|> Rumbl.Auth.call(repo: Rumbl.Repo)

Any request that goes to your app will be run through that pipeline first. You don’t have to explicitly call call/2.

Edit: actually my explanation is missing some details. Before going through the pipeline, the connection from first arrives at the Endpoint and went through a series of Plugs before going to the Router and, subsequently, the pipeline.

Does that make sense?