The plug specification.

There are two kind of plugs: function plugs and module plugs.

Function plugs

A function plug is any function that receives a connection and a set of options and returns a connection. Its type signature must be:

(Plug.Conn.t, Plug.opts) :: Plug.Conn.t

Module plugs

A module plug is an extension of the function plug. It is a module that must export:

• a call/2 function with the signature defined above
• an init/1 function which takes a set of options and initializes it.

The result returned by init/1 is passed as second argument to call/2. Note that init/1 may be called during compilation and as such it must not return pids, ports or values that are not specific to the runtime.

The API expected by a module plug is defined as a behaviour by the Plug module (this module).

Examples

Here’s an example of a function plug:

def json_header_plug(conn, opts) do
  Plug.Conn.put_resp_content_type(conn, "application/json")
end

Here’s an example of a module plug:

defmodule JSONHeaderPlug do
  import Plug.Conn

  def init(opts) do
    opts
  end

  def call(conn, _opts) do
    put_resp_content_type(conn, "application/json")
  end
end

The Plug pipeline

The Plug.Builder module provides conveniences for building plug pipelines.