Plug v1.6.0 Plug behaviour View Source

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).


Here’s an example of a function plug:

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

Here’s an example of a module plug:

defmodule JSONHeaderPlug do
  import Plug.Conn

  def init(opts) do

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

The Plug pipeline

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

Link to this section Summary

Link to this section Types

Link to this type opts() View Source
opts ::
  binary |
  tuple |
  atom |
  integer |
  float |
  [opts] |
  %{optional(opts) => opts}

Link to this section Callbacks