Plug behaviour (Plug v1.11.1) 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 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


Forwards requests to another Plug setting the connection to a trailing subpath of the request.

Run a series of Plugs at runtime.

Link to this section Types


opts() ::
  | tuple()
  | atom()
  | integer()
  | float()
  | [opts()]
  | %{optional(opts()) => opts()}
  | MapSet.t()

Link to this section Functions

Link to this function

forward(conn, new_path, target, opts)

View Source


forward(Plug.Conn.t(), [String.t()], atom(), opts()) :: Plug.Conn.t()

Forwards requests to another Plug setting the connection to a trailing subpath of the request.

The path_info on the forwarded connection will only include the trailing segments of the request path supplied to forward, while conn.script_name will retain the correct base path for e.g. url generation.


defmodule Router do
  def init(opts), do: opts

  def call(conn, opts) do
    case conn do
      # Match subdomain
      %{host: "admin." <> _} ->, opts)

      # Match path on localhost
      %{host: "localhost", path_info: ["admin" | rest]} ->
        Plug.forward(conn, rest, AdminRouter, opts)

      _ ->, opts)
Link to this function

run(conn, plugs, opts \\ [])

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  [{module(), opts()} | (Plug.Conn.t() -> Plug.Conn.t())],
) :: Plug.Conn.t()

Run a series of Plugs at runtime.

The plugs given here can be either a tuple, representing a module plug and their options, or a simple function that receives a connection and returns a connection.

If any of the plugs halt, the remaining plugs are not invoked. If the given connection was already halted, none of the plugs are invoked either.

While Plug.Builder works at compile-time, this is a straight-forward alternative that works at runtime.

Examples, [{Plug.Head, []}, &IO.inspect/1])


  • :log_on_halt - a log level to be used if a Plug halts

Link to this section Callbacks


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


init(opts()) :: opts()