View Source Plug behaviour (Plug v1.16.0)

The plug specification.

Types of plugs

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

Function plugs

A function plug is by definition any function that receives a connection and a set of options and returns a connection. Function plugs must have the following type signature:

(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.



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

Run a series of plugs at runtime.


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


@callback call(conn :: Plug.Conn.t(), opts()) :: Plug.Conn.t()
@callback init(opts()) :: opts()


Link to this function

forward(conn, new_path, target, opts)

View Source
@spec forward(Plug.Conn.t(), [String.t()], atom(), opts()) :: Plug.Conn.t()

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

The path_info on the forwarded connection will only include the request path trailing segments supplied to the forward function. The conn.script_name attribute retains the correct base path, 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 \\ [])

View Source
@spec run(
  [{module(), opts()} | (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 plug halts, the connection won't invoke the remaining plugs. If the given connection was already halted, none of the plugs are invoked either.

While Plug.Builder is designed to operate at compile-time, the run function serves as a straightforward alternative for runtime executions.

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


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