Plug

Plug is:

1. A specification for composing web applications with functions
2. Connection adapters for different web servers in the Erlang VM

In other words, Plug allows you to build web applications from small pieces and run them on different web servers. Plug is used by web frameworks such as Phoenix to manage requests, responses, and websockets. This documentation will show some high-level examples and introduce the Plug's main building blocks.

Installation

In order to use Plug, you need a webserver and its bindings for Plug. There are two options at the moment:

1. Use the Cowboy webserver (Erlang-based) by adding the plug_cowboy package to your mix.exs:

   def deps do
     [
       {:plug_cowboy, "~> 2.0"}
     ]
   end
   

2. Use the Bandit webserver (Elixir-based) by adding the bandit package to your mix.exs:

   def deps do
     [
       {:bandit, "~> 1.0"}
     ]
   end
   

Hello world: request/response

This is a minimal hello world example, using the Cowboy webserver:

Mix.install([:plug, :plug_cowboy])

defmodule MyPlug do
  import Plug.Conn

  def init(options) do
    # initialize options
    options
  end

  def call(conn, _opts) do
    conn
    |> put_resp_content_type("text/plain")
    |> send_resp(200, "Hello world")
  end
end

require Logger
webserver = {Plug.Cowboy, plug: MyPlug, scheme: :http, options: [port: 4000]}
{:ok, _} = Supervisor.start_link([webserver], strategy: :one_for_one)
Logger.info("Plug now running on localhost:4000")
Process.sleep(:infinity)

Save that snippet to a file and execute it as elixir hello_world.exs. Access http://localhost:4000/ and you should be greeted!

In the example above, we wrote our first module plug, called MyPlug. Module plugs must define the init/1 function and the call/2 function. call/2 is invoked with the connection and the options returned by init/1.

Hello world: websockets

Plug v1.14 includes a connection upgrade API, which means it provides WebSocket support out of the box. Let's see an example, this time using the Bandit webserver and the websocket_adapter project for the WebSocket bits. Since we need different routes, we will use the built-in Plug.Router for that:

Mix.install([:bandit, :websock_adapter])

defmodule EchoServer do
  def init(options) do
    {:ok, options}
  end

  def handle_in({"ping", [opcode: :text]}, state) do
    {:reply, :ok, {:text, "pong"}, state}
  end

  def terminate(:timeout, state) do
    {:ok, state}
  end
end

defmodule Router do
  use Plug.Router

  plug Plug.Logger
  plug :match
  plug :dispatch

  get "/" do
    send_resp(conn, 200, """
    Use the JavaScript console to interact using websockets

    sock  = new WebSocket("ws://localhost:4000/websocket")
    sock.addEventListener("message", console.log)
    sock.addEventListener("open", () => sock.send("ping"))
    """)
  end

  get "/websocket" do
    conn
    |> WebSockAdapter.upgrade(EchoServer, [], timeout: 60_000)
    |> halt()
  end

  match _ do
    send_resp(conn, 404, "not found")
  end
end

require Logger
webserver = {Bandit, plug: Router, scheme: :http, port: 4000}
{:ok, _} = Supervisor.start_link([webserver], strategy: :one_for_one)
Logger.info("Plug now running on localhost:4000")
Process.sleep(:infinity)

Save that snippet to a file and execute it as elixir websockets.exs. Access http://localhost:4000/ and you should see messages in your browser console.

This time, we used Plug.Router, which allows us to define the routes used by our web application and a series of steps/plugs, such as plug Plug.Logger, to be executed on every request.

Furthermore, as you can see, Plug abstracts the different webservers. When booting up your application, the difference is between choosing Plug.Cowboy or Bandit.

For now, we have directly started the server in a throw-away supervisor but, for production deployments, you want to start them in application supervision tree. See the Supervised handlers section next.

Supervised handlers

On a production system, you likely want to start your Plug pipeline under your application's supervision tree. Start a new Elixir project with the --sup flag:

$ mix new my_app --sup

Add :plug_cowboy (or :bandit) as a dependency to your mix.exs:

def deps do
  [
    {:plug_cowboy, "~> 2.0"}
  ]
end

Now update lib/my_app/application.ex as follows:

defmodule MyApp.Application do
  # See https://hexdocs.pm/elixir/Application.html
  # for more information on OTP Applications
  @moduledoc false

  use Application

  def start(_type, _args) do
    # List all child processes to be supervised
    children = [
      {Plug.Cowboy, scheme: :http, plug: MyPlug, options: [port: 4001]}
    ]

    # See https://hexdocs.pm/elixir/Supervisor.html
    # for other strategies and supported options
    opts = [strategy: :one_for_one, name: MyApp.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

Finally create lib/my_app/my_plug.ex with the MyPlug module.

Now run mix run --no-halt and it will start your application with a web server running at http://localhost:4001.

Plugs and the Plug.Conn struct

In the hello world example, we defined our first plug called MyPlug. There are two types of plugs, module plugs and function plugs.

A module plug implements an init/1 function to initialize the options and a call/2 function which receives the connection and initialized options and returns the connection:

defmodule MyPlug do
  def init([]), do: false
  def call(conn, _opts), do: conn
end

A function plug takes the connection, a set of options as arguments, and returns the connection:

def hello_world_plug(conn, _opts) do
  conn
  |> put_resp_content_type("text/plain")
  |> send_resp(200, "Hello world")
end

A connection is represented by the %Plug.Conn{} struct:

%Plug.Conn{
  host: "www.example.com",
  path_info: ["bar", "baz"],
  ...
}

Data can be read directly from the connection and also pattern matched on. Manipulating the connection often happens with the use of the functions defined in the Plug.Conn module. In our example, both put_resp_content_type/2 and send_resp/3 are defined in Plug.Conn.

Remember that, as everything else in Elixir, a connection is immutable, so every manipulation returns a new copy of the connection:

conn = put_resp_content_type(conn, "text/plain")
conn = send_resp(conn, 200, "ok")
conn

Finally, keep in mind that a connection is a direct interface to the underlying web server. When you call send_resp/3 above, it will immediately send the given status and body back to the client. This makes features like streaming a breeze to work with.

Plug.Router

To write a "router" plug that dispatches based on the path and method of incoming requests, Plug provides Plug.Router:

defmodule MyRouter do
  use Plug.Router

  plug :match
  plug :dispatch

  get "/hello" do
    send_resp(conn, 200, "world")
  end

  forward "/users", to: UsersRouter

  match _ do
    send_resp(conn, 404, "oops")
  end
end

The router is a plug. Not only that: it contains its own plug pipeline too. The example above says that when the router is invoked, it will invoke the :match plug, represented by a local (imported) match/2 function, and then call the :dispatch plug which will execute the matched code.

Plug ships with many plugs that you can add to the router plug pipeline, allowing you to plug something before a route matches or before a route is dispatched to. For example, if you want to add logging to the router, just do:

plug Plug.Logger
plug :match
plug :dispatch

Note Plug.Router compiles all of your routes into a single function and relies on the Erlang VM to optimize the underlying routes into a tree lookup, instead of a linear lookup that would instead match route-per-route. This means route lookups are extremely fast in Plug!

This also means that a catch all match block is recommended to be defined as in the example above, otherwise routing fails with a function clause error (as it would in any regular Elixir function).

Each route needs to return the connection as per the Plug specification. See the Plug.Router docs for more information.

Testing plugs

Plug ships with a Plug.Test module that makes testing your plugs easy. Here is how we can test the router from above (or any other plug):

defmodule MyPlugTest do
  use ExUnit.Case, async: true
  use Plug.Test

  @opts MyRouter.init([])

  test "returns hello world" do
    # Create a test connection
    conn = conn(:get, "/hello")

    # Invoke the plug
    conn = MyRouter.call(conn, @opts)

    # Assert the response and status
    assert conn.state == :sent
    assert conn.status == 200
    assert conn.resp_body == "world"
  end
end

Available plugs

This project aims to ship with different plugs that can be re-used across applications:

• Plug.BasicAuth - provides Basic HTTP authentication;
• Plug.CSRFProtection - adds Cross-Site Request Forgery protection to your application. Typically required if you are using Plug.Session;
• Plug.Head - converts HEAD requests to GET requests;
• Plug.Logger - logs requests;
• Plug.MethodOverride - overrides a request method with one specified in the request parameters;
• Plug.Parsers - responsible for parsing the request body given its content-type;
• Plug.RequestId - sets up a request ID to be used in logs;
• Plug.RewriteOn - rewrite the request's host/port/protocol from x-forwarded-* headers;
• Plug.Session - handles session management and storage;
• Plug.SSL - enforces requests through SSL;
• Plug.Static - serves static files;
• Plug.Telemetry - instruments the plug pipeline with :telemetry events;

You can go into more details about each of them in our docs.

Helper modules

Modules that can be used after you use Plug.Router or Plug.Builder to help development:

• Plug.Debugger - shows a helpful debugging page every time there is a failure in a request;
• Plug.ErrorHandler - allows developers to customize error pages in case of crashes instead of sending a blank one;

Contributing

We welcome everyone to contribute to Plug and help us tackle existing issues!

Use the issue tracker for bug reports or feature requests. Open a pull request when you are ready to contribute. When submitting a pull request you should not update the CHANGELOG.md.

If you are planning to contribute documentation, please check our best practices for writing documentation.

Finally, remember all interactions in our official spaces follow our Code of Conduct.

Supported Versions

License

Plug source code is released under Apache License 2.0. Check LICENSE file for more information.