Plug

Plug is:

1. A specification for composable modules between web applications
2. Connection adapters for different web servers in the Erlang VM

Documentation for Plug is available online.

Hello world

defmoduleMyPlugdoimportPlug.Conndefinit(options)do# initialize optionsoptionsenddefcall(conn,_opts)doconn|>put_resp_content_type("text/plain")|>send_resp(200,"Hello world")endend

The snippet above shows a very simple example on how to use Plug. Save that snippet to a file and run it inside the plug application with:

$ iex -S mix
iex> c "path/to/file.ex"
[MyPlug]
iex> {:ok, _} = Plug.Adapters.Cowboy.http MyPlug, []
{:ok, #PID<...>}

Access "http://localhost:4000/" and we are done! For now, we have directly started the server in our terminal but, for production deployments, you likely want to start it in your supervision tree. See the "Supervised handlers" section below.

Installation

You can use plug in your projects in two steps:

1. Add plug and your webserver of choice (currently cowboy) to your mix.exs dependencies:

   defdepsdo[{:cowboy,"~> 1.0.0"},{:plug,"~> 1.0"}]end

2. List both :cowboy and :plug as your application dependencies:

   defapplicationdo[applications: [:cowboy,:plug]]end

Supported Versions

Branch	Support
v1.4	In development (master)
v1.3	Bug fixes
v1.2	Security patches only
v1.1	Security patches only
v1.0	Unsupported from 05/2017

The Plug.Conn

In the hello world example, we defined our first plug. What is a plug after all?

A plug takes two shapes. A function plug receives a connection and a set of options as arguments and returns the connection:

defhello_world_plug(conn,_opts)doconn|>put_resp_content_type("text/plain")|>send_resp(200,"Hello world")end

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:

defmoduleMyPlugdodefinit([]),do: falsedefcall(conn,_opts),do: connend

As per the specification above, 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.

The Plug Router

In practice, developers rarely write their own plugs. For example, Plug ships with a router that allows developers to quickly match on incoming requests and perform some action:

defmoduleMyRouterdousePlug.Routerplug:matchplug:dispatchget"/hello"dosend_resp(conn,200,"world")endforward"/users",to: UsersRoutermatch_dosend_resp(conn,404,"oops")endend

The router is a plug and, 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 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:

plugPlug.Loggerplug:matchplug: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 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 Plug.Router docs for more information.

Supervised handlers

On a production system, you likely want to start your Plug application under your application's supervision tree. Plug provides the child_spec/3 function to do just that. Start a new Elixir project with the --sup flag:

$ mixnewmy_app--sup

and then update lib/my_app.ex as follows:

defmoduleMyAppdouseApplication# See https://hexdocs.pm/elixir/Application.html# for more information on OTP Applicationsdefstart(_type,_args)doimportSupervisor.Specchildren=[# Define workers and child supervisors to be supervisedPlug.Adapters.Cowboy.child_spec(:http,MyRouter,[],[port: 4001])]# See https://hexdocs.pm/elixir/Supervisor.html# for other strategies and supported optionsopts=[strategy: :one_for_one,name: MyApp.Supervisor]Supervisor.start_link(children,opts)endend

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):

defmoduleMyPlugTestdouseExUnit.Case,async: trueusePlug.Test@optsMyRouter.init([])test"returns hello world"do# Create a test connectionconn=conn(:get,"/hello")# Invoke the plugconn=MyRouter.call(conn,@opts)# Assert the response and statusassertconn.state==:sentassertconn.status==200assertconn.resp_body=="world"endend

Available Plugs

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

• 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 headers;
• 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.Session - handles session management and storage;
• Plug.SSL - enforce requests through SSL;
• Plug.Static - serves static files;

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. You may also start a discussion on the mailing list or the #elixir-lang channel on Freenode IRC. 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.

License

Plug source code is released under Apache 2 License. Check LICENSE file for more information.