# Plug
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[](http://inch-ci.org/github/elixir-plug/plug)
Plug is:
1. A specification for composing web applications with functions
2. Connection adapters for different web servers in the Erlang VM
[Documentation for Plug is available online](http://hexdocs.pm/plug/).
## 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`:
```elixir
def deps do
[
{:plug_cowboy, "~> 2.0"}
]
end
```
2. Use the Bandit webserver (Elixir-based) by adding the `bandit` package to your `mix.exs`:
```elixir
def deps do
[
{:bandit, "~> 0.6"}
]
end
```
## Hello world: request/response
This is a minimal hello world example, using the Cowboy webserver:
```elixir
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: Router, scheme: :http, port: 4000}
{:ok, _} = Supervisor.start_link([webserver], strategy: :one_for_one)
Logger.info("Plug now running on localhost:4000")
```
Save that snippet to a file and execute it as `elixir --no-halt hello_world.exs`.
Access <http://localhost:4000/> and you should be greeted!
## 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:
```elixir
Mix.install([:plug, :bandit, :websock_adapter])
defmodule EchoServer do
def init(args) do
{:ok, []}
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")
```
Save that snippet to a file and execute it as `elixir --no-halt websockets.exs`.
Access <http://localhost:4000/> and you should see messages in your browser
console.
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](#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:
```shell
$ mix new my_app --sup
```
Add both `:plug` and `:plug_cowboy` as dependencies in your `mix.exs`:
```elixir
def deps do
[
{:plug, "~> 1.14"},
{:plug_cowboy, "~> 2.0"}
]
end
```
Now update `lib/my_app/application.ex` as follows:
```elixir
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>.
## Supported Versions
| Branch | Support |
|--------|--------------------------|
| v1.14 | Bug fixes |
| v1.13 | Security patches only |
| v1.12 | Security patches only |
| v1.11 | Security patches only |
| v1.10 | Security patches only |
| v1.9 | Security patches only |
| v1.8 | Unsupported from 01/2023 |
| v1.7 | Unsupported from 01/2022 |
| v1.6 | Unsupported from 01/2022 |
| v1.5 | Unsupported from 03/2021 |
| v1.4 | Unsupported from 12/2018 |
| v1.3 | Unsupported from 12/2018 |
| v1.2 | Unsupported from 06/2018 |
| v1.1 | Unsupported from 01/2018 |
| v1.0 | Unsupported from 05/2017 |
## The `Plug.Conn` struct
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:
```elixir
def hello_world_plug(conn, _opts) do
conn
|> 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:
```elixir
defmodule MyPlug do
def init([]), do: false
def call(conn, _opts), do: conn
end
```
As per the specification above, a connection is represented by the `Plug.Conn` struct:
```elixir
%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:
```elixir
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`:
```elixir
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:
```elixir
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):
```elixir
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](http://hexdocs.pm/plug/).
## 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][issues] for bug reports or feature requests. Open a [pull request][pulls] 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][writing-docs].
Finally, remember all interactions in our official spaces follow our [Code of Conduct][code-of-conduct].
## License
Plug source code is released under Apache License 2.0.
Check LICENSE file for more information.
[issues]: https://github.com/elixir-plug/plug/issues
[pulls]: https://github.com/elixir-plug/plug/pulls
[code-of-conduct]: https://github.com/elixir-lang/elixir/blob/master/CODE_OF_CONDUCT.md
[writing-docs]: https://hexdocs.pm/elixir/writing-documentation.html
