Plug
Plug is:
- A specification for composing web applications with functions
- Connection adapters for different web servers in the Erlang VM
Documentation for Plug is available online.
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!
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:
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:
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:
%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 usingPlug.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 fromx-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;