Plug.Router

We’ll most often use the Plug.Router to match and dispatch requests.

Plug.Router is a plug that contains its own plug pipeline.

When the router is invoked, it will invoke the :match plug as we see above, which is represented by the match/2 function we see in our example.

defmodule MyApp.Router do
  use Plug.Router
  plug :match
  plug :dispatch

  def child_spec(_opts) do
    %{
      id: __MODULE__,
      start: {__MODULE__, :start_link, []},
      type: :worker,
      restart: :permanent,
      shutdown: 500
    }
  end

  def start_link() do
    {:ok, _} = Plug.Adapters.Cowboy2.http(MyApp.Router, [], port: 4000)
  end

  match _ do
    conn
    |> send_resp(404, "Not found")
  end
end

Thw match/2 function will check against any of our HTTP requests to see which one matches.

Then it calls the :dispatch plug which will execute the matched code.

Our start_link function will be called by the Supervisor.

Supporting APIs

Plug ships with several parsers that our application to parse different content-types.

plug Plug.Parsers,
  parsers: [:urlencoded, :multipart, :json],
  pass: ["*/*"],
  json_decoder: Poison

We will use Poison, an JSON library for Elixir, to decode JSON.

Let’s add parsers to our router - open up the router file and copy the following before the :dispatch function

plug Plug.Parsers,
  parsers: [:urlencoded, :multipart, :json],
  pass: ["*/*"],
  json_decoder: Poison

Our router should now look like this -

defmodule MyApp.Router do
  use Plug.Router
  plug :match

  plug Plug.Parsers,
  parsers: [:json],
  pass: ["*/*"],
  json_decoder: Poison

  plug :dispatch

  def child_spec(_opts) do
    %{
      id: __MODULE__,
      start: {__MODULE__, :start_link, []},
      type: :worker,
      restart: :permanent,
      shutdown: 500
    }
  end

  def start_link() do
    {:ok, _} = Plug.Adapters.Cowboy2.http(MyApp.Router, [], port: 4000)
  end

  get "/" do
    conn
    |> send_resp(200, "yay")
    |> halt
  end

  match _ do
    conn
    |> send_resp(404, "Not found")
  end
end

Sending Responses

We will want to be able to send responses to our client.

conn
  |> send_resp(200, "Ok")
  |> halt

The send_resp function, defined on our Plug.Conn, allows us to send response back to our client.

The halt function halts the Plug pipeline by preventing further plugs downstream from being invoked.

Forwarding

forward "/hello", to: MyApp.Plugs.HelloWorld

forward is invoked by Plug.Router to forward requests to another plug.