How To Set Up An API Using Plug

This is a quick and easy tutorial to get you started with setting up a basic API in Elixir using Plug.

Start a new Elixir project:

$ mix new api --sup

Add the plug_cowboy dependency to your mix.exs:

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

And install it using the command: mix deps.get. Update lib/my_app/application.ex as follows:

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

  use Application
  require Logger

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

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

To write a “router” plug that dispatches based on the path and method of incoming requests, Plug provides Plug.Router (lib/api_router.ex):

defmodule ApiRouter do
  use Plug.Router

  plug Plug.Logger
  plug :match
  plug :dispatch

  get "/" do
    send_resp(conn, 200, "Hello, API!")
  end

  get "/hello/:name" do
    send_resp(conn, 200, "Hello, #{name}")
  end

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

And then start the application with the command:

$ iex --erl "+S 1" -S mix

The option --erl "+S 1" specifies the number of scheduler threads available to the application.