Routes, Plugs, and Pipelines

Notes & Exercises

Code Snapshot

A snapshot of the code at the end of the video is in the raffley/03-plugs-pipelines directory of the code bundle.

Recap: Request Processing

Module Plugs

As we said, plugs come in two flavors: function plugs and module plugs. 🍦

To recap, a function plug accepts a %Plug.Conn{} struct and a list of options, transforms the conn struct or checks for something in it, and returns the updated conn struct:

def my_plug(conn, opts) do
  # transforms `conn` or checks for something in it

  conn
end

By comparison, a module plug is any module that defines two functions: init/1 and call/2, like this:

defmodule MyPlug do
  def init(opts) do
    opts
  end

  def call(conn, opts) do
    # transforms `conn` or checks for something in it

    conn
  end
end

The init/1 function accepts a list of options which it can transform, or simply pass through unchanged. The value returned by init/1 is then passed to call/2 along with the %Plug.Conn{} struct. It's in call/2 that you transform the %Plug.Conn{} struct or check for something in it. In this way, call/2 is just a function plug.

Since init/1 is invoked at compile time and call/2 is invoked at runtime, you can perform relatively expensive operations in init/1 to initialize the options without a penalty at runtime when call/2 is invoked.

Exercise

Snoop on each request as it flows through the HTTP request pipeline of your heads_up app. 👀

1. Write a plug function named snoop that assigns a key/value pair to the %Plug.Conn{} struct, inspects the transformed struct, and returns it. For the key/value pair, assign the key answer a value of "Yes", "No", or "Maybe" which you can randomly pick using:

   answer = ~w(Yes No Maybe) |> Enum.random()
   

2. Add snoop as the final plug in the browser pipeline defined in the router.

3. Reload the welcome page and verify that the %Plug.Conn{} struct was written to the server log when the request was processed. The struct's assigns field should include a random answer value that we'll use a bit later on.

Solution

The solution for the exercise is in the heads_up/03-plugs-pipelines directory of the code bundle.

📚 Dive Deeper

• See the offical docs for more details about the %Plug.Conn{} struct.

• See more details on scope/2, pipeline/2, and pipe_through/1.