Server Timing Response Headers for Elixir / Phoenix

Bring Phoenix server-side performance metrics 📈 to Chrome's Developer Tools (and other browsers that support the Server Timing API) via the plug_server_timing package. Production Safe™.

Metrics are collected from the scout_apm package. A Scout account is not required.

Browser Support

• Chrome 65+ (Chrome 64 uses an old format of the server timing headers. This isn't supported by the gem).
• Firefox 59+
• Opera 52+

Installation

To install and use PlugServerTiming, add it as a dependency in your Mixfile:

# mix.exs
  def deps do
    [
      # ...
+     {:plug_server_timing, "~> 0.0.2"}
    ]
  end

Add PlugServerTiming.Plug to your Plug pipeline:

# lib/my_app_web/router.ex
defmodule MyAppWeb.Router do
  pipeline :browser do
    # ...
+   plug PlugServerTiming.Plug
  end
end

✋Whoa! We're not done yet ... we need to instrument our app's function calls.

Instrumentation

Performance metrics are collected via the scout_apm gem. There's just a couple of steps to instrument your app.

Instrument controllers:

# lib/my_app_web.ex
defmodule MyAppWeb do
  # ...
  def controller do
    quote do
      use Phoenix.Controller, namespace: MyAppWeb
+     use ScoutApm.Instrumentation
      # ...
    end
  end
end

Instrument Ecto queries:

# config/dev.exs
+config :my_app, MyApp.Repo,
+ loggers: [{Ecto.LogEntry, :log, []}, {ScoutApm.Instruments.EctoLogger, :log, []}]

Instrument templates:

# config/dev.exs
+config :phoenix, :template_engines,
+ eex: ScoutApm.Instruments.EExEngine,
+ exs: ScoutApm.Instruments.ExsEngine

Additional instrumentation

To instrument HTTPoison, MongoDB Ecto, and more see the Scout docs.

Custom instrumentation

Collect performance data on additional function calls by adding custom instrumentation via scout_apm. See the docs for instructions.

Security

If you'd like to conditionally include Server-Timing headers depending on authorization, the Plug can be included in a Plug.Builder pipeline or you can directly use PlugServerTiming.Plug.register_before_send_headers/1 in an existing Plug.

defmodule MyExistingAuthPlug do
  @behaviour Plug
  import Plug.Conn

  def init(opts), do: opts

  def call(conn, _opts) do
    case AuthModule.verify_auth(conn) do
      {:ok, :admin} ->
        PlugServerTiming.Plug.register_before_send_headers(conn)
      {:ok, :user} ->
        conn
      {:error, _} ->
        conn
        |> put_status(:unauthorized)
        |> halt()
    end
  end
end

Overhead

The scout_apm package, a dependency of plug_server_timing, applies low overhead instrumentation designed for production use.

Thanks!

Special thank you goes to @OleMchls for writing up https://blog.dnsimple.com/2018/02/server-timing-with-phoenix/ and inspiring this package 💖

Documentation on HexDocs.