Plug.Metadata: Set Logger.metadata to provide conn request/response information

[Stephan Epping]

Hi,

I need to log additional request/response information, thus I came up with something like that:

defmodule Plug.Metadata do

  @moduledoc """

  A plug for setting request meta data for logging

  """

  require Logger

  alias Plug.Conn

  @behaviour Plug

  def init(opts) do

    Keyword.get(opts, :metadata, [:request_path])

  end

  def call(conn, metadata) do

    conn |> collect(metadata) |> Logger.metadata

    Conn.register_before_send(conn, fn conn ->

      conn |> collect([:status]) |> Logger.metadata

      conn

    end)

    conn

  end

  defp collect(conn, metadata) do

    for key <- metadata do

      {key, conn |> Map.get(key)}

    end

  end

end

Then I can use the plug as follows:

  plug Plug.RequestId

  plug Plug.Metadata

  plug Plug.Logger

One Issue here is that the Conn.register_before_send will be called after the Plug.Logger is being called, thus the response metadata are not available for the logger. One Solution would be to have a RequestMetadata and ResponseMetadata Plug like

  plug Plug.RequestId

  plug Plug.RequestMetadata

  plug Plug.Logger
  plug Plug.ResponseMetadata

Or would it be better to add a custom plug Plug.CommonLogger (like in rails/sinatra)?  The CommonLogger could in turn use the Plug.Logger? Is this something generally useful for the standard plugs? Then I would be happy to contribute.