Scout Absinthe (GraphQL) Instrumentation

scout_apm_absinthe_plug.ex

defmodule ScoutApm.Absinthe.Plug do
  alias ScoutApm.Internal.Layer

  require Logger

  @error_prefix "GraphQL query document could not be parsed"
  @endpoint_prefix "GraphQL"
  @default_action_name "unknown"

  def init(default), do: default

  def call(conn, _default) do
    ScoutApm.TrackedRequest.start_layer("Controller", endpoint_name(conn))

    conn
    |> Plug.Conn.register_before_send(&before_send/1)
  end

  defp before_send(conn) do
    endpoint_name = endpoint_name(conn)
    uri = "#{conn.request_path}/#{action_name(conn)}"

    ScoutApm.TrackedRequest.stop_layer(fn layer ->
      layer
      |> Layer.update_name(endpoint_name)
      |> Layer.update_uri(uri)
    end)

    conn
  end

  defp endpoint_name(conn), do: "#{@endpoint_prefix}##{action_name(conn)}"

  defp action_name(%{params: %{"operationName" => operation_name}} = _conn) when is_binary(operation_name) do
    operation_name
  end
  defp action_name(%{params: %{"query" => query_bin}} = _conn) when is_binary(query_bin) do
    query_chars = :erlang.binary_to_list(query_bin)
      
    with \
      {:ok, tokens, _line_count} <- :absinthe_lexer.string(query_chars),
      {:ok, query_doc} <- :absinthe_parser.parse(tokens)
    do
      get_first_field_from_query_doc(query_doc)
    else
      {:error, raw_error} ->
        log_raw_parse_error(raw_error)
        @default_action_name
    end
  end
  defp action_name(_conn), do: @default_action_name

  defp get_first_field_from_query_doc(query_doc) do
    query_doc
    |> Map.get(:definitions, [%{}])
    |> List.first()
    |> Map.get(:selection_set, %{})
    |> Map.get(:selections, [%{}])
    |> List.first()
    |> Map.get(:name, @default_action_name)
  end

  defp log_raw_parse_error({_line, :absinthe_parser, msgs}) do
    message = msgs |> Enum.map(&to_string/1) |> Enum.join("")

    Logger.error("#{@error_prefix}: #{message}")
  end
  defp log_raw_parse_error({_line, :absinthe_lexer, {problem, field}}) do
    message = "#{problem}: #{field}"

    Logger.error("#{@error_prefix}: #{message}")
  end
  defp log_raw_parse_error(_unknown), do: Logger.error("#{@error_prefix}: unknown error")
end

For even more detail, I customized the context by adding the GraphQL query that was run for the endpoint/action by adding ScoutApm.Context.add("GraphQL Query", String.replace(query_bin, ~r/ +|\n/, " ")) to line 38 in the action_name function.

For anyone who is using Absinthe 1.5.x (https://github.com/absinthe-graphql/absinthe/blob/37d72813e1d89b05413e6f58f4563f89a84c94f8/CHANGELOG.md#v150-alpha), :absinthe_lexer is no longer used. You'll have to make a slight modification to #action_name/2. Check out my fork, which has been updated to work with Absinthe 1.5.x: https://gist.github.com/ryancurtin/60c6e147a346f9155befa82ff1a1b199