Elixir Absinthe Instrumentation

Question

I'm looking to instrument my absinthe/phoenix server. I would like to find out how long it takes to encode data as json using Jason.

My endpoint.ex file looks like following:

defmodule AssessmentApi.Web.Endpoint do
  use Phoenix.Endpoint, otp_app: :assessment_api

  socket "/socket", AssessmentApi.Web.UserSocket

  # Serve at "/" the static files from "priv/static" directory.
  #
  # You should set gzip to true if you are running phoenix.digest
  # when deploying your static files in production.
  plug Plug.Static,
    at: "/", from: :assessment_api, gzip: false,
    only: ~w(css fonts images js favicon.ico robots.txt)

  if code_reloading? do
    socket "/phoenix/live_reload/socket", Phoenix.LiveReloader.Socket
    plug Phoenix.LiveReloader
    plug Phoenix.CodeReloader
  end

  plug Plug.RequestId
  plug Plug.Logger

  plug Plug.Parsers,
    parsers: [:urlencoded, :multipart, :json, Absinthe.Plug.Parser],
    pass: ["*/*"],
    json_decoder: Jason

  plug Plug.MethodOverride
  plug Plug.Head

  # The session will be stored in the cookie and signed,
  # this means its contents can be read but not tampered with.
  # Set :encryption_salt if you would also like to encrypt it.
  plug Plug.Session,
    store: :cookie,
    key: "adfadfasdfasdfadsf",
    signing_salt: "asdsfasdfasdfasdf"

  plug CORSPlug
  plug AssessmentApi.Web.Logger
  plug AssessmentApi.Web.Router
end

And my router.ex looks like the following:

defmodule AssessmentApi.Web.Router do
  use AssessmentApi.Web, :router

  pipeline :api do
    plug AssessmentApi.Guardian.AuthPipeline

    plug :accepts, ["json"]
  end

  scope "/" do
    pipe_through :api

    forward "/api", Absinthe.Plug,
      schema: AssessmentApi.Web.Schema,
      json_codec: Jason

    forward "/graphiql", Absinthe.Plug.GraphiQL,
      schema: AssessmentApi.Web.Schema,
      json_codec: Jason
  end
end

How do I instrument the json_decoder portion of the application to find out how long it takes to make json out of the response data? Maybe using telemetry?

Thanks

Yes, nowadays the community mostly uses telemetry for that. Also, this question is clearly the off-topic, you’d better ask this here: elixirforum.com — Aleksei Matiushkin

Maarten van Vliet Maarten van Vliet · Accepted Answer · 2021-05-03T14:59:23

If it is just the encoding of JSON you could add a custom module for it.

defmodule InstrumentedJason do
  def encode!(body, opts) do
    :telemetry.span(
      [:your_app, :encode_absinthe_result],
      %{},
      fn ->
        result = Jason.encode!(body, opts)

        {result, %{}}
      end
    )
    
  end
end

# router.ex
  forward "/api", Absinthe.Plug,
      schema: AssessmentApi.Web.Schema,
      json_codec: InstrumentedJason

Elixir Absinthe Instrumentation

1 Answers