Documentation

Telemetry is a dynamic dispatching library for metrics and instrumentations. It is lightweight, small and can be used in any Erlang or Elixir project.

In a nutshell, you register a custom module and function to be invoked for certain events, which are executed whenever there is such event. Event name is a list of atoms. Each event is composed of a numeric value and can have metadata attached to it. Let's see an example.

Imagine that you have a web application and you'd like to log latency and response status for each incoming request. With Telemetry, you can build a module which does exactly that whenever a response is sent. The first step is to execute a measurement.

In Elixir:

:telemetry.execute(
  [:web, :request, :done],
  %{latency: latency},
  %{request_path: path, status_code: status}
)

In Erlang:

telemetry:execute(
  [web, request, done],
  #{latency => Latency},
  #{request_path => Path, status_code => Status}
)

Then you can create a module to be invoked whenever the event happens.

In Elixir:

defmodule LogResponseHandler do
  require Logger

  def handle_event([:web, :request, :done], measurements, metadata, _config) do
    Logger.info("[#{metadata.request_path}] #{metadata.status_code} sent in #{measurements.latency}")
  end
end

In Erlang:

-module(log_response_handler).

-include_lib("kernel/include/logger.hrl")

handle_event([web, request, done], #{latency := Latency}, #{request_path := Path,
                                                            status_code := Status}, _Config) ->
  ?LOG_INFO("[~s] ~p sent in ~p", [Path, Status, Latency]).

Important note:

The handle_event callback of each handler is invoked synchronously on each telemetry:execute call. Therefore, it is extremely important to avoid blocking operations. If you need to perform any action that it is not immediate, consider offloading the work to a separate process (or a pool of processes) by sending a message.

Finally, all you need to do is to attach the module to the executed event.

In Elixir:

:ok = :telemetry.attach(
  # unique handler id
  "log-response-handler",
  [:web, :request, :done],
  &LogResponseHandler.handle_event/4,
  nil
)

In Erlang:

ok = telemetry:attach(
  %% unique handler id
  <<"log-response-handler">>,
  [web, request, done],
  fun log_response_handler:handle_event/4,
  []
)

You might think that it isn't very useful, because you could just as well write a log statement instead of Telemetry.execute/3 call - and you would be right! But now imagine that each Elixir library would publish its own set of events with information useful for introspection. Currently each library rolls their own instrumentation layer - Telemetry aims to provide a single interface for these use cases across whole ecosystem.

See the documentation for details.

Telemetry is available on Hex. To install, just add it to your dependencies in mix.exs:

defp deps() do
  [
    {:telemetry, "~> 0.4.1"}
  ]
end

or rebar.config:

{deps, [{telemetry, "~> 0.4.1"}]}.

Telemetry is copyright (c) 2018 Chris McCord and Erlang Solutions.

Telemetry source code is released under Apache License, Version 2.0.

See LICENSE and NOTICE files for more information.