What is telemetry all about by Qing Wu 

What's covered • Why telemetry • Journey to understanding • Further references 

Background ! Ecto 3 moved my cheese! 

In Ecto 2 config :my_app, MyApp.Repo, url: System.get_env("DATABASE_URL"), loggers: [Ecto.LogEntry] 

Custom Logger for Ecto 2 config :my_app, MyApp.Repo, url: System.get_env("DATABASE_URL"), loggers: [Ecto.LogEntry, {SpandexEcto.EctoLogger, :trace, ["my_repo"]}] 

Telemetry Dynamic dispatching library for metrics and instrumentations 

Telemetry telemetry allows you to invoke certain functions whenever a particular event is emitted. 

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Telemetry API attach(HandlerId, EventName, Function, Config) -> ok | {error, already_exists} HandlerId = handler_id() EventName = event_name() Function = handler_function() Config = handler_config() execute(EventName, Measurements, Metadata) -> ok EventName = event_name() Measurements = event_measurements() | event_value() Metadata = event_metadata() 

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Leap year defmodule Leapyear do def leapyear?(year) do (rem(year, 4) == 0 and rem(year, 100) != 0) or rem(year, 400) == 0 end end 

Leap year defmodule Leapyear do def leapyear?(year) when is_integer(year) do (rem(year, 4) == 0 and rem(year, 100) != 0) or rem(year, 400) == 0 end end 

Leap year defmodule Leapyear do require Logger def leapyear?(year) when is_integer(year) do (rem(year, 4) == 0 and rem(year, 100) != 0) or rem(year, 400) == 0 end def leapyear?(year) do Logger.warn("Invalid year: #{inspect(year)}") false end end 

Sharing as hex package def deps do [ {:leapyear, "~> 0.1.0"} ] end 

Sharing as hex package • hex.pm: https:/ /hex.pm/packages/leapyear • https:/ /github.com/wiserfirst/leapyear 

! defmodule Leapyear do require Logger def leapyear?(year) when is_integer(year) do (rem(year, 4) == 0 and rem(year, 100) != 0) or rem(year, 400) == 0 end def leapyear?(year) do Logger.warn("Invalid year: #{inspect(year)}") Appsignal.send_error(%RuntimeError{}, "Invalid year: #{inspect(year)}") false end end 

defmodule Leapyear do require Logger def leapyear?(year, _) when is_integer(year) do (rem(year, 4) == 0 and rem(year, 100) != 0) or rem(year, 400) == 0 end def leapyear?(year, handler) do Logger.warn("Invalid year: #{inspect(year)}") handler.("Invalid year: #{inspect(year)}") false end end 

handler = fn msg -> Appsignal.send_error(%RuntimeError{}, msg) end # client code Leapyear.leapyear?(2009, handler) Leapyear.leapyear?("whatever", handler) 

Ad hoc function passing 

Ad hoc function passing • Damage to library code ! 

Ad hoc function passing • Damage to library code ! • Damage to client code " 

Ad hoc function passing • Damage to library code ! • Damage to client code " • Consistency # 

What do we want here? • data from the library • function from end user • clean API 

With telemetry defmodule Leapyear do def leapyear?(year) when is_integer(year) do (rem(year, 4) == 0 and rem(year, 100) != 0) or rem(year, 400) == 0 end def leapyear?(year) do metrics = %{message: "Invalid year: #{inspect(year)}} :telemetry.execute([:leapyear, :invalid_input], metrics) false end end 

# in application.ex :telemetry.attach( "leapyear_events", [:leapyear, :invalid_input], &MyApp.TelemetryAdapter.handler/4, nil ) Leapyear.leapyear?(2009) Leapyear.leapyear?("whatever") 

defmodule MyApp.TelemetryAdapter do def handler([:leapyear, :invalid_input], %{message: message}, _meta, _config) Logger.warn(message) Appsignal.send_error(%RuntimeError{}, message) end end 

Example for Ecto and Datadog # in application.ex :telemetry.attach( "spandex-ecto", [:my_app, :repo, :query], &SpandexEcto.TelemetryAdapter.handle_event/4, nil ) 

Example for Ecto and Datadog defmodule SpandexEcto.TelemetryAdapter do alias SpandexEcto.EctoLogger def handle_event([_, repo_name, :query], measurements, metadata, _config) do log_entry = %{ query: metadata.query, source: metadata.source, params: metadata.params, query_time: Map.get(measurements, :query_time, 0), decode_time: Map.get(measurements, :decode_time, 0), queue_time: Map.get(measurements, :queue_time, 0), result: wrap_result(metadata.result) } EctoLogger.trace(log_entry, "#{repo_name}_database") end end 

Summary • Recommended for library author • Overkill for application code? 

References Official Documentation Telemetry and metrics for all by Arkadiusz Gil The "How"s, "What"s, and "Why"s of Elixir Telemetry 

Thank you! @wiserfirst