Today I Learned

A Hashrocket project

Telemetry Attach and Execute

The telemetry api is simpler than the name tends to imply. There are only two primary functions, attach/4 and execute/4. Check out the telemetry docs to see the full api.

Attach is simple. Essentially, you want a function to be called when a certain event occurs:

handler = fn [:a_certain_event], _measuremnts, _metadata, _config ->
  IO.puts("An event occurred!!")
end

:telemetry.attach(
  :unique_handler_id,
  [:a_certain_event],
  handler,
  :no_config
)

The first argument is handler_id and can be anything be must be unique. The last argument is config and can also be anything. It will be passed along, untouched, to the handler function as the last argument.

Execute is simple. When the program calls execute, the handler that matches the event is called and the measurements and metadata are passed to the handler.

measurements = %{}
metadata = %{}
:telemetry.execute([:a_certain_event], measurements, metadata)

It’s important to note that the event name must be a list of atoms.

A simple test for attach and execute would look like this:

test "attach and execute" do
  handler = fn _, _, _, _ ->
    send(self(), :test_message)
  end

  :telemetry.attach(
    :handler_id,
    [:something_happened],
    handler,
    :no_config
  )

  :telemetry.execute([:something_happened], %{}, %{})

  assert_receive :test_message
end
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