TelemetryChildInit

Add telemetry events to your supervisors to track child startup times.

Installation

The package can be installed by adding telemetry_child_init to your list of dependencies in mix.exs:

def deps do
  [
    {:telemetry_child_init, "~> 0.1.0"}
  ]
end

Usage

Instrument your supervisor:

defmodule DummySupervisor do
  use Supervisor

  def start_link(init_arg) do
    Supervisor.start_link(__MODULE__, init_arg, name: __MODULE__)
  end

  @impl true
  def init(_init_arg) do
    children =
      [{DummyChild, []}]
      |> TelemetryChildInit.instrument(__MODULE__)

    Supervisor.init(children, strategy: :one_for_one)
  end
end

Collect telemetry events

:telemetry_test.attach_event_handlers(self(), [
  [:supervisor, :startup, :start],
  [:supervisor, :startup, :stop],
  [:supervisor, :child, :init, :start],
  [:supervisor, :child, :init, :stop],
  [:supervisor, :child, :init, :exception]
])

Telemetry Events

The following Events are emitted

• [:supervisor, :startup, :start] — before the first child is started
• [:supervisor, :startup, :stop] — after all children have been started
• [:supervisor, :child, :init, :start] — at the point the child's init call is called
• [:supervisor, :child, :init, :stop] — after the child's init call has completed
• [:supervisor, :child, :init, :exception] — when a child's init callback has had an exception

The following chart shows which metadata you can expect for each event:

Metadata

• :supervisor_module — The module name for the supervisor itself
• :telemetry_span_context — A ref that corresponds to one instance of the supervisor, can be used to correlate start and stop events

For :exception events the metadata also includes details about what caused the failure. The :kind value is determined by how an error occurred. Here are the possible kinds:

• :error — from an {:error, error} return value. Some Erlang functions may also throw an :error tuple, which will be reported as :error
• :exit — from a caught process exit
• :throw — from a caught value, this doesn't necessarily mean that an error occurred and the error value is unpredictable