Settings View Source Telemetry

Dataloader uses telemetry to instrument its activity.

Call :telemetry.attach/4 or :telemetry.attach_many/4 to attach your handler function to any of the following event names:

• [:dataloader, :source, :run, :start] when the dataloader processing starts
• [:dataloader, :source, :run, :stop] when the dataloader processing finishes
• [:dataloader, :source, :batch, :run, :start] when the dataloader starts processing a single batch
• [:dataloader, :source, :batch, :run, :stop] when the dataloader finishes processing a single batch

Telemetry handlers are called with measurements and metadata. For details on what is passed, checkout Dataloader.

interactive-telemetry

Interactive Telemetry

As an example, you could attach a handler in an iex -S mix shell. Paste in:

:telemetry.attach_many(
  :demo,
  [
    [:dataloader, :source, :run, :stop]
  ],
  fn event_name, measurements, metadata, _config ->
    %{
      event_name: event_name,
      measurements: measurements,
      metadata: metadata
    }
    |> IO.inspect()
  end,
  []
)

After a query is executed, you'll see something like:

%{
  event_name: [:dataloader, :source, :run, :stop],
  measurements: %{duration: 112151},
  metadata: %{
    dataloader: %Dataloader{
      options: [get_policy: :raise_on_error],
      sources: ...
    },
    id: -576460752303420441
  }
}

opentelemetry

Opentelemetry

When using Opentelemetry, one usually wants to correlate spans that are created in spawned tasks with the main trace. For example, you might have a trace started in a Phoenix endpoint, and then have spans around database access.

One can correlate manually by attaching the OTel context the task function:

ctx = OpenTelemetry.Ctx.get_current()

Task.async(fn ->
  OpenTelemetry.Ctx.attach(ctx)

  # do stuff that might create spans
end)

When using Dataloader, the tasks are spawned by the loader itself, so you can't attach the context manually.

Instead, you can add the :opentelemetry_process_propagator package to your dependencies, which has suitable wrappers that will attach the context automatically. If the package is installed, Dataloader will use it in place of the default Task.async/1 and Task.async_stream/3.