OpentelemetryProcessPropagator provides helpers for dealing with context propagation across process boundaries.

Context Propagation

Erlang and Elixir do not have a mechanism for transparently passing context between processes. This requires the user to explicitly pass data between processes. In order to continue a trace across processes, the user must start a new span and pass it to the spawned process.

span_ctx = OpenTelemetry.Tracer.start_span("child")
ctx = OpenTelemetry.Ctx.get_current()

task =
  Task.async(fn ->
    OpenTelemetry.Ctx.attach(ctx)
    OpenTelemetry.Tracer.set_current_span(span_ctx)
    # do work here

    OpenTelemetry.Tracer.end_span(span_ctx)
  end)

_result = Task.await(task)

Reverse Propagation

It's not always possible to have full control over traces, such as when using telemetry events emitted from a library you don't control to create a span. In such cases, a mechanism to fetch a context from a calling process is necessary. This is effectively context propagation in reverse.

As an example, Ecto uses the Task module to execute preloads which are each a separate query. Since a task is a spawned process, creating an otel span results in orphan spans. To correctly connect these spans we must find the otel context which spawned the process.

Usage

Example of using fetch_parent_ctx/1 to find a parent context.

OpenTelemetry.with_span :span_started_in_your_app do
  # some span being created in a process spawned by a library
  # you don't control, e.g. Ecto preloads

  Task.async(fn ->
    parent_ctx = OpentelemetryProcessPropagator.fetch_parent_ctx(1, :"$callers")

    OpenTelemetry.Ctx.attach(parent_ctx)

    attrs = %{some_attr: :from_telemetry_event}

    span =
      OpenTelemetry.Tracer.start_span(:span_created_in_lib, %{attributes: attrs})

    OpenTelemetry.Span.end_span(span)
  end)
end

?with_span(span_started_in_your_app, , fun() ->
  %% some span being created in a process spawned by a library
  %% you don't control

  proc_lib:spawn_link(fun() ->
    Tracer = opentelemetry:get_tracer(test_tracer),
    ParentCtx = opentelemetry_process_propagator:fetch_parent_ctx(),
    otel_ctx:attach(ParentCtx),
    Span = otel_tracer:start_span(Tracer, span_created_in_lib, ),
    otel_tracer:end_span(Span).
  ).
end