How to link to an incoming trace context

[RoadRunnr]

When using the otel_propagator extract functions, the current trace is switched to the span context of the incoming request. Any further spans then become children of that span.

What I want is to extract the span context of the incoming request and then only link to that context.

I can work around the limit of otel_propagator_trace_context:extract by playing tricks with the span context, but I was hoping for a solution that is cleaner than this:

extract_trace_id(IEs) ->
    Ctx = otel_ctx:get_current(),
    CurrentSpan = otel_tracer:current_span_ctx(Ctx),
    Ctx1 = otel_propagator_trace_context:extract(Ctx, IEs, undefined, fun trace_get/2, #{}),
    TraceSpan = otel_tracer:current_span_ctx(Ctx1),
    otel_traceqr:set_current_span(Ctx, CurrentSpan),
    TraceSpan.

The root oy my problem is actually this line: https://github.com/open-telemetry/opentelemetry-erlang/blob/main/apps/opentelemetry_api/src/otel_propagator_trace_context.erl#L97

Why does otel_propagator_trace_context:extract make the extracted span context the current span context?

[tsloughter]

You likely don't want to use otel_propagator_trace_context directly here. A user is meant to use otel_propagator_text_map which has extract_to: https://github.com/open-telemetry/opentelemetry-erlang/blob/main/apps/opentelemetry_api/src/otel_propagator_text_map.erl#L164

But it acts essentially the same in the end.

I think your issue is some confusion around how context works and the process dictionary. The code you show can be done like:

extract_trace_id(IEs) ->
    Ctx = otel_propagator_trace_context:extract(otel_ctx:new(), IEs, undefined, fun trace_get/2, #{}),
   otel_tracer:current_span_ctx(Ctx).

If you just want the span ctx from the headers then you can create a new ctx otel_ctx:new() and then get the span ctx from it.

Also, note in your code that otel_tracer:set_current_span(Ctx, CurrentSpan), doesn't actually do anything. CurrentSpan is still the current span in Ctx because variables are immutable.