How to make a custom SamplingDecision for a started span?

[un000]

Hi!

I have a service that starts tracing with a given ratio:

	jaeger.WithSDK(&trace.Config{DefaultSampler: trace.TraceIDRatioBased(0.001)}),

So I want to override this Sampler with a trace.AwlaysSampler like I did it with opencensus.

		opts := []trace.SpanOption{trace.WithNewRoot(), trace.WithSpanKind(trace.SpanKindServer)}
		forceTrace := len(reqCtx.Request.Header.Peek("X-Force-Trace")) > 0
		if forceTrace {
			opts = append(opts, trace.WithRecord())
		}

		tr := otel.Tracer(tracer.DefaultInstrumentationName)
		ctxWithSpan, span := tr.Start(ctx, name, opts...)

trace.WithRecord() enables recording but doesn't affect the sampling decision.

How to achieve 100% sampling by a flag?

[MrAlias]

Currently the OpenTelemetry specification does not allow this type of sampling override option, it would provide the instrumentation author a mechanism that could drastically conflict with what an operator would want regarding sampling.

The Sampler interface can be implemented if you want to create custom sampling algorithms. This would allow you to implement the described algorithm if you wanted to write your own sampler.

[atombender]

Hijacking this old thread: Is this the officially recommended solution? I noticed that samplers get the attributes as an array of KeyValue structs. This means a linear search on every single span rather than a direct boolean flag lookup. Is there a more optimal approach?

I have also noticed that sampling appears to only affect spans and their children, not parents. So if I have a chain span1 -> span2 -> span3 and I flag span2 as "sampled", this only causes span2 and span3 to be recorded. (I'm just assuming span3 is included here, with the ParentBased sampler; I definitely get span2, but I don't have a scenario that creates child spans.) But this means that the root is lost. As far as I know, with OpenTracing you get the whole chain. Maybe I'm doing something wrong?