Expected behaviour when X-B3-Sampled is not set

[paulgrav]

As the title suggests should I expect traces to be sampled when only X-B3-TraceId and X-B3-SpanId are set?

We have some runtime environment constraints (long story, don’t ask) and we are still running SDK/agent 0.8.0. When X-B3-Sampled is set traces are exported, when that header is missing traces are not exported. Is there a way to force them to be exported besides setting X-B3-Sampled?

[mateuszrzeszutek]

Here's what the b3 spec says about the X-B3-Sampled header:

An accept sampling decision is encoded as X-B3-Sampled: 1 and a deny as X-B3-Sampled: 0. Absent means defer the decision to the receiver of this header.

It seems OTel SDK has decided to drop those spans that don't have that header.

Is there a way to force them to be exported besides setting X-B3-Sampled?

You can set the otel.traces.sampler property to always_on - you'll get all the spans though.

[jkwatson]

This might be a bug in the B3 Propagator. Here is the code : https://github.com/open-telemetry/opentelemetry-java/blob/main/extensions/trace-propagators/src/main/java/io/opentelemetry/extension/trace/propagation/Common.java#L36-L39

This code is being used by both the B3 Propagator and the W3C TraceContext propagator. The flag isn't nullable coming from W3C TraceContext, but it is from B3, so we probably need to explicitly handle the null sampled value coming from B3. Should null -> sampled, then, to meet the requirements from B3?

[paulgrav]

Right. I don’t think otel.traces.sampler was available in the 0.8.0 version.

[mateuszrzeszutek]

Right. I don’t think otel.traces.sampler was available in the 0.8.0 version.

Oh, right, that's a pretty old version. Hmm, if you were using just SDK you would be able to set the always-on sampler yourself, but with the agent it might be near impossible to do so.

[jkwatson]

open-telemetry/opentelemetry-java#3517 . I do think this is a deeper issue, though. Our propagation APIs don't have a way to handle this "you decide on the sampling" case from an upstream trace.