`SpanBuilder` and `Baggage`

[cijothomas]

Discussed in #2631

^{Originally posted by mladedav February 7, 2025}
I have a SpanBuilder, a parent context with baggage and then I tried calling

let span = builder.start_with_context(&tracer, &parent_cx);

However, the new span did not have the same baggage as the parent context. From what I could see, the context is just parsed for parent ID, trace flags, sampling and such.

Is that by design? I though this was a way to create a child span which as I understand it should have almost the same context (except for the active span).

[mladedav]

I asked it as a discussion instead of creating an issue because I don't know how baggage really works so I wanted to make sure this isn't how it's supposed to work.

So can you please clarify whether opentelemetry says whether the propagation should or should not work like this? From the spec I'm not entirely sure that baggage isn't a different thing alongside context which we would need to explicitly link when a new context is made.

[cijothomas]

@mladedav I added #2732 to facilitate discussion on this. Baggage (or anything in the Context) is indeed passed to OnStart callback of the SpanProcessor. Span itself only uses the SpanContext portion from the Context.

Please check if this clarifies the question you have.

[bantonsson]

I'll chime in here with my understanding of the things.

At the bottom you have Context that can contain different things including Baggage and Span (which are separate). A Context is what gets propagated locally.

When you create and start a Span based on a Context, the Tracer will extract all the things specific to a Span from the Context and use that as the starting point for the parent of the new Span. A Span does not contain Baggage, so if you want to propagate the Baggage with the new Span, you need to attach that Span to the Context that contains the Baggage.

[cijothomas]

@bantonsson Agreed. I have tried to capture that in this PR #2732 as as test. Please review that too.