[FEATURE]: Support DD_TRACE_PROPAGATION_EXTRACT_FIRST

[sdwa0]

Describe the goal of the feature

dd-trace-cpp doesn’t currently support DD_TRACE_PROPAGATION_EXTRACT_FIRST environment variable to configure the context extraction logic to stop when a valid one is found. Other dd-trace libraries support it.

Is there a particular reason it is not supported in dd-trace-cpp? Is there a plan to support it in dd-trace-cpp for consistency with other tracing libraries?

Is your feature request related to a problem?

The current context extraction logic takes the W3C headers into account regardless of the order of the propagation styles. So there is no way to configure it to prioritize other propagation styles e.g. Datadog over W3C.

Describe alternatives you've considered

No response

Additional context

No response

[sdwa0]

@dmehala @dgoffredo Apologies for the direct mention. Wanted to know your thoughts on this.

[dmehala]

Hi @sdwa0,

No worries about the direct mention. I just wanted to note that David has left the company, I know he's more than willing to help but I'd like to respect his time.

Thank you for bringing up this discrepancy with dd-trace-cpp. To my knowledge, there isn’t a specific reason why DD_TRACE_PROPAGATION_EXTRACT_FIRST isn’t supported, it seems to be more of a coverage gap. For context, this environment variable was introduced for performance reasons when multiple tracing contexts could be extracted.

By default, context extraction follows the Datadog, tracecontext order. This means that Datadog headers are extracted first, followed by W3C tracecontext headers. If the tracing contexts from both match, the context is enhanced with additional W3C information. If they don’t match, the behavior differs slightly, though I don’t have the full details at the moment.

That said, I am not against implementing DD_TRACE_PROPAGATION_EXTRACT_FIRST but I’d love to understand more about the issue you're encountering. That's probably hiding a bigger issue. Could you share what specific challenge led you to need DD_TRACE_PROPAGATION_EXTRACT_FIRST?