tracing-opentelemetry: why is SpanBuilder::start only called in on_close instead of new_span?

[LukeMathWalker]

This is not really a feature request, but more of an attempt to understand the design decisions taken in tracing-opentelemetry.
When new_span gets invoked, it assembles a SpanBuilder which is injected into the tracing's Span context (wrapped in OtelData) and manipulated throughout the lifecycle of the corresponding tracing's Span. We only build an opentelemetry::Span in on_close, using SpanBuilder::start_with_context.

Am I correct in assuming that this means that Context::current().span() will not return the OTEL counterpart to the currently active opentelemetry::Span given that we do not call start_with_context until the on_close callback is invoked (i.e. the corresponding tracing::Span is no longer active)?

[jtescher]

You are correct that otel's global Context is not used to track current spans as they are instead paired with tracing spans and tracked through the tracing registry. The main reason that an otel span is not started until the associated tracing span ends is to support setting a parent trace id / context after the associated tracing span has been created, which the otel API does not support.

Cases where you would want Context::current().span() are generally handled via either tracing's Span::current() for setting attributes, and OpenTelemetrySpanExt::context to read / inject the current context into a client request or otherwise bridge back to otel.

[LukeMathWalker]

Thanks for clarifying!
I assume the reason for set_parent is that there is no way to do that via a "special" field when the span is declared, as you do for the span name and other OTEL-specific information.

[jtescher]

Yep, exactly.