How do I create a nested span?

[ramgdev]

Using open telemetry::trace::get_active_span, I get the parent span. But I don't see any way to create a child span off of it.

[lalitb]

@ramgdev - You can find the example as rustdoc here -

opentelemetry-rust/opentelemetry/src/trace/tracer.rs

Line 21 in ff64e21

/// let mut child = tracer.start_with_context("bar", &parent_cx);

Also, the server part of tracing-http-propagator example demonstrate two ways to create child span:

• Within the same thread through Tracer::start_with_context()
• Within async code through Futures::with_context()

[ramgdev]

Thanks for the quick reply. It works. However, is this only useful when we are tracing with context propagation? Or is there another way to establish parent/child relations between spans? Thanks again.

[lalitb]

Not without having the Context containing the parent span. So you need to use something like this:

   let parent_cx = Context::current_with_span(parent_span);
   let mut child = tracer.start_with_context("bar", &parent_cx);

This is in accordance with the API Specification:

The parent Context or an indication that the new Span should be a root Span. The API MAY also have an option for implicitly using the current Context as parent as a default behavior. This API MUST NOT accept a Span or SpanContext as parent, only a full Context.