How to rehydrate current span in Rust tracing/otel

Question

I'm writing a Pingora proxy instance. It uses multiple async functions from a trait to indicate various stages. There is a common context that I can provide. I am trying to instrument the whole request lifecycle, however, since Tokio/Pingora can call the stage functions at anytime or on any thread. This does present some challenges to measuring the stages underneath the whole request parent span.

How can I get the current parent span to be active once the functions are called back?

What I'm using:

The Open Telemetry crates
The Tokio tracing crate

I've tried:

storing the various flavors of Span in my context
storing the current tracing context in my context then calling .attach()

What I'm looking for in a tracing graph:

|-- Request/Parent Span ---------------------------------------|
  |-- Stage 1 Span --|
                         |-- Stage 2 Span --|
                                             |-- Upstream --|

Are you primarily interfacing with tracing within your application which is configured to export OpenTelemetry data via the tracing-opentelemetry subscriber? Or are you doing something else? — kmdreko
– kmdreko, Commented Aug 18 at 19:50
@kmdreko I’m up for anything. Would be nice to stick with the tracing crate to allow for more stuff later — Daniel A. White
– Daniel A. White, Commented Aug 18 at 21:59

kmdreko · Accepted Answer · 2025-08-19 14:11:35Z

In the tracing crate, you can get a handle to the current span (via Span::current) and store that with your request (or keep a hold of one created by info_span! and friends). When your request is being handled you can .enter() the span or use it as the parent of a sub-span.

For you I would recommend using #[instrument] on your methods and use the stored span as the parent. There is a similar example listed in the docs but here's how it might look for you:

struct Request {
    span: tracing::Span
}

impl Request {
    #[instrument(parent = &self.span, skip(self))]
    async fn stage_1(&self) {}

    #[instrument(parent = &self.span, skip(self))]
    async fn stage_2(&self) {}

    #[instrument(parent = &self.span, skip(self))]
    async fn upstream(&self) {}
}

The outer span will close when all references to it are dropped.

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