HTTPTextFormat propagator formats should support chaining propagators

[toumorokoshi]

I think an important capability for HTTPTextFormat propagators is the ability to attach more than one. I.E. Tracer.GetHTTPTextFormat could return a proxy object that executes extract / inject on multiple HTTPTextFormats.

This would be invaluable when migrating propagation standards, such as from b3 to tracestate.

[carlosalberto]

This was mentioned in the Java repo some months ago, through a "multiformat" text adapter (to be added in the contrib section), precisely to support multiple formats: open-telemetry/opentelemetry-java#119

[toumorokoshi]

Cool! It's not clear to me why we wouldn't just want to make this a feature exposed by a contrib package rather than just in the API itself. I think it's relatively straightforward to make them chainable by allowing for a proxy object, and making the propagators have an api like:

inject(context: SpanContext, getter: Getter, object: object) -> None

[carlosalberto]

There's always the worry of making the API too cumbersome to use, as well as increasing the API surface overall.

Probably having an actual testcase/example could get things moving ;)

[toumorokoshi]

Sounds good, am working on an example in opentelemetry-python.

[a-feld]

@toumorokoshi I wonder if it would be helpful to have the context tracked in the propagator instance itself (either by having the context in the constructor or via calls to the context API within the propagator).

If the propagator is context-aware, an API called ExtractIntoCurrentContext or similar could be added where the propagator itself references the current context.

If the propagator understands how to set the context, the propagator extract becomes easily chainable by simply calling each propagator's extract 1 at a time, especially in case where both SpanContext and DistributedContext should be extracted.

This could also simplify the inject API by providing the current context through an attribute on the propagator rather than through the interface.

[tedsuo]

FWIW we have an example of this behavior here, in the LightStep OT client:
https://github.com/lightstep/lightstep-tracer-java-common/blob/master/common/src/main/java/com/lightstep/tracer/shared/PropagatorStack.java

Extract: try each header type in a list, until one is found.
Inject: Only inject the configured header type(s).

[toumorokoshi]

My concern here will be addressed somewhat by the ability to enable multiple propagators here: open-telemetry/oteps#66. I think I'll potentially pick this issue up again around implementation details once that's merged in and there's a spec to build on.

[carlosalberto]

@toumorokoshi Closing it after OTEP 66 was merged. Feel free to re-open it if you think we still need to track this ;)