Follow trace from frontend to backend

[pascalbe-dev]

Hey guys,
first things first. I expect this to be the right spot to ask questions. I saw the gitter chat as well and GitHub issues, but if I understood correctly, you want to have questions here. If that's not correct, let me know and I'll post my question via a different channel.

I'm quite new to opentelemetry and tracing in general. My goal is to follow user journeys through the whole technology stack (FE, BE, maybe other BEs, any kind of DB). I guess, this is what tracing is all about.
I tried to implement this with opentelemetry. I'm using the opentelemetry java auto instrumentation in my backend and opentelemetry-js in my (angular) frontend with the XML http request instrumentation.

Both sides alone work fine. The traces are delivered to Jaeger where I can see them.
Now, I want to be able to follow a trace from my frontend to my backend. This seems to be the same question like the one which has been asked in this issue .
I followed the advice to use the jaeger http trace propagator from the contrib repo. However, I can not seem to get it to work.

Do you have any tipps? I would have expected the frontend http calls to the backend to have any kind of headers about the trace, but there are none. I got all my code in this repo. The frontend part is here and the backend instrumentation is done during the start of the java app here.

I'd be glad for any kind of help. As I noted, the whole idea of opentelemetry and tracing in general is quite new to me. Links to resources are helpful as well of course. Thanks in advance.

[obecny]

To "connect" frontend and backend you need to configure some propagator - it doesn't look like you have it.
Check examples/tracer-web you will find things like B3Propagator
for example

provider.register({
  propagator: new B3Propagator(),
});

Not sure about java backend if they propagate this by default or not, I can assume only it should work fine.

[pascalbe-dev]

Ah, i see. I was trying to set the propagator this way. api.propagation.setGlobalPropagator(new SomePropagator());. This alone was not working. But the way, you proposed works fine. It adds the B3 header.
On backend side (I'm testing with java auto instrumentation), I had to configure which propagation is used, but once that was done, I get my distributed traces.
That's nice. Thanks.

[Flarna]

That's somewhat strange because TracerProvider.register() is also just calling api.propagation.setGlobalPropagator(config.propagator) - see

opentelemetry-js/packages/opentelemetry-tracing/src/BasicTracerProvider.ts

Line 99 in 27a8d45

api.propagation.setGlobalPropagator(config.propagator);

[pascalbe-dev]

Strange. Maybe I should check again. I used the Jaeger propagator first. Maybe my issue had nothing to do with setting the propagator in general.