How to use the example demo propagator?

[danhngo-lx]

Hi,

I'm trying understand the extension, specifically the demo propagator.
I've built my jar file from the example (I haven't changed anything yet), then configured my service to use this extension:

            -javaagent:/app/opentelemetry_javaagent-1.9.1.jar
            -Dotel.javaagent.extensions=/app/opentelemetry-java-instrumentation-extension-demo-1.0-all.jar

Add added this env var:

        - name: OTEL_PROPAGATORS
          value: b3multi,demo

So I was expecting to see an attribute named X-demo-field (https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/main/examples/extension/src/main/java/com/example/javaagent/DemoPropagator.java#L24) but the there's no such attribute in the traces.
Is there anything else I need to do to configure this demo propagator?

[mateuszrzeszutek]

Hey @danhngo-lx ,
A custom propagator will add headers to outgoing messages (and read headers in incoming messages), but it does not add any attributes to any span by itself.
Is there something specific that you need to achieve?

[danhngo-lx]

Hi Mateusz,
Thank you for your reply.

What I'm trying achieve is to use the custom propagator to propagate a specific header from a HTTP request to all the services involved in that request, and I'm expecting to see that header in Tempo, like these attributes (tags):

So is that possible with a custom propagator or I should look at something else?

[danhngo-lx]

@mateuszrzeszutek By the way, is demo the correct name of the propagator? It is required to specify the custom propagator name or we don't need to?

[mateuszrzeszutek]

By the way, is demo the correct name of the propagator? It is required to specify the custom propagator name or we don't need to?

Yes, it's demo - you can see it here.
Yes, it is required to specify the custom propagator if you want to use it.

Anyway, from what you described in your previous post I suspect that a propagator won't solve your use case -- since a propagator cannot add attributes to spans produced by the downstream services; it just propagates the context.

Instead, consider using Baggage - you can add whatever info you need to the baggage, and it will automatically get propagated across services (assuming that you enable the baggage propagator, it is enabled by default by the way).
The values you put into the Baggage will not get automatically added to spans, but you can implement a SpanProcessor that does that, e.g.

public void onStart(Context context, ReadWriteSpan span) {
  Baggage.fromContext(context).forEach((key, entry) ->
      span.setAttribute(key, entry.getValue());
}

[danhngo-lx]

Thank you very much Mateusz 😄
I'm going to give baggage a try.