Example of adding Baggage to a Trace

[daschneider]

I need to add some domain specific baggage values to our traces. I've reviewed the Javadoc and examples and I still don't see how to go about doing this. Can anyone point me to a working example of how to add baggage to a trace?

Thanks in advance.

[jack-berg]

You can create a span inside of a scope which has had the baggage added to it, like so:

    try (var baggageScope = Baggage.builder()
        .put("user.name", "jack")
        .build()
        .storeInContext(Context.current()).makeCurrent()) {
      var span = TRACER.spanBuilder("my-span").startSpan();
      try (var scope = span.makeCurrent()) {
        // do work
      } finally {
        span.end();
      }
    }

Then you can add a SpanProcessor to your SdkTracerProvider config which extracts the baggage and appends as span attributes. Here's an example which appends all baggage keys as span attributes:

    SdkTracerProvider.builder()
            .addSpanProcessor(new SpanProcessor() {
              @Override
              public void onStart(Context parentContext, ReadWriteSpan span) {
                Baggage.current().asMap().forEach((s, baggageEntry) -> span.setAttribute(s, baggageEntry.getValue()));
              }

              @Override
              public boolean isStartRequired() {
                return true;
              }

              @Override
              public void onEnd(ReadableSpan span) {
              }

              @Override
              public boolean isEndRequired() {
                return false;
              }
            })

[jkwatson]

You will need to propagate that context, by making that context "current" for whatever code executes. Jack's example spells that out, as the // do work in the scope of the baggage is where the downstream code needs to live in order for the baggage to be propagated.

[pavel-luhin]

@jkwatson Hello! Thank you for the answer. Is there any other way to propagate new baggage fields? I need to do this in spring-cloud-gateway filter and seems it does not work well with its filter chain.

try (var baggageScope = Baggage.builder()
        .put("user.name", "jack")
        .build()
        .storeInContext(Context.current()).makeCurrent()) {
        // do work in the scope of the baggage
        return chain.filter(updatedExchange)
}

And using debug in W3CBaggagePropagator I see that baggage is empty. In configuration I set up propagation type to W3C, so the problem is that context is changed. Also consuming service does not receive baggage header.

[jkwatson]

This code doesn't look right to me. storeInContext returns a context, not a scope. I think this code should look something like this:

var baggage = Baggage.builder().put("user.name", "jack").build();
try (Scope ignored = baggage.makeCurrent()) {
  return chain.filter(updatedExchange);
}

[pavel-luhin]

@jkwatson Hello.
I am doing storeInContext().makeCurrent(), so makeCurrent returns Scope.
The code that you provided does not work too. I am thinking, maybe the reason is because I am using Kotlin and its function .use.

let baggage = Baggage.builder()
            .put("enable-extra", enableExtra)
            .build()

baggage.makeCurrent().use {
            return chain.filter(updatedExchange)
}

In debug I see that even on 1 level higher after return Context.current() has empty baggage.
But in .use function Context.current() has baggage with enable-extra field.

[jkwatson]

I don't know what you mean by "1 level higher" in this context. The current Context will only have the new value in it until it falls out of scope, so if it has the value inside your use function, then it is working as it should.