WebClient Observations are missing error

[Xiphoseer]

I'm using WebClient in a setup with

• org.springframework:spring-webflux:6.1.4
• io.micrometer:micrometer-tracing-bridge-otel:1.2.3
• io.opentelemetry:opentelemetry-exporter-zipkin:1.31.0

and am missing status/error metadata in the spans that show up in Zipkin.

When a HTTP GET request fails, the outcome in zipkin is a http get span with (among others) the tags:

• exception: WebClientRequestException
• outcome: UNKNOWN
• status: CLIENT_ERROR
• otel.library.name, otel.library.version, otel.scope.name, and otel.scope.version

What is missing is both:

• error (Convention for Zipkin to mark the span in red)
• otel.status_code (from io.opentelemetry.exporter.zipkin.OtelToZipkinSpanTransformer)

Working backwards, the OtelToZipkinSpanTransformer would set both of these, if there was a status
set on the OpenTelemetry Span / SpanData, which was not the case in debugging.

These would be set in io.micrometer.tracing.otel.bridge.OtelSpan::error, which is an implementation of
io.micrometer.tracing.Span. Going further back, that method is called by io.micrometer.tracing.handler.TracingObservationHandler,
either when the ERROR tag is set, or when onError is called.

That should be called by io.micrometer.observation.SimpleObservation#notifyOnError, which is an implementation detail of io.micrometer.observation.SimpleObservation#error.

But it turns out WebClient doesn't call that function! Instead, at

spring-framework/spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultWebClient.java

Line 474 in 7f0ab22

.doOnError(observationContext::setError)

it only calls org.springframework.web.reactive.function.client.ClientRequestObservationContext#setError, whereas
SimpleObservation both sets the error in the contex and dispatches the notification:

@Override
public Observation error(Throwable error) {
    this.context.setError(error);
    notifyOnError();
    return this;
}

To me this last part seems like the best place to fix this issue, i.e. to replace .doOnError(observationContext::setError) with something like .doOnError(e -> observation.error(e)).

[Xiphoseer]

Note: technically it's io.micrometer.tracing.handler.PropagatingSenderTracingObservationHandler not TracingObservationHandler, which is provided by Spring Boot in MicrometerTracingAutoConfiguration, but the mechanism stays the same.

[Xiphoseer]

I protoyped a fix locally with the following bean, which works to make zipkin recognize the span as failed, but outcome is still UNKNOWN, which is probably fine.

@Bean
@ConditionalOnMissingBean
@ConditionalOnClass(ClientRequestObservationContext.class)
@Order(SENDER_TRACING_OBSERVATION_HANDLER_ORDER)
public PropagatingSenderTracingObservationHandler<?> propagatingSenderTracingObservationHandler(Tracer tracer, Propagator propagator) {
    return new PropagatingSenderTracingObservationHandler<>(tracer, propagator) {
        @Override
        public void customizeSenderSpan(SenderContext context, Span span) {
            // Fix for: https://github.com/spring-projects/spring-framework/issues/32389
            if (ClientRequestObservationContext.class.isAssignableFrom(context.getClass())) {
                Throwable error = context.getError();
                if (error != null) {
                    span.error(error);
                }
            }
            super.customizeSenderSpan(context, span);
        }
    };
}