otel-bridge: add trace_flags to MDC

[olivierboudet]

In Slf4JEventListenerof micrometer-tracing-otel-bridge, span_id and trace_id are added to MDC.

tracing/micrometer-tracing-bridges/micrometer-tracing-bridge-otel/src/main/java/io/micrometer/tracing/otel/bridge/Slf4JEventListener.java

Line 58 in 4f14a45

MDC.put(traceIdKey, span.getSpanContext().getTraceId());

Would it be possible to add also trace_flags which allow to know when a request is sampled (see https://www.w3.org/TR/trace-context/#trace-flags)

[jonatan-ivanov]

I think so (we used to have it in Sleuth) but what would be the use-case for that?
Can you get the whole span through the Tracer: tracer.currentSpan()?

[olivierboudet]

I would like to log the traceFlags field to know if the trace is sampled and if it worth searching it in storage backend. I am currently using the opentelemetry java agent and have a log pattern like this :
%clr(%d{yyyy-MM-dd HH:mm:ss.SSS}){faint} %X{correlation-id:--} %X{trace_id:--} %X{trace_flags:--} %clr(${LOG_LEVEL_PATTERN:%5p}) %clr(${PID:0}){magenta} %clr(---){faint} %clr([%15.15t]){faint} %clr(%-40.40logger{39}){cyan} %clr(:){faint} %m%n${LOG_EXCEPTION_CONVERSION_WORD:%wEx}}
I am just trying to have a similar behavior with micrometer tracing (with Spring Boot).

[jonatan-ivanov]

Let me ping @marcingrzejszczak to get some background about why this was removed from Sleuth.

[marcingrzejszczak]

There's no point in having this in the logs out of the box. If we're talking about log correlation it doesn't matter if the span is sampled or not.

[olivierboudet]

If it does not make sense to have it out of the box, can you explain a bit how can I have it with custom code please ?

I though to write a custom EventListener to insert traceFlags information in MDC put all methods of Scope*Event are protected.
I am trying to do something like that :

package com.cooperl.csuite.pass.eleveur.common;

import io.micrometer.tracing.otel.bridge.EventListener;
import io.micrometer.tracing.otel.bridge.EventPublishingContextWrapper;
import org.slf4j.MDC;
import org.springframework.stereotype.Component;

@Component
public class SLF4JEventListener implements EventListener {

    public static final String TRACE_FLAGS = "trace_flags";

    @Override
    public void onEvent(Object event) {
        if (event instanceof EventPublishingContextWrapper.ScopeAttachedEvent) {
            onScopeAttached((EventPublishingContextWrapper.ScopeAttachedEvent) event);
        }
        else if (event instanceof EventPublishingContextWrapper.ScopeClosedEvent) {
            onScopeClosed();
        }
        else if (event instanceof EventPublishingContextWrapper.ScopeRestoredEvent) {
            onScopeRestored((EventPublishingContextWrapper.ScopeRestoredEvent) event);
        }
    }

    private void onScopeClosed() {
        MDC.remove(TRACE_FLAGS);
    }

    private void onScopeAttached(EventPublishingContextWrapper.ScopeAttachedEvent event) {
        MDC.put(TRACE_FLAGS,  String.valueOf(event.getSpan().getSpanContext().getTraceFlags())); // does not work, getSpan() is not public

    }

    private void onScopeRestored(EventPublishingContextWrapper.ScopeRestoredEvent event) {
        MDC.put(TRACE_FLAGS,  String.valueOf(event.getSpan().getSpanContext().getTraceFlags())); // does not work, getSpan() is not public
    }
}

[marcingrzejszczak]

I've opened the scope of getters for tracing 1.1.0. Until then you need to either use reflection or create a class in the io.micrometer.tracing.otel.bridge that will be able to access the package scope methods.

[olivierboudet]

Thanks a lot !