agent does not export trace_id and span_id mdc values as attributes to collector

[maxxedev]

Describe the bug

Javaagent can be configured to include trace_id and span_id in app's MDC. However, these MDC values are not exported to collector.

Example:
app might log something like:

app-1        | 2025-10-06 04:57:24.022Z [http-nio-8080-exec-1] INFO i.o.example.javagent.Controller - \
trace_id=f0167d362d66c6c76a88ce08b4f4312c, trace_flags=01, span_id=fd55bf7512bc0a69, foo=bar - \ 
A sample log message! 

where trace_id, span_id and foo are MDC names.

foo MDC key-value is exported to collector as attributes but not trace_id/span_id:

collector-1  | Body: Str(A sample log message!)
collector-1  | Attributes:
collector-1  |      -> foo: Str(bar)
                          (trace_id, span_id??)
collector-1  | Trace ID: f0167d362d66c6c76a88ce08b4f4312c
collector-1  | Span ID: fd55bf7512bc0a69
collector-1  | Flags: 1

Steps to reproduce

• git clone https://github.com/open-telemetry/opentelemetry-java-examples
• git checkout f04cb6fb4bc45cd9f1aaff2daf3b6c1b38b3f283
• make these changes:

+++ b/javaagent/docker-compose.yml
@@ -9,6 +9,8 @@ services:
       OTEL_EXPORTER_OTLP_LOGS_ENDPOINT: "http://collector:4318/v1/logs"
       # Logs are disabled by default
       OTEL_LOGS_EXPORTER: "otlp"
+      OTEL_INSTRUMENTATION_LOGBACK_APPENDER_EXPERIMENTAL_CAPTURE_MDC_ATTRIBUTES: "*"
+      LOGGING_PATTERN_CONSOLE: "%d{yyyy-MM-dd HH:mm:ss.SSS'Z',GMT} [%thread] %level %logger{36} - %mdc - %msg %kvp%n"


b/javaagent/src/main/java/io/opentelemetry/example/javagent/Controller.java
@@ -44,8 +45,10 @@ public class Controller {
     Span span = tracer.spanBuilder("doWork").startSpan();
     try (Scope ignored = span.makeCurrent()) {
       Thread.sleep(sleepTime);
+      MDC.put("foo", "bar");
       LOGGER.info("A sample log message!");
     } finally {
+      MDC.remove("foo");
       span.end();
     }
   }

Expected behavior

trace_id, trace_flags and span_id should be exported as attributes

Actual behavior

trace_id, trace_flags and span_id are not exported as attributes

Javaagent or library instrumentation version

2.20.1

Environment

JDK:
OS:

Additional context

No response

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[laurit]

Javaagent can be configured to include trace_id and span_id in app's MDC. However, these MDC values are not exported to collector.

The intent for exposing trace_id and span_id in MDC is that you could use them in the application logs.
When logs are exported using OTLP then trace_id and span_id are not added as attributes because there are dedicated fields in the data format for them. See https://github.com/open-telemetry/opentelemetry-proto/blob/b8054e9abdba27c8c9b51dcb3f1d1884906d8572/opentelemetry/proto/logs/v1/logs.proto#L198
You can see them in the sample output you provided

collector-1  | Body: Str(A sample log message!)
collector-1  | Attributes:
collector-1  |      -> foo: Str(bar)
                          (trace_id, span_id??)
collector-1  | Trace ID: f0167d362d66c6c76a88ce08b4f4312c
collector-1  | Span ID: fd55bf7512bc0a69
collector-1  | Flags: 1

[maxxedev]

That behavior is different than that of otel spring-boot starter which produces:

collector-1  | Body: Str(A slf4j log message with a span)
collector-1  | Attributes:
collector-1  |      -> foo: Str(bar)
collector-1  |      -> span_id: Str(f4f32ce39c1d9baf)
collector-1  |      -> trace_flags: Str(01)
collector-1  |      -> trace_id: Str(a4a54e0502a97528d148e9905d4a5d10)
collector-1  | Trace ID: a4a54e0502a97528d148e9905d4a5d10
collector-1  | Span ID: f4f32ce39c1d9baf
collector-1  | Flags: 1

To reproduce, update log-appender/build.gradle.kts

+++ b/log-appender/build.gradle.kts
-        languageVersion.set(JavaLanguageVersion.of(11))
+        languageVersion.set(JavaLanguageVersion.of(17))
     }
 }
 
@@ -35,6 +35,10 @@ dependencies {
+    implementation("io.opentelemetry.instrumentation:opentelemetry-logback-mdc-1.0")
+
+    implementation("org.springframework.boot:spring-boot-starter-actuator:3.5.6")
+    implementation("io.opentelemetry.instrumentation:opentelemetry-spring-boot-starter:2.20.1")

and use this main class:

@SpringBootApplication
public class Application {

  private static final org.slf4j.Logger slf4jLogger = LoggerFactory.getLogger("slf4j-logger");

  public static void main(String[] args) {
    System.setProperty("otel.logs.exporter", "otlp");
    System.setProperty("otel.exporter.otlp.protocol", "grpc");
    System.setProperty("otel.instrumentation.logback-appender.experimental.capture-mdc-attributes", "*");
    
    System.setProperty("spring.main.banner-mode", "off");
    System.setProperty("spring.main.web-application-type", "none");
    var context = SpringApplication.run(Application.class, args);
    var otel = context.getBean(OpenTelemetry.class);

    // Log using slf4j API w/ logback backend
    maybeRunWithSpan(otel, () -> slf4jLogger.info("A slf4j log message with a span"), true);
  }

  private static void maybeRunWithSpan(OpenTelemetry otel, Runnable runnable, boolean withSpan) {
    if (!withSpan) {
      runnable.run();
      return;
    }
    Span span = otel.getTracer("my-tracer").spanBuilder("my-span").startSpan();
    try (Scope unused = span.makeCurrent()) {
      org.slf4j.MDC.put("foo", "bar");
      runnable.run();
    } finally {
      span.end();
    }
  }
}

[laurit]

You are using otel.instrumentation.logback-appender.experimental.capture-mdc-attributes=* which means that all values from mdc should be captured as span attributes. Did you test that adding the same flag to the agent won't give you the same behavior?

[maxxedev]

Yes I did. As mentioned in my original comment, I added this env var:

+      OTEL_INSTRUMENTATION_LOGBACK_APPENDER_EXPERIMENTAL_CAPTURE_MDC_ATTRIBUTES: "*"

I also tried with -Dotel.instrumentation.logback-appender.experimental.capture-mdc-attributes=* (and being careful to avoid shell expansion of *). Neither worked like spring-boot starter.