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Logger MDC auto-instrumentation

The Mapped Diagnostic Context (MDC) is

an instrument for distinguishing interleaved log output from different sources. — log4j MDC documentation

It contains thread-local contextual information which is later copied to each logging event captured by a logging library.

The OTel Java agent injects several pieces of information about the current span into each logging event's MDC copy:

  • traceId - the current trace id (same as TracingContextUtils.getCurrentSpan().getContext().getTraceIdAsHexString());
  • spanId - the current span id (same as TracingContextUtils.getCurrentSpan().getContext().getSpanIdAsHexString());
  • sampled - a boolean flag marking whether the current span is sampled or not (same as TracingContextUtils.getCurrentSpan().getContext().isSampled()).

Those three pieces of information can be included in log statements produced by the logging library by specifying them in the pattern/format. Example for Spring Boot configuration (which uses logback):

logging.pattern.console = %d{yyyy-MM-dd HH:mm:ss} - %logger{36} - %msg t:%X{traceId} s:%X{spanId} %n

This way any services or tools that parse the application logs can correlate traces/spans with log statements.

Supported logging libraries

Library Version
Log4j 1 1.2+
Log4j 2 2.7+
Logback 1.0+