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:

trace_id - the current trace id (same as Span.current().getSpanContext().getTraceId());
span_id - the current span id (same as Span.current().getSpanContext().getSpanId());
trace_flags - the current trace flags, formatted according to W3C traceflags format (same as Span.current().getSpanContext().getTraceFlags().asHex()).

Those three pieces of information can be included in log statements produced by the logging library by specifying them in the pattern/format.

Tip: for Spring Boot configuration which uses logback, you can add MDC to log lines by overriding only the logging.pattern.level:

logging.pattern.level = trace_id=%mdc{trace_id} span_id=%mdc{span_id} trace_flags=%mdc{trace_flags} %5p

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

Supported logging libraries