opentelemetry-java-instrumentation/docs/logger-mdc-instrumentation.md

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. Example for Spring Boot configuration (which uses logback):

logging.pattern.console = %d{yyyy-MM-dd HH:mm:ss} - %logger{36} - %msg trace_id=%X{trace_id} span_id=%X{span_id} trace_flags=%X{trace_flags} %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+