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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:

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

Note: If the current Span is invalid, the OpenTelemetry appender will not inject any trace information.

Supported logging libraries

Note: There are also log appenders for exporting logs to OpenTelemetry, not to be confused with the MDC appenders.

Library Auto-instrumented versions Standalone Library Instrumentation
Log4j 1 1.2+
Log4j 2 2.7+ opentelemetry-log4j-context-data-2.17-autoconfigure
Logback 1.0+ opentelemetry-logback-mdc-1.0

Frameworks

Spring Boot

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