39 lines
1.5 KiB
Markdown
39 lines
1.5 KiB
Markdown
# Logger MDC auto-instrumentation
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The Mapped Diagnostic Context (MDC) is
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> an instrument for distinguishing interleaved log output from different sources.
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> — <cite> [log4j MDC documentation](http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/MDC.html) </cite>
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It contains thread-local contextual information which is later copied to each logging event captured
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by a logging library.
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The OTel Java agent injects several pieces of information about the current span into each logging
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event's MDC copy:
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- `trace_id` - the current trace id
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(same as `Span.current().getSpanContext().getTraceId()`);
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- `span_id` - the current span id
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(same as `Span.current().getSpanContext().getSpanId()`);
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- `trace_flags` - the current trace flags, formatted according to W3C traceflags format
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(same as `Span.current().getSpanContext().getTraceFlags().asHex()`).
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Those three pieces of information can be included in log statements produced by the logging library
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by specifying them in the pattern/format.
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Tip: for Spring Boot configuration which uses logback, you can add MDC to log lines by overriding only the `logging.pattern.level`:
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```properties
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logging.pattern.level = trace_id=%mdc{trace_id} span_id=%mdc{span_id} trace_flags=%mdc{trace_flags} %5p
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```
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This way any services or tools that parse the application logs can correlate traces/spans with log
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statements.
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## Supported logging libraries
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| Library | Version |
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|---------|---------|
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| Log4j 1 | 1.2+ |
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| Log4j 2 | 2.7+ |
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| Logback | 1.0+ |
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