46 lines
2.5 KiB
Markdown
46 lines
2.5 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. This way any services or tools that parse the application
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logs can correlate traces/spans with log statements.
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> Note: If the current `Span` is invalid, the OpenTelemetry appender will not inject any trace information.
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## Supported logging libraries
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> Note: There are also log appenders for exporting logs to OpenTelemetry, not to be confused with the MDC appenders.
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| Library | Auto-instrumented versions | Standalone Library Instrumentation |
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| ------- | -------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------ |
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| Log4j 1 | 1.2+ | |
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| Log4j 2 | 2.7+ | [opentelemetry-log4j-context-data-2.17-autoconfigure](../instrumentation/log4j/log4j-context-data/log4j-context-data-2.17/library-autoconfigure) |
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| Logback | 1.0+ | [opentelemetry-logback-mdc-1.0](../instrumentation/logback/logback-mdc-1.0/library) |
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## Frameworks
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### Spring Boot
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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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