brave-context-log4j12

This adds trace and span IDs to the Log4J v1.2 Mapped Diagnostic Context so that you can search or aggregate logs accordingly.

To enable this, configure brave.Tracing with MDCCurrentTraceContext like so:

tracing = Tracing.newBuilder()
    .currentTraceContext(MDCCurrentTraceContext.create())
    ...
    .build();

Then, in your log configuration, you can use traceId and or spanId.

Here's an example log4j.properties pattern:

appender.console.layout.ConversionPattern = %d{ABSOLUTE} [%X{traceId}/%X{spanId}] %-5p [%t] %C{2} (%F:%L) - %m%n

When a trace is in progress, it would log statements like this:

11:01:29,799 [e2ffceb485bdfb1d/e2ffceb485bdfb1d] INFO  [main] c.a.FooController (FooController.java:30) - I got here!

Users could then copy/paste the trace ID into the zipkin UI, or use log correlation to further debug a problem.