brave-context-log4j12

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

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

correlationContext = new Log4jContext();
tracing = Tracing.newBuilder()
    .currentTraceContext(ThreadLocalCurrentTraceContext.newBuilder()
        .addScopeDecorator(MDCScopeDecorator.get())
        .build()
    )
    ...
    .build();

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

Log4J 1.2.x MDC is inheritable

Log4J 1.2.x MDC uses an inheritable thread local by default. This means trace identifiers could leak onto new threads and mislable log lines. If this is a problem, consider Log4J 2.x or SLF4J which do not inherit by default.