Useful logging mechanism: Log Markers

In my previous post, I showed a simple example about MDC. In this post, I discuss the use of log markers (http://logback.qos.ch/manual/filters.html). Like MDC filters, marker filters are also "turbo filters" in Logback. Both these filters get triggered after a log event is created but before the log appender(s) are invoked. So they are a good choice for high-performance logging. While MDC filters are ideal to trace the path of a request, log marker filters are good for logging certain use cases.

Suppose we want to log the stack trace for user authentication failures (incorrect userid, incorrect password, account locked, db failure, etc) . Typically, the stack traces are logged in DEBUG level so they don't fill your production logs. So what we want is the ability to log the stack trace only for user authentication failures.

Log markers come handy here. You can use a log marker (say LOG_STACKTRACE_USER_AUTH_FAILURE) like this:

Marker logMarker = MarkerFactory.getMarker(LOG_STACKTRACE_USER_AUTH_FAILURE);
log.debug(logMarker, exception);

And in your logback.xml:

 <turboFilter class="ch.qos.logback.classic.turbo.MarkerFilter">
    <Marker>LOG_STACKTRACE_USER_AUTH_FAILURE</Marker>
    <OnMatch>ACCEPT</OnMatch>
  </turboFilter>

Irrespective of the global logger level you may defined*, this will allow your software to log stack trace to an appender when a log event with a marker called "LOG_STACKTRACE_USER_AUTH_FAILURE" is created.

* This behaviour is particular to ch.qos.logback.classic.turbo.MarkerFilter implementation.

Here is a good stackoverflow discussion on uses for MDC and log markers: http://stackoverflow.com/questions/4165558/best-practices-for-using-markers-in-slf4j-logback