In MDCBasedDiscriminator.getDiscriminatingValue() the MDC of the current thread is referenced:

public String getDiscriminatingValue(ILoggingEvent event) {
String mdcValue = MDC.get(key);
if (mdcValue == null)

{ return defaultValue; }

else

{ return mdcValue; }

}

This doesn't work is the logging events are serialized and processed in a different thread or JVM.

Instead using SLF4Js MDC class why not using ILoggingEvent.getMdc() to access the MDC or a serialized version of it?
Maybe something like this:

public String getDiscriminatingValue(ILoggingEvent event) {
if (event != null) {
String mdcValue = event.getMdc().get(getKey());
if (mdcValue != null)

{ return mdcValue; }
}
return getDefaultValue();
}


Right now we have to workaround this by creating our own version of DeferedMDCBasedDiscriminator
Some like this:

public class DeferedMDCBasedDiscriminator extends MDCBasedDiscriminator {
public String getDiscriminatingValue(ILoggingEvent event) {
if (event != null) {
String mdcValue = event.getMdc().get(getKey());
if (mdcValue != null) { return mdcValue; }

}
return getDefaultValue();
}
}