Stuart, I understand what you mean now. It definately creates the dynamic appender on append. I've been reading the source that does that, SiftingAppenderBase, today looking at another issue. I can see why it wouldn't make sense to ido it in MDC.put(). Maybe there is another way? I take if your encoder is dynamic, so can't be specified in logback.xml. - Maybe you can do it with a logback.groovy file, I've heard that is more flexible. - Maybe you could define many appenders, one for each encoder, and then use a filter to control which one the event goes to? - Maybe you could use a variable name in the encoder class name, which is replaced by the sifter before the appender is created? Annoyingly SiftingApender only works on one key at a time, so this might not work for you. David -- View this message in context: http://logback.10977.n7.nabble.com/Timing-of-sifting-appender-dynamic-append... Sent from the Users mailing list archive at Nabble.com.