log4j 2's predecessor, log4j, supports the ability to filter log messages both at the <logger/> and <appender/> level (where a <threshold/> has been specified). This satisfies most typical scenarios, however I would find it very useful to have a slightly finer degree of control, such that it is possible to define log level filters at the <appender-ref/> level. Therefore for a given logger, you could have multiple <appender-ref/> definitions, each with their own independent log level filter. I feel this would be useful in scenarios where you wish messages for the same logger to be filtered independently depending on the destination appender, and also the ability for two separate loggers to write to the same appender at different log levels. A typical example might be were you have deployed a Web App and you require it log only important messages by default, perhaps WARN and above, to the server console (STDOUT) because you don't want to clutter it up with lots of details information - just the important stuff, whilst logging much more verbose information to a separate rolling appender - which could then be used for more complete analysis at a later date, should the need arise. You might also have a second Web App (or a 3rd partly library which supports log4j/log4j 2) which is configured to reuse some of all of the appenders detailed earlier, but with completely independent log level filtering.
To help illustrate the general concept I'm aiming for, I've included a pseudo-configuration file below. You'll notice that I've omitted the LEVEL attribute from the <logger/> nodes and added LEVEL attributes to each of
the <appender-ref> nodes - this doesn't work in log4j obviously, but I was wondering if there was some other way of emulating the behaviour that I'm after without cluttering up the logback configuration too much...? You can see from the configuration that for com.myorg.myapp.subpackage.one I would like to apply different log level restrictions depending on the destination appender. You'll also notice that for com.myorg.myapp.subpackage.two I want to reuse the same appenders, but also use different level restrictions.... hope that makes it clear enough...
<appender-ref ref="A" LEVEL="INFO"/>
<appender-ref ref="B" LEVEL="DEBUG"/>
<appender-ref ref="A" LEVEL="WARN"/>
<appender-ref ref="B" LEVEL="TRACE"/>