Current Logback support provides support for registering Filter as OSGi services . This only supports registering against known appenders.
If we need to have a logic which needs to handle/listen to all WARN and above log messages that needs to get logged then only way for now is to register a TurboFilter which has performance cost. To support such cases the Filter support should enable registering a Filter instance against all configured appenders. This would allow such a Filter to listen to all logged messages without the performance cost