[logback-user] Set Level per Logger AND Appender?

[Robert Elliot]

Hi,

A common use case for me with log4j and now LogBack is that I want to have a default logging setup appending to a (set of) file(s) - typically this will be a root logger with level warn appending to say application.log.  I will then tweak sub loggers to get exactly the output I want - info in some places, error in others, so I get sufficient feedback to see e.g. server startup and shutdown but otherwise I only see things that are genuinely a potential problem.  I consider this config part of the normal working operation of the application, and expect to be able to retrieve the expected output at any point in the future.  It's a system problem if any of that output is lost or if the application.log is polluted with other log statements.

I then frequently want to set up more detailed debug logging for a subset of the system - indeed I'm writing an Ajax based web console to allow logging to be configured and output seen on the fly, having written an aspectJ based trace logging aspect.

The problem I experience is that if I set a Logger, using the normal class based naming scheme, to level "DEBUG" then it will log at that level to application.log as well as my debug appender, littering the application.log file with message I do not want recorded there.  If I turn additivity to false for that logger then it will no longer send WARN messages to the application.log appender, losing the record of all system problems that I expect application.log to contain.

Does that make sense?  I hope it seems a reasonable use case.  If so, what would be considered the best practice for achieving this?  In log4j I would overcome this by creating a new appender that also appends to application.log, assigning it to the logger I wanted to use for debug and giving it an appropriate threshold to maintain the general contract of application.log, and then setting the logger's additivity to false.  This, however, is a very crude solution involving a great deal of additional configuration and is very fragile - any alterations to what I consider the "normal" logging behaviour of my system may well require me to alter all debug configuration.

(What I really want is to specify levels on the combination of a logger and an appender rather than just on a logger.  This would allow taking advantage of the inheritance system - the combination of root logger and application.log appender's level of WARN would then be inherited (and could be over-ridden, of course) by all loggers, and adding a new debug appender to a logger and setting the level of that logger & appender combo to DEBUG would not affect what was sent to application.log at all.  If I were to work on making that possible in Logback, would that alteration be considered, or is that seen as undesirable?)

Regards,
Robert