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From Brett Birschbach <brett.birschb...@nsighttel.com>
Subject Re: New Implementation: Logger-level Filtering
Date Fri, 25 May 2007 01:35:54 GMT

As far as I know, log4j does not have a "ForwardingAppender" concept. 
However, implementing it is relatively trivial.  I actually rolled my own a
few months back trying to solve the problem of over filtering.  

In the end, a logger level filter still makes configuration MUCH more
straight forward, as well as providing a performance benefit.  Allow me to
explain the complexity and lower efficiency of the "ForwardingAppender"

A simple configuration is like you would expect:

  <appender name="ROOT_APPENDER"
     <appender-ref ref="FILEAPPENDER"/>
     <appender-ref ref="CONSOLEAPPENDER"/>
  <appender name="MY_FILTERED_APPENDER"/>
     <filter class="com.XXXXXXXXX.log4j.MyFilterForXYZDOSTUFF"/>
     <appender-ref ref="ROOT_APPENDER"/>

  <logger name="com.xyz.dostuff" additivity="false">
     <appender-ref ref="MY_FILTERED_APPENDER"/>
      <appender-ref ref="ROOT_APPENDER"/>

On the surface the solution looks good.  However, let's say I want to add
another appender for all classes in com.xyz

  <logger name="com.xyz" additivity="true">
     <appender-ref ref="XYZFILE"/>

Since, to prevent duplicate log entries, additivity="false" for
com.xyz.dostuff, if I want the output from com.xyz.dostuff to go to XYZFILE
appender as well (since it is a subclass of com.xyz), now I need to add the
appender to com.xyz AND com.xyz.dostuff:

 <logger name="com.xyz" additivity="true">
     <appender-ref ref="NEW_COM_XYZ_APPENDER"/>
 <logger name="com.xyz.dostuff" additivity="false">
     <appender-ref ref="NEW_COM_XYZ_APPENDER"/>
     <appender-ref ref="MY_FILTERED_APPENDER"/>

Not only am I duplicating appender references and creating ever increasing
complexity, but more importantly I have the same problems as originally.  I
have to filter both MY_FILTERED_APPENDER and NEW_COM_XYZ_APPENDER.  Instead
of just filtering log messages once, I have to filter them twice, once for
each appender.  Furthermore, instead of simply filtering log events from the
com.xyz.dostuff package, as I was before, now I am filtering all logging
events from all com.xyz.* packages due to filtering NEW_COM_XYZ_APPENDER.

As I state before, a Logger filter is still simpler and more efficient.

Ron Grabowski wrote:
> In log4net we have a ForwardingAppender that takes a message and fowards
> it to one or more other appenders. To solve the problem of not having to
> apply filters twice I'd put a ForwardingAppender infront of the two other
> appenders. If the message passed through the filters on the
> FowardingAppender the message would be sent to the registered appenders
> (in your example that would be a file and console). Is there something
> equivalent to a ForwardingAppender in log4j?

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