logback - remapping a log level for a specific logger

Question

I have a logback configuration that has an appender with a threshold filter:

<appender name="SYSLOG" class="ch.qos.logback.classic.net.SyslogAppender">
  <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
    <level>INFO</level>
  </filter>
  ...
</appender>

This ensures that only info and higher (warn, error) get logged to syslog. However, one of the 3rd party libraries we use is logging a particular event at DEBUG, and I would like to log this event to syslog. The first approach I had in mind was to try remap the log level in the logger, but am not sure if this is possible? Something like:

<logger name="akka.some.Thing" level="DEBUG" logAs="INFO">
  <appender-ref ref="SYSLOG" />
</logger>

obviously, the "logAs" parameter doesn't exist, so I can't do that. What would be the best approach to logging akka.some.Thing to the SYSLOG appender while leaving the filter in place for other loggers?

The other approach would be to create a 2nd appender called SYSLOG2 that doesn't have the filter in place and set the specific logger to use that, but was wondering if there was a way to configure logback with just 1 SYSLOG appender...

Thanks,

Why do you have the Threshold filter? Are your other "logger" configurations sending levels higher than INFO to this appender? If so, why? — Darius X.
Yes, we log to 2 locations. the syslog appender ends up in an indexer with licensing constaints, so we never want to log anything lower than INFO. However when we want to debug we enable debugging and this goes to a file appender (without all the debug gunk in syslog). — Brett
Interesting. If your environment allows, the easiest would be to write your own little filter that would DENY anything that had licensing issues. Otherwise, one of the more complex filter -- that allow expressions -- might work, but a second appender will probably be easier to understand and maintain. — Darius X.

joensson joensson · Accepted Answer · 2015-01-08T12:29:07

I know this is an old question - but it is actually possible to do what the OP wants to do with a single SyslogAppender.

If others are searching for an example of how to remap you can take a look at the org.springframework.boot.logging.logback.LevelRemappingAppender class. With that appender it is possible to both remap what appender is finally used for the log event, and it is also possible to remap the level that is used for the final log event - e.g. by changing a DEBUG level into an INFO level.

Usage example in logback config file (taken from https://github.com/spring-projects/spring-boot/blob/master/spring-boot/src/main/resources/org/springframework/boot/logging/logback/defaults.xml):

<appender name="DEBUG_LEVEL_REMAPPER" class="org.springframework.boot.logging.logback.LevelRemappingAppender">
    <!-- Optional: specify the destination logger the event ends up in -->
    <destinationLogger>org.springframework.boot</destinationLogger>
    <!-- Optional: specify log level remapping  -->
    <remapLevels>INFO->DEBUG,ERROR->WARN</remapLevels>
</appender>

<logger name="org.thymeleaf" additivity="false">
    <appender-ref ref="DEBUG_LEVEL_REMAPPER"/>
</logger>

Note that remapping to a specific destination logger can make it harder to find the source code of the original log event - so use it with care.

logback - remapping a log level for a specific logger

2 Answers