[logback-user] Can the SiftingAppender discard 'unknown' log events?

[Colm.McDonnell at rbs.com]

I am using the SiftingAppender to isolate log events with a specific MDC key. This works really well, giving me high signal:noise ratio log files for specific invocations on particular process etc etc.

However the SiftingAppender (or rather the MDCBasedDiscriminator) insists on the provision of a 'default value'. If I supply this default value (e.g. "unknown") then all unmatched events will be directed to unknown.log. But the 'other' log events are already logged separately so the entries in 'unknown.log' are unwanted duplicates (for me). Ideally I would use the sifting appender for a particular use case and let everything else be logged the way it is currently logged i.e. I would lke to provide no default value and have the appender understand this to mean 'ignore any events which do not match the given key' but that seems not to be supported so I'm trying to figure out how to either discard the unwanted events or to incorporate my existing logging into the sifting appender.

I'd be grateful if someone could suggest how I might do either of the following:

 * somehow discard the 'unknown' events discriminated by the SiftingAppender i.e. to ignore them completely as they are already logged elsewhere
or
 * figure out how to control the 'unknown' log events so that Ii can direct them to an 'everything else' appender which is distinct from the 'isolated events' appender

I've included an example of the config below. The STDOUT appender is the main appender, there's a wrapper process running the java app and this wrapper process redirects STDOUT to its own log file (with its own rolling rules etc) therefore the 'everything else' events are all logged to STDOUT. I'd like the SIFTER to only log events for which it gets a match otherwise we end up with the same events in STDOUT and in "unknown.log". Alternatively, if there is a way to configure SIFTER such that it wraps the STDOUT appender and the unmatched events are only directed to that appender then that would work too. I have tried several configurations but I cannot associate the unmatched events with a specific appender.

<configuration>
    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>[%-5p] [%t] %logger{5}: %m%n</pattern>
        </encoder>
    </appender>
    <appender name="SIFTER" class="ch.qos.logback.classic.sift.SiftingAppender">
        <discriminator>
            <Key>isolatedLogName</Key>
            <defaultValue>unknown</defaultValue>
        </discriminator>
        <sift>
            <appender name="${isolatedLogName}" class="ch.qos.logback.core.FileAppender">
                <File>${log.path:-target/log}/${isolatedLogName}.log</File>
                <encoder>
                    <Pattern>%d | %-5p | %logger{5} | %m%n</Pattern>
                </encoder>
            </appender>
        </sift>
    </appender>

    <!--  ... loggers excluded for brevity ... -->

    <root level="INFO">
        <appender-ref ref="STDOUT" />
        <appender-ref ref="SIFTER"/>
    </root>
</configuration>

For example: I tried this:

    <appender name="SIFTER" class="ch.qos.logback.classic.sift.SiftingAppender">
        <discriminator>
            <Key>isolatedLogName</Key>
            <defaultValue>everythingElse</defaultValue>
        </discriminator>
        <sift>
            <appender name="isolatedLogName" class="ch.qos.logback.core.FileAppender">
                <File>${log.path:-target/log}/${isolatedLogName}.log</File>
                <encoder>
                    <Pattern>%d | %-5p | %logger{5} | %m%n</Pattern>
                </encoder>
            </appender>
            <appender name="everythingElse" class="ch.qos.logback.core.ConsoleAppender">
                <encoder>
                    <pattern>[%-5p] [%t] %logger{5}: %m%n</pattern>
                </encoder>
            </appender>
        </sift>
    </appender>

And logback created two files - everythingElse.log and <the real value associated with the isolatedLogName key>.log - then wrote all output to the "everythingElse" appender. I was trying to tell it to do this: "if the key's value is the default value then ignore it or if that is not possible then direct the output to the appender named after that value otherwise direct to the other appender".

Any help appreciated, thanks.

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