[jira] [Commented] (LOG4J2-643) LogEventFactory not called when using AsyncContextSelector

["Remko Popma (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/LOG4J2-643?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14005482#comment-14005482 ] 

Remko Popma commented on LOG4J2-643:
------------------------------------

If you make all loggers asynchronous by setting system property {{Log4jContextSelector}} to {{org.apache.logging.log4j.core.async.AsyncLoggerContextSelector}}, there are no new log events created for each log message: the ringbuffer is pre-allocated with RingBufferLogEvents which are reused. 
So this is a different mechanism where LogEventFactory does not come into play.

There is an alternative though: you can use what the documentation calls ["Mixing Synchronous and Asynchronous Loggers"|http://logging.apache.org/log4j/2.x/manual/async.html#MixedSync-Async]. This has a different ringbuffer and does use the LogEventFactory to create log events, so the solution you mention to always put values in the MDC should work.

To do this, remove the  {{Log4jContextSelector}} system property and instead change your log4j2.xml configuration: replace {{<Logger>}} by {{<AsyncLogger>}} and replace {{<Root>}} by {{<AsyncRoot>}}. Performance-wise this is fairly close to making all loggers asynchronous (performance comparison is on the same doc page).

Please take a look at the docs and let me know if you run into any problems.

> LogEventFactory not called when using AsyncContextSelector
> ----------------------------------------------------------
>
>                 Key: LOG4J2-643
>                 URL: https://issues.apache.org/jira/browse/LOG4J2-643
>             Project: Log4j 2
>          Issue Type: Improvement
>          Components: Core
>    Affects Versions: 2.0-rc2
>            Reporter: Stefan Wehner
>            Priority: Minor
>
> We're using the JDBCAppender to write log messages to a MySQL database - including some data provided by the ThreadContext.
> We wanted to make sure that at least some default values are available in the Event's MDC in case the values are not provided in the ThreadContext We also calculate some extra data (delta of the log event's time from the request start time - stored in the ThreadContext).
> We thought a good idea would be to do this in a custom LogEventFactory, that makes sure that values are set, provides defaults, and calculates the offset.
> When switching to using the AsyncLoggerContext this doesn't work anymore, because the LogEventFactory is not used (unless the RingBuffer is full - see AsyncLogger:248).
> I'm not sure if we're abusing the LogEventFactory here for something that it wasn't meant to do...
> If you think this is a legit use case: shouldn't the async logging also use this factory somehow? I understand the biggest problem of using the LogEventFactory to create an event, and then pass the values to the translator would be the context stack, which is handled differently in the AsyncLogger and the Log4jLogEvent constructor.



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