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