How to write some logs to a different log file

Issue

We would like to move a trace from the main log file to a separate log file.

Environment

DXP 7.3+
Quarterly Release

Resolution

We can use a portal-log4j-ext.xml file to to add a new appender where we specify the name of the additional log file:

1. Edit portal-log4j-ext.xml.

2. Add a new appender for your log file (myLog....log), i.e.:

<appender class="org.apache.log4j.rolling.RollingFileAppender" name="MY_LOG_TEXT_FILE">
<rollingPolicy class="org.apache.log4j.rolling.TimeBasedRollingPolicy">
<param name="FileNamePattern" value="@liferay.home@/logs/myLog@spi.id@.%d{yyyy-MM-dd}.log" />
</rollingPolicy>
   <layout class="org.apache.log4j.EnhancedPatternLayout">
      <param name="ConversionPattern" value="%d{yyyy-MM-dd HH:mm:ss.SSS} %-5p [%t][%c{1}:%L] %m%n" />
   </layout>
</appender>

3. Add your new appender to the <root> tag:

<root>
   <priority value="INFO" />

   <appender-ref ref="CONSOLE" />
   <appender-ref ref="TEXT_FILE" />
   <appender-ref ref="XML_FILE" />
   <appender-ref ref="MY_LOG_TEXT_FILE" />
</root>

4. One way to filter the traces could be by text, this can be achieved by adding filters to each appender to display or not the traces containing that text:
Displays the trace in the log:

<filter class="org.apache.log4j.varia.StringMatchFilter">
   <param name="StringToMatch" value="Tex to filter" />
   <param name="AcceptOnMatch" value="true" />
</filter>

Not displays the trace in the log:

<filter class="org.apache.log4j.varia.StringMatchFilter">
   <param name="StringToMatch" value="Tex to filter" />
   <param name="AcceptOnMatch" value="false" />
</filter>

Additional Information

Check https://help.liferay.com/hc/en-us/articles/360050830051-Adjusting-Log-Levels-to-persist-after-portal-restart, steps 1 to 4, to see how to use a portal-log4j-ext.xml file.
Attached a portal-log4j-ext.xml as an example for 7.3.x.