log4j - on ERROR, log DEBUG?

Question

I'm relatively new to log4j (v2), but I believe I've exhausted my resources, so I come to ask my question here.

I have a working log4j configuration, which I will append to the end of my question. The application is run every x minutes using a cronjob.

The log4j behaviour I'd like to see is the following:

1. All INFO (and up) logging is done to file A
2. All ERROR (and up) logging is done to file B
3. In case of an ERROR (or up), a mail is sent containing the DEBUG (and up) lines of the given execution.

I have achieved 1 and 2 using the configuration below. Is number 3 also possible using log4j (if not, why not?) and if so, how?

The current configuration:

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="INFO">
  <Appenders>
    <Console name="Console" target="SYSTEM_OUT">
      <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
    </Console>
    <RandomAccessFile name="LogFile" fileName="/var/log/fileA.log">
      <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
    </RandomAccessFile>
    <RandomAccessFile name="ErrorLogFile" fileName="/var/log/fileB.log">
      <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
    </RandomAccessFile>
    <SMTP name="MailError" subject="Error occurred" to="a@b.com" smtpHost="mail.foo.bar" from="b@c.net"/>
    <Async name="Async">
      <AppenderRef ref="LogFile"/>
    </Async>
    <Async name="Errors">
      <AppenderRef ref="ErrorLogFile"/>
      <AppenderRef ref="MailError"/>
    </Async>
  </Appenders>
  <Loggers>
    <Root level="DEBUG">
      <AppenderRef ref="Console" level="INFO"/>
      <AppenderRef ref="Async" level="INFO"/>
      <AppenderRef ref="Errors" level="ERROR"/>
    </Root>
  </Loggers>
</Configuration>

Answer 1

I think you should be able to achieve this with a ThresholdFilter. Make your root logger level TRACE so that it sends all events to all appenders, then put different threshold filters on each appender. The log4j2 manual contains an example.

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(EDIT) After understanding the requirements better, I think the SMTP Appender already does what you need: it internally keeps a buffer of the events (512 by default) that preceded the ERROR event that triggered the email. These preceding events will be included in the email.