public class DebugFileDumper extends AppenderBase<ILoggingEvent> {

  // writes a debug-timestamp.log from a cyclic buffer
  private FileAppender<ILoggingEvent> debugBufferFileDumper;
  // writes a debug-timestamp.meta containing corresponding meta infos stored in MDC in an easy to parse key=value format for further processing by external tools.
  private FileAppender<ILoggingEvent> mdcFileDumper;
  
  // ... additional properties

  public DebugFileDumper() {
    mdcFileDumper.setEncoder(new MdcOnlyEncoder());
  }

  public void setEncoder(Encoder<ILoggingEvent> encoder) {
    debugBufferFileDumper.setEncoder(encoder);
  }

  public Encoder<ILoggingEvent> getEncoder() {
    return debugBufferFileDumper.getEncoder();
  }

  // .. additional methods
}

<appender name="debugDumper" class="com.example.logging.logback.extensions.DebugFileDumper">

  <discriminator class="ch.qos.logback.classic.sift.MDCBasedDiscriminator">
    <key>SESSION_ID</key>
    <defaultValue>server</defaultValue>
  </discriminator>

  <evaluator class="ch.qos.logback.classic.boolex.OnErrorEvaluator" />

  <cyclicBufferTracker class="ch.qos.logback.core.spi.CyclicBufferTracker">
    <bufferSize>128</bufferSize>
    <maxComponents>128</maxComponents>
    <timeout>1800000</timeout>
  </cyclicBufferTracker>

  <folder>${logback.logs}/dumps</folder>

  <encoder>
    <pattern>[SOME PATTERN FOR DEBUG LOG FILE]</pattern>
  </encoder>

</appender>