code: protected void initIndexPersistableListener() {
          _index.setPersistableListener(new PersistableListener() {
              @Override
              public void beforePersist() {
                  try {
                      PersistableListener l = _listener;
                      if(l != null) l.beforePersist();
                  } catch (Exception e) {
                      _logger.error("failed on calling beforePersist", e);
                  }
                  
                  try {
                      _bytesDB.persist();
                  } catch (Exception e) {
                      _logger.error("failed on calling beforePersist", e);
                  }
              }
              
              @Override
              public void afterPersist() {
                  try {
                      PersistableListener l = _listener;
                      if(l != null) l.afterPersist();
                  } catch(Exception e) {
                      _logger.error("failed on calling afterPersist", e);
                  }
              }
          });
      }
docs: Initialize the Index persistable listener.