docs: public String changeSessionIdOnMemcachedFailover( final String requestedSessionId ) {
  
      	if ( !_memcachedNodesManager.isEncodeNodeIdInSessionId() ) {
      		return null;
      	}
  
          try {
              if ( _sticky ) {
                  /* We can just lookup the session in the local session map, as we wouldn't get
                   * the session from memcached if the node was not available - or, the other way round,
                   * if we would get the session from memcached, the session would not have to be relocated.
                   */
                  final MemcachedBackupSession session = _manager.getSessionInternal( requestedSessionId );
  
                  if ( session != null && session.isValid() ) {
                  	final String newSessionId = _memcachedNodesManager.getNewSessionIdIfNodeFromSessionIdUnavailable( session.getId() );
                      if ( newSessionId != null ) {
                          _log.debug( "Session needs to be relocated, setting new id on session..." );
                          session.setIdForRelocate( newSessionId );
                          _statistics.requestWithMemcachedFailover();
                          return newSessionId;
                      }
                  }
              } else {
  
                  /* for non-sticky sessions we check the validity info
                   */
                  final String nodeId = getSessionIdFormat().extractMemcachedId( requestedSessionId );
                  if ( nodeId == null || _memcachedNodesManager.isNodeAvailable( nodeId ) ) {
                      return null;
                  }
  
                  _log.info( "Session needs to be relocated as node "+ nodeId +" is not available, loading backup session for " + requestedSessionId );
                  final MemcachedBackupSession backupSession = loadBackupSession( requestedSessionId );
                  if ( backupSession != null ) {
                      _log.debug( "Loaded backup session for " + requestedSessionId + ", adding locally with "+ backupSession.getIdInternal() +"." );
                      addValidLoadedSession( backupSession, true );
                      _statistics.requestWithMemcachedFailover();
                      return backupSession.getId();
                  }
              }
  
          } catch ( final RuntimeException e ) {
              _log.warn( "Could not find session in local session map.", e );
          }
          return null;
      }