public function getCluster($name, $db = 0, $replicas = 128, $selectRandomSlave = true, $writeOnly = false)
    {
        if (!isset($this->_cluster[$name])) {
            $this->_cluster[$name] = $this->createCluster($name, $db, $replicas, $selectRandomSlave, $writeOnly);
        }
        return $this->_cluster[$name];
    }