code: public void setPrimary(InstanceId ofPrimary) throws IOException {
      if (ofPrimary == null) {
        // failover in progress
        primaryNode = null;
        standbyNode = null;
        return;
      }
      switch (ofPrimary) {
        case NODEZERO:
          primaryNode = getNodeZero();
          standbyNode = getNodeOne();
        case NODEONE:
          primaryNode = getNodeOne();
          standbyNode = getNodeZero();
      }
    }
docs: The purpose of this method is to update in memory references to primary
  and standby. Please override this method and do the zookeeper specific
  logic in the method and call this method on successful zookeeper write.