Replication on demand helps in the scenario where each node works with different pieces of data and will be combined at logical steps.
对于每个节点处理不同部分数据并在逻辑步骤中整合这些数据的情况，随需复制是很有帮助的。
During failover, one of the children becomes the master node and all replication starts flowing from that node.
在故障恢复过程中，其中一个子节点会成为主节点，而所有复制将从该节点开始。
With synchronous replication, the write on the production node does not complete until it has been received on the target node, so you are guaranteed identical copies on each side.
对于同步复制，目标节点不接收到此写入操作，生产节点上的写入操作就无法完成，这样可以保证两边的复制一致。
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