环形 sql 主从同步， 3 台机子挂掉其中一台该怎么办

在“Designing Data-Intensive Applications - CHAPTER 5 Replication - Multi-Leader Replication - Multi-Leader Replication Topologies”中，介绍了三种不同 replication topologies：1. circular topology 2. star topology 3. all-to-all topology 。

它说“A problem with circular and star topologies is that if just one node fails, it can interrupt the flow of replication messages between other nodes, causing them to be unable to communicate until the node is fixed. The topology could be reconfigured to work around the failed node, but in most deployments such reconfiguration would have to be done manually. The fault tolerance of a more densely connected topology (such as all-to-all) is better because it allows messages to travel along different paths, avoiding a single point of failure.”，也就是说如果你继续采用 circular topology 的话，如果其中一个节点失败了，你可以重新配置 topology，你也可以选择容错性更好的 all-to-all topology 。

当然不同的 topologies 都有各自的优缺点，具体的话，你可以看一下“Designing Data-Intensive Applications”这本书。

顺便说一下，你这个根本不是主从同步，而是 Multi-Leader Replication 。