Node Failure Models
- fail-stop
crash and never recover - fail-recover
crash and later recover - byzantine failure
Cases
2 phase commit，n个节点，那么需要3n个消息交换
coordinator发送proposal后crash
- 有的node收到，有的没收到
- 收到Proposal的node被block forever，它可能已经vote commit了
不能简单地timeout/abort，因为coordinator可能随时recover并启动phase2 commit
这个txn就只能blocked by coordinator，cannot make any progress - 解决办法
引入coordinator的watchdog机制，它发现coordinator crash后，接管
Phase1. 先询问每个participants，已经vote commit还是vote abort还是没有vote
Phase2. 通知每个participant Commit/Abort
但仍有局限，如果有个participant crash了，那么Phase1无法确认
worse case
coordinator本身也是participant
3PC
在propose和commit这2个phase中间，加了个prepare to commit
- 如果coordinator在prepare to commit或者proposal阶段crash
trx aborted - 如果coordinator在commit阶段crash
nodes will timeout waiting for the commit phase and commit the trx