注：部分内容参考自https://blog.csdn.net/lezg_bkbj/article/details/52149863。 

Two Phase Commit is an algorithm that coordinates transactions between one coordinator (Master) and many slaves. Transactions that change the state of the slave are considered TPC transactions and must be logged and tracked according to the TPC algorithm. TPC ensures atomicity and durability by ensuring that a write happens across ALL replicas or NONE of them. 

二阶段提交的算法思路可以概括为：参与者将操作成败通知协调者，再由协调者根据所有参与者的反馈情报决定各参与者是否要提交操作还是中止操作。 

所谓的两个阶段是指：第一阶段：准备阶段(投票阶段)和第二阶段：提交阶段（执行阶段）。

第一阶段：准备阶段

1）协调者节点向所有参与者节点询问是否可以执行提交操作(vote)，并开始等待各参与者节点的响应。

2）参与者节点执行询问发起为止的所有事务操作，并将Undo信息和Redo信息写入日志。（注意：若成功这里其实每个参与者已经执行了事务操作）。

3）各参与者节点响应协调者节点发起的询问。如果参与者节点的事务操作实际执行成功，则它返回一个”同意”消息；如果参与者节点的事务操作实际执行失败，则它返回一个”中止”消息。

第二阶段：提交阶段

如果协调者收到了参与者的失败消息或者超时，直接给每个参与者发送回滚(Rollback)消息；否则，发送提交(Commit)消息；参与者根据协调者的指令执行提交或者回滚操作，释放所有事务处理过程中使用的锁资源。(注意:必须在最后阶段释放锁资源）。

If at least one slave votes to abort, the master sends a GLOBAL-ABORT. If all slaves vote to commit, the master sends GLOBAL-COMMIT. Whenever a master receives a response from a slave, it may assume that the previous request has been recognized and committed to log and is therefore fault tolerant. (If the master receives a VOTE, the master can assume that the slave has logged the action it is voting on. If the master receives an ACK for a GLOBAL-COMMIT, it can
assume that action has been executed, saved, and logged such that it will remain consistent even if the slave dies and rebuilds.)