分布式数据库管理系统可靠性：2PC协议及其变体与故障处理

### 分布式数据库管理系统可靠性：2PC协议及其变体与故障处理 #### 1. 2PC协议基础 2PC（两阶段提交）协议是分布式事务处理中的重要协议，包含协调者（2PC - C）和参与者（2PC - P）两部分。 ##### 1.1 2PC协调者算法（2PC - C） ```plaintext Algorithm 5.6: 2PC Coordinator (2PC - C) begin repeat wait for an event switch event do case Msg Arrival do Let the arrived message be msg switch msg do case Commit do {commit command from scheduler} write begin_commit record in the log send “Prepared” message to all the involved participants set timer end case case Vote - abort do {one participant has voted to abort; unilateral abort} write abort record in the log send “Global - abort” message to the other involved participants set timer end case case Vote - commit do update the list of participants who have answered if all the participants have answered then {all must have voted to commit} write commit record in the log send “Global - commit” to all the involved participants set timer end if end case case Ack do update the list of participants who have acknowledged if all the participants have acknowledged then write end_of_transaction record in the log else send global decision to the unanswering participants end if end case end switch end case case Timeout do execute the termination protocol end case end switch until forever end ``` 其流程如下： 1. 等待事件发生。 2. 若收到消息： - 若为提交命令，记录开始提交信息，向参与者发送准备消息并设置定时器。 - 若有参与者投票中止，记录中止信息，向其他参与者发送全局中止消息并设置定时器。 - 若收到投票提交，更新已回复参与者列表，若全部回复则记录提交信息，发送全局提交消息并设置定时器。 - 若收到确认消息，更新已确认参与者列表，若全部确认则记录事务结束信息，否则向未回复者发送全局决策。 3. 若超时，则执行终止协议。 ##### 1.2 2PC参与者算法（2PC - P） ```plaintext Algorithm 5.7: 2PC Participant (2PC - P) begin repeat wait for an event switch ev do case Msg Arrival do Let the arrived message be msg switch msg do case Prepare do {Prepare command from the coordinator} if ready to commi ```