注：部分內容參考自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. 

二階段提交的演算法思路可以概括為：參與者將操作成敗通知協調者，再由協調者根據所有參與者的反饋情報決定各參與者是否要提交操作還是中止操作。 

所謂的兩個階段是指：第一階段：準備階段(投票階段)和第二階段：提交階段（執行階段）。

第一階段：準備階段

1）協調者節點向所有參與者節點詢問是否可以執行提交操作(vote)，並開始等待各參與者節點的響應。

2）參與者節點執行詢問發起為止的所有事務操作，並將Undo資訊和Redo資訊寫入日誌。（注意：若成功這裡其實每個參與者已經執行了事務操作）。

3）各參與者節點響應協調者節點發起的詢問。如果參與者節點的事務操作實際執行成功，則它返回一個”同意”訊息；如果參與者節點的事務操作實際執行失敗，則它返回一個”中止”訊息。

第二階段：提交階段

如果協調者收到了參與者的失敗訊息或者超時，直接給每個參與者傳送回滾(Rollback)訊息；否則，傳送提交(Commit)訊息；參與者根據協調者的指令執行提交或者回滾操作，釋放所有事務處理過程中使用的鎖資源。(注意:必須在最後階段釋放鎖資源）。

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.)