Using Consensus to Avoid Blocking After getting response from 2PC participants,coordinator can initiate distributed consensus protocol by sending its decision to a set of participants who then use consensus protocol to commit the decision If coordinator fails before informing all consensus participants Choose a new coordinator,which follows 2PC protocol for failed coordinator If a commit/abort decision was made as long as a majority of consensus participants are accessible,decision can be found without blocking If consensus process fails(e.g.,split vote),restart the consensus Split vote can happen if a coordinator send decision to some participants and then fails,and new coordinator send a different decision The three phase commit protocol is an extension of 3PC which avoids blocking under certain assumptions Ideas are similar to distributed consensus. Consensus is also used to ensure consistency of replicas of a data item Details later in the chapter Database System Concepts-7th Edition 23.16 ©Silberscha乜，Korth and SudarshanDatabase System Concepts - 7 23.16 ©Silberschatz, Korth and Sudarshan th Edition Using Consensus to Avoid Blocking ▪ After getting response from 2PC participants, coordinator can initiate distributed consensus protocol by sending its decision to a set of participants who then use consensus protocol to commit the decision • If coordinator fails before informing all consensus participants ▪ Choose a new coordinator, which follows 2PC protocol for failed coordinator ▪ If a commit/abort decision was made as long as a majority of consensus participants are accessible, decision can be found without blocking • If consensus process fails (e.g., split vote), restart the consensus ▪ Split vote can happen if a coordinator send decision to some participants and then fails, and new coordinator send a different decision ▪ The three phase commit protocol is an extension of 3PC which avoids blocking under certain assumptions • Ideas are similar to distributed consensus. ▪ Consensus is also used to ensure consistency of replicas of a data item • Details later in the chapter