Commit Protocols Commit protocols are used to ensure atomicity across sites a transaction which executes at multiple sites must either be committed at all the sites,or aborted at all the sites. cannot have transaction committed at one site and aborted at another The two-phase commit(2PC)protocol is widely used ■ Three-phase commit(3PC)protocol avoids some drawbacks of 2PC,but is more complex Consensus protocols solve a more general problem,but can be used for atomic commit More on these later in the chapter The protocols we study all assume fail-stop model-failed sites simply stop working,and do not cause any other harm,such as sending incorrect messages to other sites. Protocols that can tolerate some number of malicious sites discussed in bibliographic notes online Database System Concepts-7th Edition 23.6 ©Silberscha乜，Korth and SudarshanDatabase System Concepts - 7 23.6 ©Silberschatz, Korth and Sudarshan th Edition Commit Protocols ▪ Commit protocols are used to ensure atomicity across sites • a transaction which executes at multiple sites must either be committed at all the sites, or aborted at all the sites. ▪ cannot have transaction committed at one site and aborted at another ▪ The two-phase commit (2PC) protocol is widely used ▪ Three-phase commit (3PC) protocol avoids some drawbacks of 2PC, but is more complex ▪ Consensus protocols solve a more general problem, but can be used for atomic commit • More on these later in the chapter ▪ The protocols we study all assume fail-stop model – failed sites simply stop working, and do not cause any other harm, such as sending incorrect messages to other sites. • Protocols that can tolerate some number of malicious sites discussed in bibliographic notes online