Distributed Timestamps A node with a slow clock will assign smaller timestamps Still logically correct:serializability not affected ·But:“disadvantages”transactions 。To fix this problem Keep clocks synchronized using network time protocol Or,define within each node N;a logical clock(LC),which generates the unique local timestamp Require that N,advance its logical clock whenever a request is received from a transaction Ti with timestamp x,y>and x is greater that the current value of LC. In this case,site N,advances its logical clock to the value x+1 Database System Concepts-7th Edition 23.35 ©Silberscha乜，Korth and SudarshanDatabase System Concepts - 7 23.35 ©Silberschatz, Korth and Sudarshan th Edition Distributed Timestamps ▪ A node with a slow clock will assign smaller timestamps • Still logically correct: serializability not affected • But: “disadvantages” transactions ▪ To fix this problem • Keep clocks synchronized using network time protocol • Or, define within each node Ni a logical clock (LCi ), which generates the unique local timestamp ▪ Require that Ni advance its logical clock whenever a request is received from a transaction Ti with timestamp < x,y> and x is greater that the current value of LCi . ▪ In this case, site Ni advances its logical clock to the value x + 1