False Cycles (Cont.) Suppose that starting from the state shown in figure, 1.T2 releases resources at Si resulting in a message remove T1->T2 message from the Transaction Manager at site S1 to the coordinator) 2.And then T2 requests a resource held by T3 at site S2 resulting in a message insert 72->T3 from S2 to the coordinator ■ Suppose further that the insert message reaches before the delete message this can happen due to network delays The coordinator would then find a false cycle T1→T2→T3→T1 The false cycle above never existed in reality. False cycles cannot occur if two-phase locking is used. Database System Concepts-7th Edition 23.30 ©Silberscha乜，Korth and SudarshanDatabase System Concepts - 7 23.30 ©Silberschatz, Korth and Sudarshan th Edition False Cycles (Cont.) ▪ Suppose that starting from the state shown in figure, 1. T2 releases resources at S1 ▪ resulting in a message remove T1 → T2 message from the Transaction Manager at site S1 to the coordinator) 2. And then T2 requests a resource held by T3 at site S2 ▪ resulting in a message insert T2 → T3 from S2 to the coordinator ▪ Suppose further that the insert message reaches before the delete message • this can happen due to network delays ▪ The coordinator would then find a false cycle T1 → T2 → T3 → T1 ▪ The false cycle above never existed in reality. ▪ False cycles cannot occur if two-phase locking is used