数据库课件lec11transaction幻灯片.pptVIP

* * * * * * * * * * * * * * * * * * * * * * * * * * * * Lock_X(A) Read(A) Lock_S(A) A: = A-50 Write(A) Lock_X(B) Unlock(A) Read(A) Lock_S(B) Read(B) B := B +50 Write(B) Unlock(B) Unlock(A) Read(B) Unlock(B) PRINT(A+B) 2PL, A= 1000, B=2000, Output =? Lock_X(A) Read(A) Lock_S(A) A: = A-50 Write(A) Lock_X(B) Read(B) B := B +50 Write(B) Unlock(A) Unlock(B) Read(A) Lock_S(B) Read(B) PRINT(A+B) Unlock(A) Unlock(B) Strict 2PL, A= 1000, B=2000, Output =? Venn Diagram for Schedules All Schedules Avoid Cascading Abort Serial View Serializable Conflict Serializable Which schedules does Strict 2PL allow? All Schedules Avoid Cascading Abort Serial View Serializable Conflict Serializable Lock Management Lock and unlock requests handled by Lock Manager. LM keeps an entry for each currently held lock. Entry contains: List of xacts currently holding lock Type of lock held (shared or exclusive) Queue of lock requests Lock Management (Contd.) When lock request arrives: Does any other transaction hold a conflicting lock? If no, grant the lock. If yes, put requestor into wait queue. Lock upgrade: A transaction with shared lock can request to upgrade to exclusive. Lock_X(A) Lock_S(B) Read(B) Lock_S(A) Read(A) A: = A-50 Write(A) Lock_X(B) Example Deadlocks Deadlock: Cycle of transactions waiting for locks to be released by each other. Ways of dealing with deadlocks: prevention detection avoidance Many systems just punt and use Timeouts What are the dangers with this approach? Deadlock Prevention Common technique in operating systems Standard approach: resource ordering Screen Network Card Printer Why is this problematic for Xacts in a DBMS? Deadlock Detection Create and maintain a “waits-for” graph Periodically check for cycles in graph Deadlock Detection (Continued) Example: T1: S(A), S(D), S(B) T2: X(B), X(C) T3: S(D), S(C), X(A) T4: X(B) T1 T2 T4 T3 Deadlock Avoidance Assign priorities based on timestamps. Say T