Chapter 16:Concurrency Control Lock-Based Protocols Timestamp-Based Protocols Validation-Based Protocols Multiple Granularity Multiversion Schemes Insert and Delete Operations Concurrency in Index Structures Database System Concepts-5th Edition,Oct 5,2006 16.2 @Silberschatz,Korth and Sudarshan
Database System Concepts - 5 16.2 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 Chapter 16: Concurrency Control Lock-Based Protocols Timestamp-Based Protocols Validation-Based Protocols Multiple Granularity Multiversion Schemes Insert and Delete Operations Concurrency in Index Structures
Lock-Based Protocols A lock is a mechanism to control concurrent access to a data item Data items can be locked in two modes: 1.exclusive (mode.Data item can be both read as well as written.X-lock is requested using lock-X instruction. 2.shared (S)mode.Data item can only be read.S-lock is requested using lock-S instruction. Lock requests are made to concurrency-control manager.Transaction can proceed only after request is granted. Database System Concepts-5th Edition,Oct 5,2006 16.3 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 5 16.3 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 Lock-Based Protocols A lock is a mechanism to control concurrent access to a data item Data items can be locked in two modes : 1. exclusive (X) mode. Data item can be both read as well as written. X-lock is requested using lock-X instruction. 2. shared (S) mode. Data item can only be read. S-lock is requested using lock-S instruction. Lock requests are made to concurrency-control manager. Transaction can proceed only after request is granted
Lock-Based Protocols (Cont.) Lock-compatibility matrix S X S true false X false false Atransaction may be granted a lock on an item if the requested lock is compatible with locks already held on the item by other transactions Any number of transactions can hold shared locks on an item, but if any transaction holds an exclusive on the item no other transaction may hold any lock on the item. If a lock cannot be granted,the requesting transaction is made to wait till all incompatible locks held by other transactions have been released. The lock is then granted. Database System Concepts-5th Edition,Oct 5,2006 16.4 @Silberschatz,Korth and Sudarshan
Database System Concepts - 5 16.4 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 Lock-Based Protocols (Cont.) Lock-compatibility matrix A transaction may be granted a lock on an item if the requested lock is compatible with locks already held on the item by other transactions Any number of transactions can hold shared locks on an item, but if any transaction holds an exclusive on the item no other transaction may hold any lock on the item. If a lock cannot be granted, the requesting transaction is made to wait till all incompatible locks held by other transactions have been released. The lock is then granted
Lock-Based Protocols (Cont.) Example of a transaction performing locking: T2:lock-S(A); read (A); unlock(A); lock-S(B); read (B); unlock(B); display(A+B) Locking as above is not sufficient to guarantee serializability-if A and B get updated in-between the read of A and B,the displayed sum would be wrong. A locking protocol is a set of rules followed by all transactions while requesting and releasing locks.Locking protocols restrict the set of possible schedules. Database System Concepts-5th Edition,Oct 5,2006 16.5 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 5 16.5 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 Lock-Based Protocols (Cont.) Example of a transaction performing locking: T2 : lock-S(A); read (A); unlock(A); lock-S(B); read (B); unlock(B); display(A+B) Locking as above is not sufficient to guarantee serializability — if A and B get updated in-between the read of A and B, the displayed sum would be wrong. A locking protocol is a set of rules followed by all transactions while requesting and releasing locks. Locking protocols restrict the set of possible schedules
Pitfalls of Lock-Based Protocols Consider the partial schedule T3 Ta lock-X(B) read(B) B:=B-50 write(B) lock-S(A) read(A) lock-S(B) lock-X(A) Neither T3 nor T4 can make progress-executing lock-S(B)causes T4 to wait for T3 to release its lock on B,while executing lock-X(A)causes T3 to wait for T4 to release its lock on A. Such a situation is called a deadlock. To handle a deadlock one of Ta or T4 must be rolled back and its locks released. Database System Concepts-5th Edition,Oct 5,2006 16.6 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 5 16.6 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 Pitfalls of Lock-Based Protocols Consider the partial schedule Neither T3 nor T4 can make progress — executing lock-S(B) causes T4 to wait for T3 to release its lock on B, while executing lock-X(A) causes T3 to wait for T4 to release its lock on A. Such a situation is called a deadlock. To handle a deadlock one of T3 or T4 must be rolled back and its locks released