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
Pitfalls of Lock-Based Protocols (Cont.) The potential for deadlock exists in most locking protocols.Deadlocks are a necessary evil. Starvation is also possible if concurrency control manager is badly designed.For example: A transaction may be waiting for an X-lock on an item,while a sequence of other transactions request and are granted an S-lock on the same item. The same transaction is repeatedly rolled back due to deadlocks. Concurrency control manager can be designed to prevent starvation. Database System Concepts-5th Edition,Oct 5,2006 16.7 @Silberschatz,Korth and Sudarshan
Database System Concepts - 5 16.7 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 Pitfalls of Lock-Based Protocols (Cont.) The potential for deadlock exists in most locking protocols. Deadlocks are a necessary evil. Starvation is also possible if concurrency control manager is badly designed. For example: A transaction may be waiting for an X-lock on an item, while a sequence of other transactions request and are granted an S-lock on the same item. The same transaction is repeatedly rolled back due to deadlocks. Concurrency control manager can be designed to prevent starvation
The Two-Phase Locking Protocol This is a protocol which ensures conflict-serializable schedules. Phase 1:Growing Phase transaction may obtain locks transaction may not release locks Phase 2:Shrinking Phase transaction may release locks transaction may not obtain locks The protocol assures serializability.It can be proved that the transactions can be serialized in the order of their lock points (i.e. the point where a transaction acquired its final lock). Database System Concepts-5th Edition,Oct 5,2006 16.8 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 5 16.8 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 The Two-Phase Locking Protocol This is a protocol which ensures conflict-serializable schedules. Phase 1: Growing Phase transaction may obtain locks transaction may not release locks Phase 2: Shrinking Phase transaction may release locks transaction may not obtain locks The protocol assures serializability. It can be proved that the transactions can be serialized in the order of their lock points (i.e. the point where a transaction acquired its final lock)
The Two-Phase Locking Protocol (Cont.) Two-phase locking does not ensure freedom from deadlocks Cascading roll-back is possible under two-phase locking.To avoid this, follow a modified protocol called strict two-phase locking.Here a transaction must hold all its exclusive locks till it commits/aborts. Rigorous two-phase locking is even stricter:here all locks are held till commit/abort.In this protocol transactions can be serialized in the order in which they commit. Database System Concepts-5th Edition,Oct 5,2006 16.9 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 5 16.9 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 The Two-Phase Locking Protocol (Cont.) Two-phase locking does not ensure freedom from deadlocks Cascading roll-back is possible under two-phase locking. To avoid this, follow a modified protocol called strict two-phase locking. Here a transaction must hold all its exclusive locks till it commits/aborts. Rigorous two-phase locking is even stricter: here all locks are held till commit/abort. In this protocol transactions can be serialized in the order in which they commit
The Two-Phase Locking Protocol (Cont.) There can be conflict serializable schedules that cannot be obtained if two-phase locking is used. However,in the absence of extra information(e.g.,ordering of access to data),two-phase locking is needed for conflict serializability in the following sense: Given a transaction T that does not follow two-phase locking,we can find a transaction T that uses two-phase locking,and a schedule for T; and T that is not conflict serializable. Database System Concepts-5th Edition,Oct 5,2006 16.10 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 5 16.10 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 The Two-Phase Locking Protocol (Cont.) There can be conflict serializable schedules that cannot be obtained if two-phase locking is used. However, in the absence of extra information (e.g., ordering of access to data), two-phase locking is needed for conflict serializability in the following sense: Given a transaction Ti that does not follow two-phase locking, we can find a transaction Tj that uses two-phase locking, and a schedule for Ti and Tj that is not conflict serializable
Lock Conversions Two-phase locking with lock conversions: First Phase: can acquire a lock-S on item can acquire a lock-X on item can convert a lock-S to a lock-X(upgrade) Second Phase: can release a lock-S can release a lock-X can convert a lock-X to a lock-S (downgrade) This protocol assures serializability.But still relies on the programmer to insert the various locking instructions. Database System Concepts-5th Edition,Oct 5,2006 16.11 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 5 16.11 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 Lock Conversions Two-phase locking with lock conversions: – First Phase: can acquire a lock-S on item can acquire a lock-X on item can convert a lock-S to a lock-X (upgrade) – Second Phase: can release a lock-S can release a lock-X can convert a lock-X to a lock-S (downgrade) This protocol assures serializability. But still relies on the programmer to insert the various locking instructions