《数据库系统概念 Database System Concepts》原书教学资源（第七版，PPT课件讲稿，英文版）Chapter 18 Concurrency Control

Outline Lock-Based Protocols Timestamp-Based Protocols Validation-Based Protocols ■ Multiple Granularity Multiversion Schemes Insert and Delete Operations ■ Concurrency in Index Structures Database System Concepts-7th Edition 18.2 ©Silberscha乜，Korth and Sudarshan

Database System Concepts - 7 18.2 ©Silberschatz, Korth and Sudarshan th Edition Outline ▪ 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-7th Edition 18.3 ©Silberscha乜，Korth and Sudarshan

Database System Concepts - 7 18.3 ©Silberschatz, Korth and Sudarshan th Edition 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-Xinstruction. 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 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. Database System Concepts-7th Edition 18.4 @Silberschatz,Korth and Sudarshan

Database System Concepts - 7 18.4 ©Silberschatz, Korth and Sudarshan th Edition 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

Schedule With Lock Grants Grants omitted in rest of T T concurrency-control manager chapter lock-X(B) ·Assume grant grant-X(B,T) happens just before read(B) the next instruction B:=B-50 write(B) following lock unlock(B) request lock-S(A) grant-S(A4,T2) ■This schedule is not read() serializable (why?) unlock(4) lock-S(B) A locking protocol is a grant-S(B,T2) set of rules followed by read(B) unlock(B) all transactions while display(4+B) requesting and releasing lock-X(4) locks. grant-X(A,T) read(4) ■Locking protocols A:=A+50 enforce serializability by write() unlock() restricting the set of possible schedules. Database System Concepts-7th Edition 18.6 @Silberschatz,Korth and Sudarshan

Database System Concepts - 7 18.6 ©Silberschatz, Korth and Sudarshan th Edition Schedule With Lock Grants ▪ Grants omitted in rest of chapter • Assume grant happens just before the next instruction following lock request ▪ This schedule is not serializable (why?) ▪ A locking protocol is a set of rules followed by all transactions while requesting and releasing locks. ▪ Locking protocols enforce serializability by restricting the set of possible schedules

Deadlock Consider the partial schedule T3 Ta lock-X(B) read(B) B:=B-50 write(B) lock-S(A) read() lock-S(B) lock-X(4) ■ 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 Ta 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. Database System Concepts-7th Edition 18.7 ©Silberscha乜，Korth and Sudarshan

Database System Concepts - 7 18.7 ©Silberschatz, Korth and Sudarshan th Edition Deadlock ▪ 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

Deadlock (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-7th Edition 18.8 ©Silberscha乜，Korth and Sudarshan

Database System Concepts - 7 18.8 ©Silberschatz, Korth and Sudarshan th Edition Deadlock (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 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 Time 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-7th Edition 18.9 @Silberschatz,Korth and Sudarshan

Database System Concepts - 7 18.9 ©Silberschatz, Korth and Sudarshan th Edition The Two-Phase Locking Protocol ▪ 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). Time Locks

The Two-Phase Locking Protocol (Cont.) Two-phase locking does not ensure freedom from deadlocks Extensions to basic two-phase locking needed to ensure recoverability of freedom from cascading roll-back Strict two-phase locking:a transaction must hold all its exclusive locks till it commits/aborts. Ensures recoverability and avoids cascading roll-backs Rigorous two-phase locking:a transaction must hold al/locks till commit/abort. Transactions can be serialized in the order in which they commit. Most databases implement rigorous two-phase locking,but refer to it as simply two-phase locking Database System Concepts-7th Edition 18.10 ©Silberscha乜，Korth and Sudarshan

Database System Concepts - 7 18.10 ©Silberschatz, Korth and Sudarshan th Edition The Two-Phase Locking Protocol (Cont.) ▪ Two-phase locking does not ensure freedom from deadlocks ▪ Extensions to basic two-phase locking needed to ensure recoverability of freedom from cascading roll-back • Strict two-phase locking: a transaction must hold all its exclusive locks till it commits/aborts. ▪ Ensures recoverability and avoids cascading roll-backs • Rigorous two-phase locking: a transaction must hold all locks till commit/abort. ▪ Transactions can be serialized in the order in which they commit. ▪ Most databases implement rigorous two-phase locking, but refer to it as simply two-phase locking

The Two-Phase Locking Protocol (Cont.) T T Two-phase locking is not a necessary condition for serializability lock-X(B) There are conflict serializable read(B) schedules that cannot be obtained B:=B-50 if the two-phase locking protocol is write(B) used. unlock(B) In the absence of extra information lock-S(4) (e.g.,ordering of access to data),two- read() phase locking is necessary for conflict unlock() serializability in the following sense: lock-S(B) Given a transaction Ti that does not follow two-phase locking,we read(B) can find a transaction T;that uses unlock(B) two-phase locking,and a schedule display(A +B) for Ti and Ti that is not conflict lock-X(4) serializable. read() A:=A+50 write() unlock() Database System Concepts-7th Edition 18.11 @Silberschatz,Korth and Sudarshan

Database System Concepts - 7 18.11 ©Silberschatz, Korth and Sudarshan th Edition The Two-Phase Locking Protocol (Cont.) ▪ Two-phase locking is not a necessary condition for serializability • There are conflict serializable schedules that cannot be obtained if the two-phase locking protocol is used. ▪ In the absence of extra information (e.g., ordering of access to data), two￾phase locking is necessary 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

Locking Protocols Given a locking protocol (such as 2PL) A schedule S is legal under a locking protocol if it can be generated by a set of transactions that follow the protocol A protocol ensures serializability if all legal schedules under that protocol are serializable Database System Concepts-7th Edition 18.12 @Silberschatz,Korth and Sudarshan

Database System Concepts - 7 18.12 ©Silberschatz, Korth and Sudarshan th Edition Locking Protocols ▪ Given a locking protocol (such as 2PL) • A schedule S is legal under a locking protocol if it can be generated by a set of transactions that follow the protocol • A protocol ensures serializability if all legal schedules under that protocol are serializable