Chapter 15 - Concurrency control. This chapter focuses on concurrency control and presents several techniques for ensuring serializability, including locking, timestamping, and optimistic (validation) techniques. The chapter also covers deadlock issues. Alternatives to serializability are covered, most notably the widely-used snapshot isolation, which is discussed in detail. | Chapter 15 : Concurrency Control Database System Concepts, 6th Ed. ©Silberschatz, Korth and Sudarshan See for conditions on re-use Chapter 15: 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 - 6th Edition ©Silberschatz, Korth and Sudarshan 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. Database System Concepts - 6th Edition ©Silberschatz, Korth and Sudarshan 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. Database System Concepts - 6th Edition ©Silberschatz, Korth and Sudarshan 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 .
