tailieunhanh - Lecture Database system concepts - Chapter 16: Concurrency control

This chapter describes how to control concurrent execution in a database, in order to ensure the isolation properties of transactions. A variety of protocols are described for this purpose. This chapter presents the following content: Lock­based protocols, timestamp­based protocols, validation­based protocols, multiple granularity,. | Chapter 16 : Concurrency Control Version: Oct 5, 2006 Database System Concepts 5th Ed. © Silberschatz, Korth and Sudarshan, 2005 See ­ for conditions on re­use Chapter 16: Concurrency Control s Lock­Based Protocols s Timestamp­Based Protocols s Validation­Based Protocols s Multiple Granularity s Multiversion Schemes s Insert and Delete Operations s Concurrency in Index Structures Database System Concepts ­ 5th Edition 16. ©Silberschatz, Korth and Sudarshan Lock­Based Protocols s A lock is a mechanism to control concurrent access to a data item s Data items can be locked in two modes : 1. exclusive (X) mode. Data item can be both read as well as written. X­lock is requested using lock­X instruction. 2. shared (S) mode. Data item can only be read. S­lock is requested using lock­S instruction. s Lock requests are made to concurrency­control manager. Transaction can proceed only after request is granted. Database System Concepts ­ 5th Edition 16. ©Silberschatz, Korth and Sudarshan Lock­Based Protocols (Cont.) s Lock­compatibility matrix s 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 s Any number of transactions can hold shared locks on an item, q but if any transaction holds an exclusive on the item no other transaction may hold any lock on the item. s 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 16. ©Silberschatz, Korth and Sudarshan Lock­Based Protocols (Cont.) s .

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• Database system concepts
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