©Silberschatz, Korth and Sudarshan16.3Database System Concepts - 5
th
Edition, Sep 12, 2005
Lock-Based Protocols
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.
