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. 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 15.4 ©Silberschat乜，Korth and SudarshanDatabase System Concepts - 6 15.4 ©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. 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