what is lock what are different types of locks in database management system
A lock is a variable associated with a data item that describes the status of the item with respect to possible operations that can be applied to it. Generally, there is one lock for each data item in the database.Locks are used as a means of synchronizing the access by concurrent transactions to the database item. Types of Locks. Several types of locks are used in concurrency control.inary Locks. A binary lock can have two states or values: locked and unlocked. A distinct lock is associated with each database item A. If the value of the lock on A is 1, item A cannot be accessed by a database operation that requests the item.
Lock modes specify different kinds of locks. The choice of which lock mode to apply depends on the resource that needs to be locked. The following three lock types are used for row-and page-level locking: 1  Shared (S). 2  Exclusive (X).3  Update (U).nce an exclusive lock is set for the page (or row), no other lock can be placed on the same resource. NOTE. The database system automatically chooses the appropriate lock mode according to the operation type (read or write). An update lock can be placed only if no other update or exclusive lock exists.