Concurrency control-locks Dbms Sem-IV
Lock-Based Protocols
Oneway to ensure isolation is to require that data items be accessed in a mutually
exclusive manner; that is, while one transaction is accessing a data item, no
other transaction can modify that data item. The most common method used to
implement this requirement is to allow a transaction to access a data item only if
it is currently holding a lock on that item. We introduced the concept of locking
in
Locks
There are various modes in which a data item may be locked. In this section, we
restrict our attention to two modes:
1. Shared. If a transaction Ti has obtained a shared-mode lock (denoted by S)
on item Q, then Ti can read, but cannot write, Q.
2. Exclusive. If a transaction Ti has obtained an exclusive-mode lock (denoted
by X) on item Q, then Ti can both read and write Q.