Locking Methods of Concurrency Control
"A lock is a variable that is used to control the access of the data item."
Locking is the mostly used in the concurrency control. Lock is used when multiple users need to access a database concurrently. This protect data from being corrupted when multiple users try to read while others write to the database.
Locks are divided into three fields:
- Lock Granularity
- Lock Types
- Deadlocks
Lock Granularity-: The size of the data item chosen as the unit of protection by a concurrency control program is called GRANULARITY and it also indicates the level of lock use.
Locking can take place at following table.
Ø Database level.
Ø Table level.
Ø Page level.
Ø Row (Tuple) level.
Ø Attributes (fields) level.
Database level-: In this entire database is locked
Table level-: Entire table is locked.
Page level-: Entire disk page is locked.
Row (Tuple) level-: Allow concurrent transaction to access different rows of same table. Even if rows are located on the same page.
Attributes (fields) level-: Allow concurrent transaction to access same rows. As long as they require use of different field (attributes) within that row.
Lock Types-: The DBMS mainly uses following types of locking techniques.
- Binary Locking
- Shared / Exclusive Locking
- Two - Phase Locking (2PL)
Binary Locking-: It have only two states
- Locked
- Unlocked
Shared Locking -: These locks are known as read locks, and it is denoted by 'S'. Concurrent transaction are granted read access on the basis of a common lock.
Exclusive Locking-: Access is specifically reserved for transaction that locked object. These locks are referred as Write locks, and denoted by 'X'.
Two - Phase Locking (2PL)-: It define how transaction acquire and relinquish locks. It ensure serializability but it does not prevent deadlocks.
Growing phase-: Transaction acquires all required locks without unlocking any data.
Shrinking phase-: Transaction release all the locks and cannot obtain any new lock.
Deadlocks-: It is the condition where two transactions are waiting for each other to unlock data.
Controlling of Deadlock.
Ø Avoidance
Ø Prevention
