viewquestions/12479/ Write a short Notes Database Management Systems 10CS54 VTU notes Dec Jan 11Jan 2016

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. Write a short Notes Database Management Systems - 10CS54 VTU notes (Dec /Jan 11/Jan 2016)

a) 2PL Lock: 



By:anonymousguy7799

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Answers

a) 2PL Lock: Locking is an operation which secures
(a) permission to Read or (b) permission to Write a data item for a transaction. Example: Lock (X). Data item X is locked in behalf of the requesting transaction.
 Unlocking is an operation which removes these permissions from the data item. 
Example: Unlock (X). Data item X is made available to all other transactions. Lock and Unlock are Atomic operations. Two locks modes (a) shared (read) and (b) exclusive (write). Shared mode: shared lock (X). 
More than one transaction can apply share lock on X for reading its value but no write lock can be applied on X by any other transaction. Exclusive mode: Write lock (X). Only one write lock on X can exist at any time and no shared lock can be applied by any other transaction on X. Lock Manager: Managing locks on data items. Lock table: Lock manager uses it to store the identify of transaction locking a data item, the data item, lock mode and pointer to the next data item locked. One simple way to implement a lock table is through linked list.  

Nikhil-bharadwaj

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