Explain the problems that can occur when concurrent transaction are executed give examples Database Management Systems 10CS54 VTU notes JULY 2013 Jan 2014July 2015

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Explain the problems that can occur when concurrent transaction are executed give examples. Database Management Systems - 10CS54 VTU notes (JULY 2013/ Jan 2014/July 2015)

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Why Concurrency Control is needed: 
The Lost Update Problem This occurs when two transactions that access the same database items have their operations interleaved in a way that makes the value of some database item incorrect. The Temporary Update (or Dirty Read) Problem This occurs when one transaction updates a database item and then the transaction fails for some reason (see Section 17.1.4). The updated item is accessed by another transaction before it is changed back to its original value.
 The Incorrect Summary Problem If one transaction is calculating an aggregate summary function on a number of records while other transactions are updating some of these records, the aggregate function may calculate some values before they are updated and others after they are updated Two-Phase Locking Techniques
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-Phase Locking Techniques: Essential components 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. Conflict matrix conflict matrix
Two-Phase Locking Techniques: Essential components 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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