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DBMS BCA Unit-5 Transaction - Definition Transaction - Examples COMMIT and ROLLBACK Properties of a Transaction States of Transactions The Concurrent Transactions Transactions - Lost Updates Transactions - Unrepeatable Reads Transactions - Dirty Reads Transactions - Inconsistent Analysis Serialisable Schedule Serial and Interleaved Schedule Precedence Graph Algorithm Precedence Graph Construction Non Serializable Schedule Locking Protocol Locks and Types of Locks Binary Locks Multiple-Mode Locks Shared and Exclusive Locks Two Phase Locking (2PL) Types of Two Phase Locking Database Security Goals Types of Database Security Threats to Databases Database Security & Authorization Subsystem Discretionary Security Mechanisms Mandatory Security Mechanisms Control Measures Access Control Inference Control Flow Control Data Encryption Database Security and the DBA DBA - Privileged Commands DBA Role Access Control, User Accounts and Database Audits System Log Database Audit Audit Trail Sensitive Data and Types of Disclosures Sensitive Data - Classification Factors Factors - Data is Safe Discretionary Access Control in a Database Account Level Privileges Relation Level Privileges Access Matrix Model Privileges on Relations


The Concurrent Transactions

Almost all the commercial DBMS support multi-user environment. Thus, allowing multiple transactions to proceed simultaneously.

The concurrency related problem may occur in databases only if “two transactions are contending for the same data item and at least one of the concurrent transactions wishes to update a data value in the database”. In case, the concurrent transactions only read same data item and no updates are performed on these values, then it does not cause any concurrency related problem. 

Consider Transaction T1:
	Read X
	Subtract 100
	Write X

	Read Y
	Add 100
	Write Y

Consider Transaction T2 :
	Read X
	Read Y
	Display X + Y

If both of these transactions are issued simultaneously, then the execution of these instructions can be mixed in many ways. This is called Schedule.

A schedule is defined as the sequential ordering of the operations of n interleaved transactions. A schedule maintains the order of operations within the individual transaction. In a schedule, two operations of different transactions conflict if they access the same data item and one of them is a write operation.