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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


Two Phase Locking (2PL)

The two-phase locking protocol consists of two phases:

  • Phase 1: The Lock Acquisition Phase:
    If a transaction T wants to read an object, it needs to obtain the S(shared) lock. If T wants to modify an object, it needs to obtain X(exclusive) lock. No conflicting locks are granted to a transaction.
    New locks on items can be acquired but no lock can be released till all the locks required by the transaction are obtained.

  • Phase 2: Lock Release Phase:
    The existing locks can be released in any order but no new lock can be acquired after a lock has been released. The locks are held only till they are required.