Chapter 18-6
Copyright © 2004 Ramez Elmasri and Shamkant Navathe
Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition
Database Concurrency Control
lTwo-Phase Locking Techniques: Essential components
l 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