Chapter 18-35
Copyright © 2004 Ramez Elmasri and Shamkant Navathe
Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition
Database Concurrency Control
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lMultiversion Two-Phase Locking Using Certify Locks
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Steps
1.X is the committed version of a data item.
2.T creates a second version X’ after obtaining a write lock on X.
3.Other transactions continue to read X.
4.T is ready to commit so it obtains a certify lock on X’.
5.The committed version X becomes X’.
6.T releases its certify lock on X’, which is X now.
read/write locking scheme          read/write/certify locking scheme
Compatibility tables for