Database Systems: The Complete Book- P11: Database Systems and Database Design and Application courses offered at the junior, senior and graduate levels in Computer Science departments. Written by well-known computer scientists, this introduction to database systems offers a comprehensive approach, focusing on database design, database use, and implementation of database applications and database management systems | 980 CHAPTER 18. CONCURRENCY CONTROL 1. START the set of transactions that have started but not yet completed validation. For each transaction T in this set the scheduler maintains START T the time at which T started. 2. VAL the set of transactions that have been validated but not yet finished the writing of phase 3. For each transaction T in this set the scheduler maintains both START T and VAL T the time at which T validated. Note that VAL T is also the time at which T is imagined to execute in the hypothetical serial order of execution. 3. FIA the set of transactions that have completed phase 3. For these transactions T the scheduler records START T VAL T and FIX T the time at which T finished. In principle this set grows but as we shall see. we do not have to remember transaction T if FIN T START T for any active transaction U . for any U in START or E4L . The scheduler may thus periodically purge the FIN set to keep its size from growing beyond bounds. The Validation Rules If maintained by the scheduler the information of Section is enough for it to detect any potential violation of the assumed serial order of the transactions the order in which the transactions validate. To understand the rules let us first consider what can be wrong when we try to validate a transaction T. T reads X U writes X B . CONCURRENCY CONTROL BY VALIDATION 981 V cj rsiT n ws F is not empty in particular let it contain database element A . Then it is possible that U wrote A after T read A. In fact U may not even have written A yet. A situation where U wrote A but. not in time T is shown in Fig. . To interpret the figure note that the dotted lines connect the events in real time with the time at which they would have . occurred had transactions been executed at the moment thev validated i Sine we don t know whether or not T got to read b s value we must rollback T to avoid a risk that the actions of T and U will not be consistent with the assumed serial order.
