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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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24 Advanced Transaction Processing
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The McGraw Hill Companies, 2001
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Subtasks An interactive transaction may consist of a set of subtasks initiated by the user The user may wish to abort a subtask without necessarily causing the entire transaction to abort Recoverability It is unacceptable to abort a long-duration interactive transaction because of a system crash The active transaction must be recovered to a state that existed shortly before the crash so that relatively little human work is lost Performance Good performance in an interactive transaction system is de ned as fast response time This de nition is in contrast to that in a noninteractive system, in which high throughput (number of transactions per second) is the goal Systems with high throughput make ef cient use of system resources However, in the case of interactive transactions, the most costly resource is the user If the ef ciency and satisfaction of the user is to be optimized, response time should be fast (from a human perspective) In those cases where a task takes a long time, response time should be predictable (that is, the variance in response times should be low), so that users can manage their time well In Sections 2451 through 2455, we shall see why these ve properties are incompatible with the techniques presented thus far, and shall discuss how those techniques can be modi ed to accommodate long-duration interactive transactions
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The properties that we discussed make it impractical to enforce the requirement used in earlier chapters that only serializable schedules be permitted Each of the concurrency-control protocols of 16 has adverse effects on long-duration transactions: Two-phase locking When a lock cannot be granted, the transaction requesting the lock is forced to wait for the data item in question to be unlocked The duration of this wait is proportional to the duration of the transaction holding the lock If the data item is locked by a short-duration transaction, we expect that the waiting time will be short (except in case of deadlock or extraordinary system load) However, if the data item is locked by a long-duration transaction, the wait will be of long duration Long waiting times lead to both longer response time and an increased chance of deadlock Graph-based protocols Graph-based protocols allow for locks to be released earlier than under the two-phase locking protocols, and they prevent deadlock However, they impose an ordering on the data items Transactions must lock data items in a manner consistent with this ordering As a result, a transaction may have to lock more data than it needs Furthermore, a transaction must hold a lock until there is no chance that the lock will be needed again Thus, long-duration lock waits are likely to occur
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24 Advanced Transaction Processing
The McGraw Hill Companies, 2001
24
Advanced Transaction Processing
Timestamp-based protocols Timestamp protocols never require a transaction to wait However, they do require transactions to abort under certain circumstances If a long-duration transaction is aborted, a substantial amount of work is lost For noninteractive transactions, this lost work is a performance issue For interactive transactions, the issue is also one of user satisfaction It is highly undesirable for a user to nd that several hours worth of work have been undone Validation protocols Like timestamp-based protocols, validation protocols enforce serializability by means of transaction abort Thus, it appears that the enforcement of serializability results in long-duration waits, in abort of long-duration transactions, or in both There are theoretical results, cited in the bibliographical notes, that substantiate this conclusion Further dif culties with the enforcement of serializability arise when we consider recovery issues We previously discussed the problem of cascading rollback, in which the abort of a transaction may lead to the abort of other transactions This phenomenon is undesirable, particularly for long-duration transactions If locking is used, exclusive locks must be held until the end of the transaction, if cascading rollback is to be avoided This holding of exclusive locks, however, increases the length of transaction waiting time Thus, it appears that the enforcement of transaction atomicity must either lead to an increased probability of long-duration waits or create a possibility of cascading rollback These considerations are the basis for the alternative concepts of correctness of concurrent executions and transaction recovery that we consider in the remainder of this section
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