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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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For certain applications, it may be desirable to allow active transactions to display data to users, particularly for long-duration transactions that run for minutes or hours Unfortunately, we cannot allow such output of observable data unless we are willing to compromise transaction atomicity Most current transaction systems ensure atomicity and, therefore, forbid this form of interaction with users In 24, we discuss alternative transaction models that support long-duration, interactive transactions
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The recovery-management component of a database system can support atomicity and durability by a variety of schemes We rst consider a simple, but extremely inef cient, scheme called the shadow copy scheme This scheme, which is based on making copies of the database, called shadow copies, assumes that only one transaction is active at a time The scheme also assumes that the database is simply a le on disk A pointer called db-pointer is maintained on disk; it points to the current copy of the database In the shadow-copy scheme, a transaction that wants to update the database rst creates a complete copy of the database All updates are done on the new database copy, leaving the original copy, the shadow copy, untouched If at any point the transaction has to be aborted, the system merely deletes the new copy The old copy of the database has not been affected If the transaction completes, it is committed as follows First, the operating system is asked to make sure that all pages of the new copy of the database have been written out to disk (Unix systems use the ush command for this purpose) After the operating system has written all the pages to disk, the database system updates the pointer db-pointer to point to the new copy of the database; the new copy then becomes the current copy of the database The old copy of the database is then deleted Figure 152 depicts the scheme, showing the database state before and after the update
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Shadow-copy technique for atomicity and durability
Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
V Transaction Management
15 Transactions
The McGraw Hill Companies, 2001
15
Transactions
The transaction is said to have been committed at the point where the updated dbpointer is written to disk We now consider how the technique handles transaction and system failures First, consider transaction failure If the transaction fails at any time before db-pointer is updated, the old contents of the database are not affected We can abort the transaction by just deleting the new copy of the database Once the transaction has been committed, all the updates that it performed are in the database pointed to by dbpointer Thus, either all updates of the transaction are re ected, or none of the effects are re ected, regardless of transaction failure Now consider the issue of system failure Suppose that the system fails at any time before the updated db-pointer is written to disk Then, when the system restarts, it will read db-pointer and will thus see the original contents of the database, and none of the effects of the transaction will be visible on the database Next, suppose that the system fails after db-pointer has been updated on disk Before the pointer is updated, all updated pages of the new copy of the database were written to disk Again, we assume that, once a le is written to disk, its contents will not be damaged even if there is a system failure Therefore, when the system restarts, it will read db-pointer and will thus see the contents of the database after all the updates performed by the transaction The implementation actually depends on the write to db-pointer being atomic; that is, either all its bytes are written or none of its bytes are written If some of the bytes of the pointer were updated by the write, but others were not, the pointer is meaningless, and neither old nor new versions of the database may be found when the system restarts Luckily, disk systems provide atomic updates to entire blocks, or at least to a disk sector In other words, the disk system guarantees that it will update db-pointer atomically, as long as we make sure that db-pointer lies entirely in a single sector, which we can ensure by storing db-pointer at the beginning of a block Thus, the atomicity and durability properties of transactions are ensured by the shadow-copy implementation of the recovery-management component As a simple example of a transaction outside the database domain, consider a textediting session An entire editing session can be modeled as a transaction The actions executed by the transaction are reading and updating the le Saving the le at the end of editing corresponds to a commit of the editing transaction; quitting the editing session without saving the le corresponds to an abort of the editing transaction Many text editors use essentially the implementation just described, to ensure that an editing session is transactional A new le is used to store the updated le At the end of the editing session, if the updated le is to be saved, the text editor uses a le rename command to rename the new le to have the actual le name The rename, assumed to be implemented as an atomic operation by the underlying le system, deletes the old le as well Unfortunately, this implementation is extremely inef cient in the context of large databases, since executing a single transaction requires copying the entire database Furthermore, the implementation does not allow transactions to execute concurrently with one another There are practical ways of implementing atomicity and durability that are much less expensive and more powerful We study these recovery techniques in 17
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