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2025-03-24 11:57本頁(yè)面
【正文】 formulated against a private database． The multipledatabase model is more appropriate than the singledatabase model in an environment where it is easy to determine in advance logical partitions of the database tha t correspond to work to be performed. The singledatabase model has received considerably more attention than the multiple database model． The focus of research into singledatabase model has been on addressing the twin problem of long waits and loss of work that the long duration of transactions brings about. The singledatabase model requires the introdiction of a notion of database consistency, and protocols for concurrency control and recovery, that are different from those supported in traditional database systems． If a reasonable notion of database consistency is to be supported (I． c. the database system is to enforce it automatically), there are bound to be” conflict situations where one transaction es into an access conflict against some other transaction. If await is to be avoided， some means of a negotiated settlement of the conflict must be provided, thereby dragging the users into the details of concurrency control． The singledatabase model is more appropriateate than the multipledatabase model in an environment where it is difficult to determine in advance logical partitions of the database that correspond to work to be performed, and the users closely cooperate. The objective of longduration transactions is to model longduration, interactive Database access sessions in application environments． The fundamental assumption about shortduration of transactions that underlies the traditional model of transactions is inappropriate for longduration transactions． The implementation of the traditional model of transactions may cause intolerably long waits when transactions aleph to acquire locks before accessing data, and may also cause a large amount of work to be lost when transactions are backed out in response to userinitiated aborts or system failure situations． The objective of a transaction model is to provide a rigorous basis for automatically enforcing criterion for database consistency for a set of multiple concurrent read and write accesses to the database in the presence of potential system failure situations． The consistency criterion adopted for traditional transactions is the notion of scrializability． Scrializability is enforced in conventional database systems through the use of locking for automatic concurrency control, and logging for automatic recovery from system failure situations． A transaction”that doesn’t provide a basis for automatically enforcing database consistency is not really a transaction. To be sure, a longduration transaction need not adopt serializability as its consistency criterion． However. there must be some consistcricv criterion． Despite a large number of proposals on version support in the context of puter aided design and software engineering, the absence of a consensus on version semantics has been a key impediment to version support in database systems． Because of the differences between files and databases, it is intuitively c
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