barcode generator excel 2013 ean13 Maintenance of Integrity in Software

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remote transaction has to reinitiate any canceled subtransaction Since the remote transaction must in any case deal with communication failures, we can safely assume that a remote transaction has already the capability to restart its subtransactions The fourth solution, avoidance of circular request sequences, requires a global naming scheme This scheme appears mainly of value in tightly controlled and strongly bound systems, since otherwise it is di cult to assume that all objects can be ordered, since the name to object correspondence may be a local option
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Distributed Deadlock Detection Distributed deadlock detection is initiated when a node nds that it is deadlocked If the deadlock involves subtransactions submitted by remote nodes, a node has to collect information of all dependencies in the network, as shown for a local net in Fig 13-6 The information needed can be always sent along when a subtransaction is submitted, or has to be obtained when a problem is detected by querying the other nodes To resolve the deadlock a transaction involved in the chain has to be preempted The selection of transaction can be based again on minimal loss, or on features of certain sites Some sites may have good rollback capability or be deemed unimportant If the information is available to all nodes, all deadlocked nodes can come to the same conclusion and resolve the deadlock identically Queries for the information to resolve a deadlock after the deadlock has occurred must be answerable without access to any data which may be blocked These strategies are liable to fail if there is another system failure The simple local priority scheme leading to preemption and avoiding deadlocks caused by remote transactions seems to be more robust
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The previous sections have shown how integrity can be violated, and how such violations can be avoided or their e ect restored These discussions are of necessity based on expectations of perfect hardware and software performance Well managed system design and programming techniques can considerably improve system reliability
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A large multiuser on-line le system [Frey71 ] developed under management by the author operated for more than 6 years without loss of data stored on les Control over reliability, security, and integrity was achieved by integrating access, access protection, and backup The granule for all operations was the block; elds could be protected only by encryption (Sec 14-3) Periodic monitoring was performed to locate errors Backup procedures provided manually initiated rollback
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Continued integrity of a database is essential for successful operation We would like to have assurance that the mechanisms used to protect shared data cannot fail In practical systems today correctness of integrity protection is impossible to prove Many levels of hardware and software have to work without failure to permit integrity protection mechanisms to do their work We covered in earlier Chaps 11 and 12 the prerequisite reliability and access protection issues Secs 13-1 and 13-2 presented the locking mechanisms used to avoid problems due to shared access We will now present some further considerations which can help designing and operating a database so that integrity is kept at a high level
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Most systems allow the programmers to make decisions regarding the use of locks Problems may be caused by having one individual make decisions which a ect programs written by others A system organization to prevent update interference can be achieved if: 1 The access protection system allows only one user, the owner of the data, to modify a protected object or to set a lock for the object 2 The access security system allows only one instance of the user to exist 3 The le management system locks objects which are synonymous with the objects identi ed for ownership by the access protection system The last condition is rarely true Ownership is allocated to logical entities: elds, records, les, databases, whereas physical control is exercised over physical objects: blocks, tracks, les, and devices If interference problems can be resolved only on the common le level, shared database access is severely restricted The rules outlined above do not solve read interference if query regions involve les of more than one user If the programmer is not restricted by the access system from potential interference with the activities of others, a higher level of control may be required Update operations may be permitted only if a claim has preceded the operation This removes the decision to lock or not to lock from the purview of the programmer, although it does not guarantee the locks will cover the correct region Some system automatically precede update operations with a claim which will lock the object being updated, but an additional claim facility is required to let the programmer claim regions containing multiple objects For the sake of programming consistency explicit claims are desirable The decision whether a read operation is of the audit or free type is local to a transaction and program It is desirable to let the end user know which choice was made when the results are being transmitted The general problem of assuring the integrity of databases has not been solved An initial step is the de nition of the application requirements by collecting semantic constraints
13-3-2 Integrity Monitoring Since a single consistency failure can be gradually copied throughout the database, a strategy of regular monitoring of the database is essential wherever long term data are kept If results based on data stored for many years are in error, the user s con dence in all the work of past years is suddenly lost Monitoring is possibly on two levels: structural and content-oriented Structural monitoring can be carried out by the le system without user participation Content-oriented monitoring requires the user to provide assertions regarding data relationships In either case monitoring is possible only if there is some redundancy in a database
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