barcode generator excel 2013 ean13 Hibernation and Deadlock in Software

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Hibernation and Deadlock
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Another problem to be addressed in communicating transactions is to avoid message loss when the arrival of a new message coincides with the clearing of an earlier message This problem is solved using semaphores to introduce blocking of coincident arrivals and is not a primary database concern The use of locks to resolve problems due to resource sharing does interact, however, with the management of locks in database systems Deadlocks can be caused by interfering demands on objects from distinct resources classes:
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Deadlocks Regions across Structural Levels
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Transaction P1 needs a tape drive to write results from its disk region Transaction P2 cannot release a tape drive until it is no longer blocked from accessing a record in this same disk region Mechanisms to handle deadlocks hence must involve all lockable objects This may include tape units, printers, and other devices, bu ers and memory areas allocated by the operating system, and les, blocks, and records controlled by the le and the database system This can cause problems in otherwise well structured systems, where devices are controlled on one system level and data objects are managed at higher system levels An unsatisfying solution is the use of potentially harmful global variables to communicate locking information Strict rules regarding access privileges to these variables, and adequate documentation can avoid most problems
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The potential for deadlock exists when all four conditions
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are true [Co man71 ]: 1: Locks 2: Blocking Access interference is resolved by setting and obeying locks An owner of an object is blocked when requesting a locked object Objects cannot be removed from their owners
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3: Completion Guarantee
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4: Circularity A circular request sequence, as shown in Example 13-4, exists All techniques to deal with deadlock attempt to change one of these conditions Techniques which resolve deadlocks, however, will also a ect hibernation Any technique adopted should be analyzed to gain an understanding of its e ect on system performance due to increased lock out and individual task response time due to blocking
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Frequency of Deadlock Occurrence Deadlock frequency will depend on the degree of interaction among transactions When regions being locked consist of large and many objects, say multiple les are locked in various combinations, then deadlock will be frequent and will cause serious problems unless constraints are applied Resolution involves isolation and duplicated les and is hence not well adapted to a database-oriented approach In a very busy system deadlocks may yet be frequent where incremental claims specify small objects A large system (WEYCOS), which has the capability to detect and recover from deadlock, experiences 100 deadlocks per hour [Bachman73 ] Other published values range to as low as ve [BrinchHansen73 ] or two per year [Frailey73 ] in systems which are programmed for limited deadlock prevention
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Integrity of Databases
Statistics of system failure due to deadlock are di cult to gather in an environment that fails to consider their existence As shared systems increase in popularity and activity it will become more di cult to ignore deadlock 13-2-3 Deadlock Avoidance Prevention is the better part of valor An ability to avoid deadlock can simplify many alternative choices Deadlock avoidance schemes impose, however, restrictions on users which can be di cult to accept Given the list of four conditions allowing deadlock to occur, there are four approaches to avoid deadlock A fth approach avoids both blocking and circularity 1: Postrepair Do not use locks and x inconsistency failures later 2: Dont Block Only advise requesters of con icting claims 3: Preempt Remove objects from their owners if there is a con ict Do not allow circular request sequences Make all claims rst and if none are blocked begin all 4: Presequence
5: Two-phase locking changes
The rst approach, repair problems due to not locking afterward, can be valid in experimental and educational systems, but is unacceptable in most commercial applications If the systems has already some means to repair damage because of errors, the approach may be tolerable
An airline, for instance, permits 15% overbooking on its routes If lack of locking contributes, say, 1% cases of overbooking, reducing the planned rate to 14% and avoiding locks can greatly improve performance Recording how a customer paid for the ight still requires locking payment les but here no or little interference is expected
Dont block The second approach puts the responsibility on the transaction The system will give a noti cation of potential interference by denying the request for exclusive access Access is still permitted The transaction may proceed, taking the risk that data will be modi ed or it may decide to restart later If the transaction wants to be sure then it will preclaim all needed resources before accessing them Since this approach also avoids circularity we will discuss it below If the transaction goes ahead it may simply warn the submitter of the fact that interference has been detected The transaction cannot count on the validity of its previous claims This choice may have to be made when systems do not have restoration or backup capabilities Whether the database is ever left in an inconsistent state depends on the programming algorithms and the types of interference encountered A KEEP statement of the 1973 version of the COBOL proposals (CODASYL78, ) will interrupt transactions which access records modi ed by currently active transactions The competing transactions have the same choice; go on or quit If auditable results are required, no interference can be tolerated; the transaction may decide to release its previous claims and restart itself after some delay or it may abort entirely and let an external restart mechanism take over
Sec 13-2
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