Bài giảng Thiết kế và quản trị cơ sở dữ liệu - Chương 6: Crash Recovery

Chương này trình bày những nội dung chính như: Transaction, how can constraints be violated? failures, transaction log, checkpoint, undo-logging rules, undo logging recovery rules, undo-logging & checkpoint,...và một số nội dung khác, mời các bạn cùng tham khảo.
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Bài giảng Thiết kế và quản trị cơ sở dữ liệu - Chương 6: Crash RecoveryCrash RecoveryVu Tuyet Trinhtrinhvt@it-hut.edu.vnDepartment of Information Systems, Faculty of Information TechnologyHanoi University of TechnologyTransactioncollection of action that preserve consistencyConsistent DBTConsistent DB’with assumptionIFTHENT starts with consistent state +T executes in isolationT leaves consistent state1How can constraints be violated?Transaction bugDBMS bugHardware failuree.g., disk crashData sharinge.g.,T1 and T2 in parallelFailuresEventsDesiredUndesiredExpectedUnexpectedprocessorCPUmemorydiskMD2RecoveryMaintaining the consistency of DB by ROLLBACK to thelast consistency state.Ensuring 2 propertiesAtomicDurabilityUsing LOGTransaction LogA sequence of log record keeping trace ofactions executed by DBMSLog the beginning of the transaction executiontransaction is already finishedTransaction is calcelTransaction makes an update actio, before update X=v, afterupdate x = w3Transaction LogHandled in main memory and put to externalmemory (disk) when possibleA = 8 16B = 8 16ActionsDataLogLogDiskMemoryCheckpointDefinition:Objectivemoment where intermediate results and a log record are savedto disk.being initiated at specified intervalsminimize the amount of time and effort wasted when restartthe process can be restarted from the latest checkpoint ratherthan from the beginning.Log record or 4Undo-loggingStepActiontMem A881616Mem BDisk ADisk B88888888815Read(B,t)8161686t:=t*216168887161616888Write(B,t)Flush log9Output(A)16161616810Output(B)1616161616234Read(A,t)t:=t*2Write(A,t)1112Mem LogFlush logUndo-Logging Rules(1) For every action generate undo log record (containingold value)(2) Before X is modified on disk, log records pertaining to Xmust be on disk (write ahead logging: WAL)(3) Before commit is flushed to log, all writes of transactionmust be reflected on disk5