پاورپوینت کامل Chapter 15: Transactions 51 اسلاید در PowerPoint

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پاورپوینت کامل Chapter 15: Transactions 51 اسلاید در PowerPoint

اسلاید ۴: ACID PropertiesAtomicity. Either all operations of the transaction are properly reflected in the database or none are.Consistency. Execution of a transaction in isolation preserves the consistency of the database.Isolation. Although multiple transactions may execute concurrently, each transaction must be unaware of other concurrently executing transactions. Intermediate transaction results must be hidden from other concurrently executed transactions. That is, for every pair of transactions Ti and Tj, it appears to Ti that either Tj, finished execution before Ti started, or Tj started execution after Ti finished.Durability. After a transaction completes successfully, the changes it has made to the database persist, even if there are system failures. A transaction is a unit of program execution that accesses and possibly updates various data items.To preserve the integrity of data the database system must ensure:

اسلاید ۵: Example of Fund TransferTransaction to transfer $50 from account A to account B:1.read(A)2.A := A – ۵۰۳.write(A)4.read(B)5.B := B + 506.write(B)Atomicity requirement — if the transaction fails after step 3 and before step 6, the system should ensure that its updates are not reflected in the database, else an inconsistency will result. Consistency requirement – the sum of A and B is unchanged by the execution of the transaction.

اسلاید ۶: Example of Fund Transfer (Cont.)Isolation requirement — if between steps 3 and 6, another transaction is allowed to access the partially updated database, it will see an inconsistent database (the sum A + B will be less than it should be).Isolation can be ensured trivially by running transactions serially, that is one after the other. However, executing multiple transactions concurrently has significant benefits, as we will see later.Durability requirement — once the user has been notified that the transaction has completed (i.e., the transfer of the $50 has taken place), the updates to the database by the transaction must persist despite failures.

اسلاید ۷: Transaction StateActive – the initial state; the transaction stays in this state while it is executingPartially committed – after the final statement has been executed.Failed — after the discovery that normal execution can no longer proceed.Aborted – after the transaction has been rolled back and the database restored to its state prior to the start of the transaction. Two options after it has been aborted:restart the transaction; can be done only if no internal logical errorkill the transactionCommitted – after successful completion.

اسلاید ۸: Transaction State (Cont.)

اسلاید ۹: Implementation of Atomicity and DurabilityThe recovery-management component of a database system implements the support for atomicity and durability.The shadow-database scheme:assume that only one transaction is active at a time.a pointer called db_pointer always points to the current consistent copy of the database.all updates are made on a shadow copy of the database, and db_pointer is made to point to the updated shadow copy only after the transaction reaches partial commit and all updated pages have been flushed to disk.in case transaction fails, old consistent copy pointed to by db_pointer can be used, and the shadow copy can be deleted.

اسلاید ۱۰: Implementation of Atomicity and Durability (Cont.)Assumes disks do not failUseful for text editors, but extremely inefficient for large databases (why)Does not handle concurrent transactions Will study better schemes in Chapter 17.The shadow-database scheme:

اسلاید ۱۱: Concurrent ExecutionsMultiple transactions are allowed to run concurrently in the system. Advantages are:increased processor and disk utilization, leading to better transaction throughput: one transaction can be using the CPU while another is reading from or writing to the diskreduced average response time for transactions: short transactions need not wait behind long ones.Concurrency control schemes – mechanisms to achieve isolation; that is, to control the interaction among the concurrent transactions in order to prevent them from destroying the consistency of the databaseWill study in Chapter 16, after studying notion of correctness of concurrent executions.

اسلاید ۱۲: SchedulesSchedule – a sequences of instructions that specify the chronological order in which instructions of concurrent transactions are executeda schedule for a set of transactions must consist of all instructions of those transactionsmust preserve the order in which the instructions appear in each individual transaction.A transaction that successfully completes its execution will have a commit instructions as the last statement (will be omitted if it is obvious)A transaction that fails to successfully complete its execution will have an abort instructions as the last statement (will be omitted if it is obvious)

اسلاید ۱۳: Schedule 1Let T1 transfer $50 from A to B, and T2 transfer 10% of the balance from A to B. A serial schedule in which T1 is followed by T2:

اسلاید ۱۴: Schedule 2 A serial schedule where T2 is followed by T1

اسلاید ۱۵: Schedule 3Let T1 and T2 be the transactions defined p