[計(jì)算機(jī)軟件及應(yīng)用]kc第16講-事務(wù)管理技術(shù)-文庫吧
2025-01-04 17:26 本頁面
【正文】 rior to the start of the transaction. Two options after it has been aborted: ? restart the transaction。 can be done only if no internal logical error ? kill the transaction ? Committed – after successful pletion. ? 直觀圖示 (事務(wù)狀態(tài)變化 ) 14 事務(wù)執(zhí)行時(shí)可能出現(xiàn)哪些狀態(tài)？ 事務(wù)的狀態(tài) Transaction State (直觀圖示 ) 事務(wù)的狀態(tài) Implementation of Atomicity and Durability –原子性 amp。持久性 ? The recoverymanagement ponent of a database system implements the support for atomicity and durability. ? The 引子 shadowdatabase 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 mit 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. ? 直觀圖示（ 數(shù)據(jù)庫指針切換 ） 15 如何實(shí)現(xiàn)事務(wù)的原子性和持久性？ 如何保證事務(wù)的 ACID性質(zhì) 問題 3答案 Implementation of Atomicity and Durability –原子性 amp。持久性 (續(xù) ) ? Assumes disks do not fail ? Useful for text editors, but ? extremely inefficient for large databases (why?復(fù)制整個(gè)數(shù)據(jù)庫開銷很大 ) ? Does not handle concurrent transactions ? Will study better schemes in Chapter 17. The shadowdatabase scheme: 如何保證事務(wù)的 ACID性質(zhì) Concurrent Executions ? Multiple 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 disk ? reduced 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 database ? Will study in Chapter 16, after studying notion of correctness of concurrent executions. 二 并發(fā)執(zhí)行與調(diào)度 21 什么是并發(fā)執(zhí)行和并發(fā)控制？ 并發(fā)執(zhí)行 Schedules ? Schedule – a sequences of instructions that specify the chronological按時(shí)間順序排列的 order in which instructions of concurrent transactions are executed ? a schedule for a set of transactions must consist of all instructions of those transactions ? must preserve the order in which the instructions appear in each individual transaction. ? A transaction that successfully pletes its execution will have a mit instructions as the last statement (will be omitted if it is obvious) ? A transaction that fails to successfully plete its execution will have an abort instructions as the last statement (will be omitted if it is obvious) ? 例子 : (Schedule14) 22 什么是調(diào)度(Schedule)？ 調(diào)度 Schedule 問題 4答案 Schedule 1 ? Let 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: 23 什么是合理的調(diào)度？ 調(diào)度 Schedule Schedule 2 ? A serial schedule where T2 is followed by T1 調(diào)度 Schedule Schedule 3 ? Let T1 and T2 be the transactions defined previously. The following schedule is not a serial schedule, but it is equivalent to Schedule 1. ? In Schedules 1, 2 and 3, the sum A + B is 調(diào)度 Schedule Schedule 4 ? The following concurrent schedule does not preserve the value of (A + B). 不 合理轉(zhuǎn)賬 24 什么是不合理的調(diào)度？ 調(diào)度 Schedule Serializability ? Basic Assumption – Each transaction preserves database consistency. ? Thus serial ex
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