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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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V Transaction Management
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Introduction
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The McGraw Hill Companies, 2001
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Transaction Management
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The term transaction refers to a collection of operations that form a single logical unit of work For instance, transfer of money from one account to another is a transaction consisting of two updates, one to each account It is important that either all actions of a transaction be executed completely, or, in case of some failure, partial effects of a transaction be undone This property is called atomicity Further, once a transaction is successfully executed, its effects must persist in the database a system failure should not result in the database forgetting about a transaction that successfully completed This property is called durability In a database system where multiple transactions are executing concurrently, if updates to shared data are not controlled there is potential for transactions to see inconsistent intermediate states created by updates of other transactions Such a situation can result in erroneous updates to data stored in the database Thus, database systems must provide mechanisms to isolate transactions from the effects of other concurrently executing transactions This property is called isolation 15 describes the concept of a transaction in detail, including the properties of atomicity, durability, isolation, and other properties provided by the transaction abstraction In particular, the chapter makes precise the notion of isolation by means of a concept called serializability 16 describes several concurrency control techniques that help implement the isolation property 17 describes the recovery management component of a database, which implements the atomicity and durability properties
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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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V Transaction Management
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15 Transactions
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Transactions
Often, a collection of several operations on the database appears to be a single unit from the point of view of the database user For example, a transfer of funds from a checking account to a savings account is a single operation from the customer s standpoint; within the database system, however, it consists of several operations Clearly, it is essential that all these operations occur, or that, in case of a failure, none occur It would be unacceptable if the checking account were debited, but the savings account were not credited Collections of operations that form a single logical unit of work are called transactions A database system must ensure proper execution of transactions despite failures either the entire transaction executes, or none of it does Furthermore, it must manage concurrent execution of transactions in a way that avoids the introduction of inconsistency In our funds-transfer example, a transaction computing the customer s total money might see the checking-account balance before it is debited by the fundstransfer transaction, but see the savings balance after it is credited As a result, it would obtain an incorrect result This chapter introduces the basic concepts of transaction processing Details on concurrent transaction processing and recovery from failures are in s 16 and 17, respectively Further topics in transaction processing are discussed in 24
151 Transaction Concept
A transaction is a unit of program execution that accesses and possibly updates various data items Usually, a transaction is initiated by a user program written in a high-level data-manipulation language or programming language (for example, SQL, COBOL, C, C++, or Java), where it is delimited by statements (or function calls) of the form begin transaction and end transaction The transaction consists of all operations executed between the begin transaction and end transaction To ensure integrity of the data, we require that the database system maintain the following properties of the transactions:
Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
V Transaction Management
15 Transactions
The McGraw Hill Companies, 2001
15
Transactions
Atomicity Either all operations of the transaction are re ected properly in the database, or none are Consistency Execution of a transaction in isolation (that is, with no other transaction executing concurrently) preserves the consistency of the database Isolation Even though multiple transactions may execute concurrently, the system guarantees that, for every pair of transactions Ti and Tj , it appears to Ti that either Tj nished execution before Ti started, or Tj started execution after Ti nished Thus, each transaction is unaware of other transactions executing concurrently in the system Durability After a transaction completes successfully, the changes it has made to the database persist, even if there are system failures These properties are often called the ACID properties; the acronym is derived from the rst letter of each of the four properties To gain a better understanding of ACID properties and the need for them, consider a simpli ed banking system consisting of several accounts and a set of transactions that access and update those accounts For the time being, we assume that the database permanently resides on disk, but that some portion of it is temporarily residing in main memory Transactions access data using two operations: read(X), which transfers the data item X from the database to a local buffer belonging to the transaction that executed the read operation write(X), which transfers the data item X from the the local buffer of the transaction that executed the write back to the database In a real database system, the write operation does not necessarily result in the immediate update of the data on the disk; the write operation may be temporarily stored in memory and executed on the disk later For now, however, we shall assume that the write operation updates the database immediately We shall return to this subject in 17 Let Ti be a transaction that transfers $50 from account A to account B This transaction can be de ned as Ti : read(A); A := A 50; write(A); read(B); B := B + 50; write(B) Let us now consider each of the ACID requirements (For ease of presentation, we consider them in an order different from the order A-C-I-D) Consistency: The consistency requirement here is that the sum of A and B be unchanged by the execution of the transaction Without the consistency requirement, money could be created or destroyed by the transaction! It can
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