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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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1 Introduction
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The McGraw Hill Companies, 2001
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base and expert systems, systems that store data with complex data types (for example, graphics data and audio data), and environment-modeling systems s 8 and 9 cover several of these applications
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One of the main reasons for using DBMSs is to have central control of both the data and the programs that access those data A person who has such central control over the system is called a database administrator (DBA) The functions of a DBA include: Schema de nition The DBA creates the original database schema by executing a set of data de nition statements in the DDL Storage structure and access-method de nition Schema and physical-organization modi cation The DBA carries out changes to the schema and physical organization to re ect the changing needs of the organization, or to alter the physical organization to improve performance Granting of authorization for data access By granting different types of authorization, the database administrator can regulate which parts of the database various users can access The authorization information is kept in a special system structure that the database system consults whenever someone attempts to access the data in the system Routine maintenance Examples of the database administrator s routine maintenance activities are: Periodically backing up the database, either onto tapes or onto remote servers, to prevent loss of data in case of disasters such as ooding Ensuring that enough free disk space is available for normal operations, and upgrading disk space as required Monitoring jobs running on the database and ensuring that performance is not degraded by very expensive tasks submitted by some users
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Often, several operations on the database form a single logical unit of work An example is a funds transfer, as in Section 12, in which one account (say A) is debited and another account (say B) is credited Clearly, it is essential that either both the credit and debit occur, or that neither occur That is, the funds transfer must happen in its entirety or not at all This all-or-none requirement is called atomicity In addition, it is essential that the execution of the funds transfer preserve the consistency of the database That is, the value of the sum A + B must be preserved This correctness requirement is called consistency Finally, after the successful execution of a funds transfer, the new values of accounts A and B must persist, despite the possibility of system failure This persistence requirement is called durability A transaction is a collection of operations that performs a single logical function in a database application Each transaction is a unit of both atomicity and consis-
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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
1 Introduction
Text
The McGraw Hill Companies, 2001
1
Introduction
tency Thus, we require that transactions do not violate any database-consistency constraints That is, if the database was consistent when a transaction started, the database must be consistent when the transaction successfully terminates However, during the execution of a transaction, it may be necessary temporarily to allow inconsistency, since either the debit of A or the credit of B must be done before the other This temporary inconsistency, although necessary, may lead to dif culty if a failure occurs It is the programmer s responsibility to de ne properly the various transactions, so that each preserves the consistency of the database For example, the transaction to transfer funds from account A to account B could be de ned to be composed of two separate programs: one that debits account A, and another that credits account B The execution of these two programs one after the other will indeed preserve consistency However, each program by itself does not transform the database from a consistent state to a new consistent state Thus, those programs are not transactions Ensuring the atomicity and durability properties is the responsibility of the database system itself speci cally, of the transaction-management component In the absence of failures, all transactions complete successfully, and atomicity is achieved easily However, because of various types of failure, a transaction may not always complete its execution successfully If we are to ensure the atomicity property, a failed transaction must have no effect on the state of the database Thus, the database must be restored to the state in which it was before the transaction in question started executing The database system must therefore perform failure recovery, that is, detect system failures and restore the database to the state that existed prior to the occurrence of the failure Finally, when several transactions update the database concurrently, the consistency of data may no longer be preserved, even though each individual transaction is correct It is the responsibility of the concurrency-control manager to control the interaction among the concurrent transactions, to ensure the consistency of the database Database systems designed for use on small personal computers may not have all these features For example, many small systems allow only one user to access the database at a time Others do not offer backup and recovery, leaving that to the user These restrictions allow for a smaller data manager, with fewer requirements for physical resources especially main memory Although such a low-cost, low-feature approach is adequate for small personal databases, it is inadequate for a medium- to large-scale enterprise
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