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Integrity is praised, and starves Juvenal Satires, vol 1 (AD 110)
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Throughout the previous chapters we have implied that a well-structured database system, using careful data entry and update procedures, and operating reliably and free from unauthorized access, will be a secure depository for the data There is, however, one area where failures can be generated while everything else seems to be proceeding satisfactorily This problem area exists because of interference among the multiple transactions which are active within one computer system The interference can be related to competition as well as to cooperation Integrity of a database means absence of inconsistent data While an update transaction is in progress, the consistency of a database can be temporarily disturbed One data element will re ect the update, while another element has not yet been changed A low level of redundancy reduces potential integrity violations, since one fact will appear only rarely in more than one place We have designed our databases with that objective, and yet nd that they contain redundancies An error in a single transaction can cause a failure if a database update is not carried to completion, but the techniques presented in Secs 11-3 and 11-4 deal with this problem successfully However, even error-free update transactions can cause problems when they access shared data in a time period where there is other activity Accessors sharing the data can pick up temporary inconsistencies caused by a concurrent update and spread the inconsistency through the database or to the output sent to the users 613
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In order to achieve rapid access we create structures which are redundant We also maintain connections between relations which employ redundancy Less obvious are redundancies due to derived data, and due to externally imposed integrity constraints
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An index is a redundant access structure The department name appears in each employee record and in one record of the department relation The departmental salary expense is equal to the sum of all its employees salaries The total number of employees on the payroll should not change when an employee transfers from one department to another
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Many of the issues of integrity maintenance are also of concern to developers of operating-system services We will in fact assume the availability of basic operating system facilities and restrict our discussions to the interactions caused by database usage An operating system will concentrate on controlling the competition for system resources due to interfering processes Performance degradation due to competing multiple processes was seen in Sec 5-4, and in Sec 6-3 multiple processes were spawned by a single computation so that devices could be activated independently Computations within a system may remain conceptually independent, and yet a ect each other For databases we will deal again within the framework of transactions, as de ned in Secs 1-7-5 and 11-3-1 Although transactions may spawn multiple processes, we have to consider only cases of interference among processes which are initiated independently or, equivalently, by di erent transactions We recall that a transaction is a computation that is initiated by a user, allowed to complete as rapidly as the available hardware permits, and then terminated The primary task of a transaction-oriented operating system is to keep one transaction as much as possible una ected by the presence of other transactions This independence is not completely realizable There will be competition for the use of processors (especially if there is only one), for the use of disks and the channels to access them, and for the use of devices for input and output Older devices for input and output (tape drives, readers, printers) are especially awkward to share since manual intervention is required to select the tapes to be mounted and to keep the paper going in and out in the proper order In a database operation there are additional resources to be shared All users will want to access the schema, many may select a particular le, or index, or hierarchical level, and several may wish to access the same data item Data are perfectly sharable, until one or more transactions want to make changes Then some users may be denied access to the data to avoid the dissemination of inconsistent information; Figs 13-1 and 13-2 will illustrate the problem Now competition for data resources ensues