软 件 工 程 Software Engineering[001].ppt

PARTONETheProductandtheProcess Chapter1TheProduct TheEvolvingRoleofSoftware Intheearlydays User ComputerSoftware Placeasequenceofinstructionstogethertogetthecomputertodosomethinguseful Inlate1950 s ComputerbecamecheaperandmorecommonHighlevellanguageswereinvented Programmer Inearly1960 s Veryfewlargesoftwareprojectsweredonebysomeexperts Hacker Cracker PARTONE TheProduct Inmiddletolate1960 s Trulylargesoftwaresystemswereattempted Case1 美国IBM公司在1963年至1966年开发的IBM360机的操作系统 这一项目花了5000人一年的工作量 最多时有1000人投入开发工作 写出了近100万行源程序 据统计 这个操作系统每次发行的新版本都是从前一版本中找出1000个程序错误而修正的结果 这个项目的负责人F P Brooks事后总结了他在组织开发过程中的沉痛教训时说 正像一只逃亡的野兽落到泥沼中做垂死的挣扎 越是挣扎 陷得越深 最后无法逃脱灭顶的灾难 程序设计工作正像这样一个泥潭 一批批程序员被迫在泥沼中拼命挣扎 谁也没有料到问题竟会陷入这样的困境 IBM360操作系统的历史教训成为软件开发项目的典型事例为人们所记取 而Brooks博士随后写出了软件工程领域的经典著作 人月神话 TheMythicalMan Month 至今畅销不衰 SoftwareCrisis NATOConference Garmisch Germany 1968 OrmaybeChronicAfflictionismoreaccurate PARTONE TheProduct DramaticimprovementsinhardwareperformanceProfoundchangesincomputingarchitecturesVastincreasesinmemoryandstoragecapacityWidevarietyofexoticinputandoutputoptions Today Software Product informationtransformer Vehiclefordeliveringaproduct OS network tools Thesamequestionsarestillaskedtoday Whydoesittakessolongtogetsoftwarefinished Whyaredevelopmentcostssohigh Whycan twefindalltheerrorsbeforewegivethesoftwaretocustomers Whydowecontinuetohavedifficultyinmeasuringprogressassoftwareisbeingdeveloped PARTONE TheProduct PARTONE TheProduct Aweeklaterthebosscalledtheengineerintohisofficeandasked Wherearewe Everything sgoingwell saidtheyoungster butI verunintoafewsmallsnags I llgetthemironedoutandbebackontracksoon Howdoesthedeadlinelook thebossasked Noproblem saidtheengineer I mcloseto90percentcomplete Ifyou vebeenworkinginthesoftwareworldformorethanafewyears youcanfinishthestory It llcomeasnosurprisethattheyoungengineerstayed90percentcompletefortheentireprojectdurationandonlyfinished withthehelpofothers onemonthlate Myth Theonlydeliverableforasuccessfulprojectistheworkingprogram Reality Aworkingprogramisonlyonepartofasoftwareconfigurationthatincludesprograms documents anddata Documentationformsthefoundationforsuccessfuldevelopmentand moreimportant providesguidanceforsoftwaresupport Managers evaluate trackprogress Programmers communicatetoeachotherMaintainers VITAL PARTONE TheProduct Case3 Intheearly1980s theUnitedStates InternalRevenueService IRS hiredSperryCorporationtobuildanautomatedfederalincometaxformprocessingsystem AccordingtotheWashingtonPost the systemhasprovedinadequatetotheworkload costnearlytwicewhatwasexpectedandmustbereplacedsoon Sawyer1985 In1985 anextra 90millionwasneededtoenhancetheoriginal 103millionworthofSperryequipment Inaddition becausetheproblempreventedtheIRSfromreturningrefundstotaxpayersbythedeadline theIRSwasforcedtopay 40 2millionininterestand 22 3millioninovertimewagesforitsemployeeswhoweretryingtocatchup In1996 thesituationhadnotimproved TheLosAngelesTimesreportedonMarch29thattherewasstillnomasterplanforthemodernizationofIRScomputers onlyasix thousand pagetechnicaldocument CongressmanJimLightfootcalledtheproject a 4 billionfiascothatisflounderingbecauseofinadequateplanning Vartabedian1996 Myth Projectrequirementscontinuallychange butchangecanbeeasilyaccommodatedbecausesoftwareisflexible PARTONE TheProduct Myth Ifwegetbehindschedule wecanaddmoreprogrammersandcatchup Reality Softwaredevelopmentisnotamechanisticprocesslikemanufacturing InthewordsofBrooks addingpeopletoalatesoftwareprojectmakesitlater PARTONE TheProduct Case4 某公园有一游船码头 负责人希望开发一游船管理系统 要求如下 当游客租船时 管理员输入S表示租船周期开始 当游客还船时 管理员输入E表示租船周期结束 一天结束时 要求系统打印出租船次数和平均租船时间 Algorithm Number Total time 0 GetMessage While End of stream if Code S Number Total time Start time elseTotal time End time GetMessage PrintNumber If Number PrintTotal time Number 新要求 输出一天中的最长租用时间 新要求 将报告分上午和下午输出 新要求 当通信线路出问题时 能从计算中删除一切不完整的租船信息 Myth Oncewewritetheprogramandgetittowork ourjobisdone Reality Someoneoncesaidthat thesooneryoubegin writingcode thelongerit lltakeyoutogetdone Industrydataindicatethatbetween60and80percentofalleffortexpendedonaprogramwillbeexpendedafteritisdeliveredtothecustomerforthefirsttime PARTONE TheProduct WhatIsSoftware Softwareisasetofitemsorobjectsthatformaconfigurationthatincludes instructions computerprograms thatwhenexecutedprovidedesiredfunctionandperformance datastructuresthatenabletheprogramstoadequatelymanipulateinformation and documentsthatdescribetheoperationanduseoftheprograms ANDMORE Softwareisdevelopedorengineered itisnotmanufacturedintheclassicalsense Highqualityisachievedthroughgooddesign Dependonpeople Requiretheconstructionofa product PARTONE TheProduct Softwaredoesn twearout Butitdoesdeteriorate Therearenosoftwarespareparts Althoughtheindustryismovingtowardcomponent basedassembly mostsoftwarecontinuestobecustombuilt Myth Softwareengineeringwillmakeuscreatevoluminousandunnecessarydocumentationandwillinvariablyslowusdown Reality Softwareengineeringisnotaboutcreatingdocuments Itisaboutcreatingquality Betterqualityleadstoreducedrework Andreducedrework