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1、HowtheWebWorksTheWebisaworldofinformationavailableattheclickofamouse.Touseit,youneedacomputer,aconnectiontotheInternet,andabrowser.Whenyourunyourbrowser,itfindsanddisplaysapageofinformation,whichmightbeheldonyourowncomputer(yourhomepage),orfetchedfromsomewhereelse,youneedn
2、'tknoworevencarewhereitcomesfrom.Itworksbyhidinganetworkhypertextlinkbehindthehighlighteditemswhichappearonthescreen.Certainwords,phrases,orevenimagesarehighlighted,andclickingonthemcausesthebrowsertogooffandfindanotherpage,whichprobablycontainsmorehighlighteditems,andsoon
3、.Webdocumentsofvariousformatsareheldonso-calledservercomputers,representedintheimagebelowbyafactory.Userscaninspectthesedocumentsbyrequestingthemfromtheirlocal(personal)clientcomputers,representedbythehouse.AllcomputersinvolvedintheWebareconnectedbytheInternet,representedb
4、ytheroads.Whenyouclickonahyperlink,yourcomputerasksaservercomputertoreturntoyouadocument.Thisislikeaparceldeliveryservice.Forexample,startingfromtheCERN"Welcomepage"inSwitzerlandyournextclickmightfetchadocumentfromaphysicslabattheothersideoftheworld.Alltheinformationseemst
5、obeinthelittleboxinfrontofyou,andinasenseitis.Whenyouclickonapieceofhighlightedtextyourbrowser"ordersadocument"fromanothercomputer,receivesitby"returnmail"anddisplaysit.Youarethenfreetoreadthenewpageatleisure,withoutfurtherconsumptionofnetworkresources.Documentsareorganize
6、dinoneormorepages.Thepagesareusuallystoredonacomputerasseparatefiles.Today,thenumberofpagesaccessibleontheWebishuge(manymillions).Butjustthisessentialideaisnotenoughforrealusage.Imagineputtingaseparatepageforeveryentryinaphonebook,andtryingtomaintainthat.Clearly,therearety
7、pesofinformationthatarebetterhandledbystoringtheminadatabase.Traditionallyalsoalargeamountofinformationhasbeenmaintainedinsidedatabases.Databaseshaveareputationofbeinghardtodealwith,andtheyarealldifferent.Ifwecanmakeaprogramthatshowsusthecontentsofthedatabaseasifitwereaset
8、ofhypertextpages,thenwewouldnothavetoknowabouthowtousethedatabaseinterfacetogetourinforma
