Introduction
Networkmodelisadatabasemodelconceivedasaflexiblewayofrepresentingobjectsandtheirrelationships.Itsuniquenessisthatitisnotlimitedtoahierarchicalstructurewhenviewedasagraphwhoseobjecttypeisnodeandrelationshiptypeisarc.
Thecommonfeatureofmanyengineeringsystemsisthattheyarecomposedofmanyunitsthatareactuallyinterwovenintoanetworkform.Typicalexamplesincludeurbantransportationsystems,urbansewagecollectionandtreatmentsystems,urbanwatersupplysystems,andurbanpowertelecommunicationsystems.Inaddition,manyengineeringdecision-makingproblemsandorganizationalsystems,althoughtheydonothavetheformofnetworkexpression,canoftenbeexplainedbynetworkmodels.Forexample,inaconstructioncompany,thedecision-makingandorderingprocesscanbedescribedbyanetworkmodel.Intheconstructionprocess,theworkschedulecanberegardedasanetworkcomposedofprocesses.Usingnetworkmodelstodescribehugeandcomplexengineeringsystemsandmanagementproblemscaneasilysolvemanyoptimizationproblemsinengineeringdesignandmanagementdecision-making.
Althoughthestructuredataofthehierarchicalmodelasatreeeachrecordhasaparentrecordandmultiplechildren,thenetworkmodelallowseachrecordtohavemultipleparentandchildrecords,formingacommongraphstructure.Thisattributeappliestotwolevels:theschemaisageneralizedgraphofrecordtypesconnectedbyarelationtype(called"collectiontype"inCODASYL),andthedatabaseitselfisageneralizedgraphofrecordtypesconnectedbyarelation(CODASYL"collection").Graphics.Bothlevelsallowlooping.Themainargumentthatsupportsthenetworkmodelcomparedtothehierarchicalmodelisthatitallowsamorenaturalmodelingoftherelationshipsbetweenentities.Althoughthismodeliswidelyimplementedandused,ithasnotbecomedominantduetotwomainreasons.First,IBMchosetoadheretothesemi-networkexpansionofexistingproductssuchasthehierarchicalmodelIMSandDL/I.Second,itwaseventuallyreplacedbytherelationalmodel,whichprovidedahigher-level,moredescriptiveinterface.Untiltheearly1980s,theperformanceadvantagesofthelow-levelnavigationinterfaceprovidedbytheclassificationandnetworkdatabaseswereconvincingformanylargeapplications,butasthehardwarebecamefaster,theadditionalproductivityandflexibilityoftherelationalmodelledtoThenetworkmodelusedbyenterprisesisgraduallyeliminated.
History
TheoriginalinventorofthenetworkmodelwasCharlesBachman(CharlesBachman),anditwasdevelopedintoastandardpublishedbytheConferenceonDataSystemsLanguage(CODASYL)in1969specification.Thiswasfollowedbythesecondpublicationin1971,whichbecamethebasisformostimplementations.Subsequentworkcontinueduntiltheearly1980s,andfinallyreachedtheISOstandard,butthishadlittleeffectontheproduct.
DatabaseSystem
Somewell-knowndatabasesystemsthatusenetworkmodelsinclude:
IntegratedDataStorage(IDS)
IDMS(IntegratedDatabaseManagementSystem)
RaimaDatabaseManager
TurboIMAGE
UnivacDMS-1100
Threeelements
Oneistocharacterizetheelementsofthesystemnode.
Thesecondisthearrow(sometimestheedge)thatreflectstherelationshipbetweentheconstituentelements.
Thethirdistheflowoftrafficinthenetwork.Ontheonehand,itreflectsthequantitativerelationshipbetweenelements,anditalsodeterminesthegoalanddirectionofnetworkmodeloptimization.
Classification
1.Networkmodelwithsubstanceastheflowrate
Whentheflowcontentinthenetworkmodelisliquid,gas,solidandothermaterialentities,itconstitutesanetworkmodelwithsubstanceastheflowrate,anditsoptimizationgoalisgenerallythemaximumflowrateOrminimumcostflow.Manypracticalproblemssuchastransportation(roads,railways,aviation,navigation),resourceallocation,andindustrialprocessinstallationscanbeabstractedintothistypeofnetworkmodel.
Ifthedigitalwellalongthelineisnotdistance,butthemaximumflowthatthecorrespondingroadcanpass,itbecomesanetworkmodelwithmaterialflow.
2.Networkmodelwithinformationastraffic
Examplesofnetworkmodelwithinformationsuchassignalsanddataastraffic,inadditiontobroadcastingandcommunicationnetworks,alsoincludeblockdiagramsorsignalflowdiagramsusedinthecontrolprocess,Socialorganizationsystemdiagram,managementinformationsystemnetwork,etc.
Figure4-2showsthecontrolsystemdiagramofconstructionenterprisemanagementforecast.Enterprisesmustfirstdeterminetheforecastgoalsandrequirementsaccordingtotheactualneedsofproductionandoperation,collectrelevantdatabasedonthis,selectappropriateforecastingmethodsforforecasting,andthenanalyzewhethertheforecastconclusionsarereasonable,ifnot,orrevisetheadvancedonationtargetandRequestorre-selecttheforecastmethod,otherwise,youcanentertheforecastimplementation,andusetheforecastconclusiontoguidetheproductionandoperationactivitiesoftheenterprise.Duringtheimplementation,youmayencounternewproductionandoperationforecastingproblems,andstartanewcycle.
3.Networkmodelwithenergyastheflowrate
Themosttypicalnetworksystemswithenergyastheflowrateareurbanpowersystemsandcentralheatingsystems.Figure4-3showsaschematicdiagramofacity'spowernetwork.
4.Networkmodelwithtime,cost,distance,etc.astraffic
ThemosttypicalnetworkmodelwithtimeastrafficisPERT(PlanEvaluationTechnology).Figure4.4isanetworkdiagramshowingtheconstructionsequenceofprefabricatedhouses.Inthefigure,eacharrowrepresentsajobandindicatestheestimatednumberofworkinghours.Usingthenetworkdiagram,youcanfindoutthebestplanintheentireconstructionprocess,andreasonablysolvetheproblemsoflaborarrangement,capitalturnover,andshorteningtheconstructionperiod.Theshortestpossibletimeinthisexampleis66h.
Aconstructioncompanyiscarryingoutaprojectinadepressionbytheriver.Theprojectsitehasbeenaffectedbytheriver'srisingwaterinthepastandhasalsoencountereddestructiveflooding.Sincetheequipmentwillnotbeusedforfourmonthsinthisproject,itisnecessarytodecideonthestorageplanoftheequipment.Therearethreeoptions:oneistotaketheequipmentawayandthentakeitback,whichwillcostatotalof1,800yuan,andtheotheristoleavetheequipmentontheconstructionsiteandbuildaplatformtoprotectit.Thecostofbuildingtheplatformis500yuan.Theplatformcandefendagainstfloods,butnotagainstdestructiveflooding.Thethirdistoleavetheequipmentontheconstructionsitewithouttakingprotectivemeasures.