آرمان حسينزاده آذر 89 1. access to data varies depending on the source of the data....
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آرمانحسينزاده89آذر
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Accesstodatavariesdependingonthesourceofthedata.
Accesstopersistentstorage,suchastoadatabase,variesgreatlydependingonthetypeofstorage(relationaldatabases,object-orienteddatabases,flatfiles,andsoforth)andthevendorimplementation.
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persistentstoragewithdifferentmechanisms
differentAPIstoaccessthesedifferentpersistentstoragemechanisms
explicitlyaccessthepersistentstorage directdependencybetweenapplication
codeanddataaccesscode tightcouplingbetweenthecomponentsand
thedatasourceimplementation difficultandtedioustomigratetheapplication
fromonetypeofdatasourcetoanother
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Components needtoretrieveandstoreinformationfrompersistentstoresandotherdatasources
PersistentstorageAPIsvarydependingontheproductvendor
ThereisalackofuniformAPIstoaddresstherequirementstoaccesssuchdisparatesystems
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PortabilityofthecomponentsisdirectlyaffectedwhenspecificaccessmechanismsandAPIsareincludedinthecomponents
Componentsneedtobetransparenttotheactualpersistentstoreordatasourceimplementation
easymigrationtodifferentvendorproducts,differentstoragetypes,anddifferentdatasourcetypes
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Use a Data Access Object (DAO) to abstract and encapsulate all access to the data source. The DAO manages the connection with the data source to obtain and store data.
Intent abstracts the retrieval of data from a data
resourcesuchasadatabase.Theconceptisto "separate a data resource's clientinterfacefromitsdataaccessmechanism
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DAOimplementstheaccessmechanismrequiredtoworkwiththedatasource
datasource persistentstorelikeanRDBMS externalservicelikeaB2Bexchange repositorylikeanLDAPdatabase businessserviceaccessedviaCORBA
DAOcompletelyhidesthedatasourceimplementationdetailsfromitsclients
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interfaceexposedbytheDAOtoclientsdoesnotchangewhentheunderlyingdatasourceimplementationchanges allowstheDAOtoadapttodifferentstorage
schemeswithoutaffectingitsclientsorbusinesscomponents
Essentially,theDAOactsasanadapterbetweenthecomponentandthedatasource
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BusinessObject (Client) objectthatrequiresaccesstothedata
sourcetoobtainandstoredata
DataAccessObject primaryobjectofthispattern abstractstheunderlyingdataaccess
implementationfortheBusinessObject enabletransparentaccesstothedata
source BusinessObjectalsodelegatesdataloadand
storeoperationstotheDataAccessObject11
DataSource representsadatasourceimplementation couldbeadatabasesuchasanRDBMS,
OODBMS,XMLrepository,flatfilesystem,andsoforth
TransferObject TransferObjectusedasadatacarrier DataAccessObjectmayuseaTransferObject
toreturndatatotheclient DataAccessObjectmayalsoreceivethedata
fromtheclientinaTransferObjecttoupdatethedatainthedatasource
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Factory for Data Access Objects Strategy highlyflexiblebyadoptingtheAbstract
FactoryandtheFactoryMethod
Whentheunderlyingstorageisnotsubjecttochangefromoneimplementationtoanother,thisstrategycanbeimplementedusingtheFactoryMethodpatterntoproduceanumberofDAOsneededbytheapplication
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Factory for Data Access Object strategy using Factory Method
Whentheunderlyingstorageissubjecttochangefromoneimplementationtoanother,thisstrategymaybeimplementedusingtheAbstractFactorypattern
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Enables Transparency implementationdetailsarehiddeninsidethe
DAO.
Enables Easier Migration migrationinvolveschangesonlytotheDAOlayer
Reduces Code Complexity in Business Objects implementation-relatedcode(suchasSQL
statements)iscontainedintheDAOandnotinthebusinessobject
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Centralizes All Data Access into a Separate Layer dataaccesslayercanbeviewedasthelayer
thatcanisolatetherestoftheapplicationfromthedataaccessimplementation
Adds Extra Layer Needs Class Hierarchy Design
hierarchyofconcreteproductsproducedbythefactoriesneedtobedesignedandimplemented
increasesthecomplexityofthedesign
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Implementing the DAO pattern
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Using Factory Method Pattern
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Using Abstract Factory Pattern
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Transfer Object ADAOusesTransferObjectstotransport
datatoandfromitsclients.
Factory Method and Abstract Factory The Factory for Data Access Objects
Strategy usestheFactoryMethodpatterntoimplementtheconcretefactoriesanditsproducts(DAOs).Foraddedflexibility,theAbstractFactorypatternmaybeemployedasdiscussedinthestrategies.
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