DEV475 Mastering Object-Oriented Analysis and Design with UML ...

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DEV475 Mastering Object-Oriented Analysis and Design with UMLTypical Layering ApproachTypical Layering ApproachSpecificfunctionalityApplication SubsystemsDistinct application subsystems that makeup an application — contains the valueadding software developed by theorganization.Business-SpecificMiddlewareSystem SoftwareBusiness specific — contains a number ofreusable subsystems specific to the type ofbusiness.Middleware — offers subsystems for utilityclasses and platform-independent servicesfor distributed object computing inheterogeneous environments and so on.System software — contains the softwarefor the actual infrastructure such asoperating systems, interfaces to specifichardware, device drivers, and so on.GeneralfunctionalityMastering Object Oriented Analysis and Design with UMLCopyright © 2003 Rational Software, all rights reserved 14Layering represents an ordered grouping of functionality, with the application-specificfunctions located in the upper layers, functionality that spans application domains inthe middle layers, and functionality specific to the deployment environment at thelower layers.The number and composition of layers is dependent upon the complexity of both theproblem domain and the solution space:• There is generally only a single application-specific layer.• In domains with existing systems, or that have large systems composed of interoperatingsmaller systems, the Business-Specific layer is likely to partially existand may be structured into several layers for clarity.• Solution spaces, which are well-supported by middleware products and in whichcomplex system software plays a greater role, have well-developed lower layers,with perhaps several layers of middleware and system software.This slide shows a sample architecture with four layers:• The top layer, Application layer, contains the application-specific services.• The next layer, Business-Specific layer, contains business-specific componentsused in several applications.• The Middleware layer contains components such as GUI-builders, interfaces todatabase management systems, platform-independent operating system services,and OLE-components such as spreadsheets and diagram editors.• The bottom layer, System Software layer, contains components such asoperating systems, databases, interfaces to specific hardware, and so on.5 - 14
Module 5 - Architectural AnalysisArchitectural Pattern: LayersArchitectural Pattern: LayersEquipment andcustomer-specificcodeProcesses and otherapplication code54ApplicationMajor abstractions,classes, etc.Mechanisms,servicesH/W specific code, O/Sspecific code, generalpurposecode (forexample, ORB, MQS)321InfrastructureApplicationFrameworkMastering Object Oriented Analysis and Design with UMLCopyright © 2003 Rational Software, all rights reserved 15ContextA large system that requires decomposition.ProblemA system that must handle issues at different levels of abstraction; for example:hardware control issues, common services issues, and domain-specific issues. Itwould be extremely undesirable to write vertical components that handle issues at alllevels. The same issue would have to be handled (possibly inconsistently) multipletimes in different components.Forces• Parts of the system should be replaceable.• Changes in components should not ripple.• Similar responsibilities should be grouped together.• Size of components — complex components may have to be decomposed.SolutionStructure the systems into groups of components that form layers on top of eachother. Make upper layers use services of the layers below only (never above). Try notto use services other than those of the layer directly below. (Do not skip layers unlessintermediate layers would only add pass-through components.)A strict layered architecture states that design elements (classes, components,packages, and subsystems) only utilize the services of the layer below them. Servicescan include event-handling, error-handling, database access, and so forth. It containsmore palpable mechanisms, as opposed to the raw operating system level callsdocumented in the bottom layer.5 - 15
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