Architectural DesignObjectivesTopics coveredSoftware architectureArchitectural designAdvantages of explicit architectureArchitecture and system characteristicsArchitectural conflictsSystem structuringPacking robot control systemBox and line diagramsArchitectural design decisionsSlide 13Architecture reuseArchitectural stylesArchitectural modelsSystem organisationThe repository modelCASE toolset architectureRepository model characteristicsClient-server modelFilm and picture libraryClient-server characteristicsAbstract machine (layered) modelVersion management systemModular decomposition stylesSub-systems and modulesModular decompositionObject modelsInvoice processing systemObject model advantagesFunction-oriented pipeliningSlide 33Pipeline model advantagesControl stylesCentralised controlCall-return modelReal-time system controlEvent-driven systemsBroadcast modelSelective broadcastingInterrupt-driven systemsInterrupt-driven controlReference architecturesSlide 45OSI reference modelCase reference modelThe ECMA reference modelKey pointsSlide 50Slide 51Architecture attributes©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 1Architectural Design©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 2ObjectivesTo introduce architectural design and to discuss its importanceTo explain the architectural design decisions that have to be madeTo introduce three complementary architectural styles covering organisation, decomposition and controlTo discuss reference architectures are used to communicate and compare architectures©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 3Topics coveredArchitectural design decisionsSystem organisationDecomposition stylesControl stylesReference architectures©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 4Software architectureThe design process for identifying the sub-systems making up a system and the framework for sub-system control and communication is architectural design.The output of this design process is a description of the software architecture.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 5Architectural designAn early stage of the system design process.Represents the link between specification and design processes.Often carried out in parallel with some specification activities.It involves identifying major system components and their communications.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 6Advantages of explicit architectureStakeholder communication•Architecture may be used as a focus of discussion by system stakeholders.System analysis•Means that analysis of whether the system can meet its non-functional requirements is possible.Large-scale reuse•The architecture may be reusable across a range of systems.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 7Architecture and system characteristicsPerformance•Localise critical operations and minimise communications. Use large rather than fine-grain components.Security•Use a layered architecture with critical assets in the inner layers.Safety•Localise safety-critical features in a small number of sub-systems.Availability•Include redundant components and mechanisms for fault tolerance.Maintainability•Use fine-grain, replaceable components.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 8Architectural conflictsUsing large-grain components improves performance but reduces maintainability.Introducing redundant data improves availability but makes security more difficult.Localising safety-related features usually means more communication so degraded performance.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 9System structuringConcerned with decomposing the system into interacting sub-systems.The architectural design is normally expressed as a block diagram presenting an overview of the system structure.More specific models showing how sub-systems share data, are distributed and interface with each other may also be developed.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 10Packing robot control systemVisionsystemObjectidentifi cationsystemArmcontrollerGrippercontrollerPackagingselectionsystemPackingsystemConveyorcontroller©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 11Box and line diagramsVery abstract - they do not show the nature of component relationships nor the externally visible properties of the sub-systems.However, useful for communication with stakeholders and for project planning.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 12Architectural design decisionsArchitectural design is a creative process so the process differs depending on the type of system being developed.However, a number of common decisions span all design processes.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 13Architectural design decisionsIs there a generic application architecture that can be used?How will the system be distributed?What architectural styles are appropriate?What approach will be used to structure the system?How will the system be decomposed into modules?What control strategy should be used?How will the architectural design be evaluated?How should the architecture be documented?©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 14Architecture reuseSystems in the same domain often have similar architectures that reflect domain concepts.Application product lines are built around a core architecture with variants that satisfy particular customer requirements.Application architectures are covered in Chapter 13 and product lines in Chapter 18.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 15Architectural stylesThe architectural model of a system may conform to a generic architectural model or style.An awareness of these styles can simplify the problem of defining system architectures.However, most large systems are heterogeneous and do not follow a single architectural style.©Ian Sommerville 2004