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Design PrinciplesDesign Principles
The design process should not suffer from ‘tunnel vision.’The design process should not suffer from ‘tunnel vision.’
The design should be traceable to the analysis model.The design should be traceable to the analysis model.
The design should not reinvent the wheel.The design should not reinvent the wheel.
The design should “minimize the intellectual distance” [DAV95] between theThe design should “minimize the intellectual distance” [DAV95] between the
software and the problem as it exists in the real world.software and the problem as it exists in the real world.
The design should exhibit uniformity and integration.The design should exhibit uniformity and integration.
The design should be structured to accommodate change.The design should be structured to accommodate change.
The design should be structured to degrade gently, even when aberrantThe design should be structured to degrade gently, even when aberrant
data, events, or operating conditions are encountered.data, events, or operating conditions are encountered.
Design is not coding, coding is not design.Design is not coding, coding is not design.
The design should be assessed for quality as it is being created, not after theThe design should be assessed for quality as it is being created, not after the
fact.fact.
The design should be reviewed to minimize conceptual (semantic) errors.The design should be reviewed to minimize conceptual (semantic) errors.
From Davis [DAV95]](https://image.slidesharecdn.com/designconcepts-151125171854-lva1-app6892/85/Design-concepts-and-principle-10-320.jpg)
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ArchitecturArchitectur
ee““The overall structure of the software andThe overall structure of the software and
the ways in which that structure providesthe ways in which that structure provides
conceptual integrity for a system.”conceptual integrity for a system.”
[SHA95a][SHA95a]Structural properties.Structural properties. This aspect of the architectural designThis aspect of the architectural design
representation defines the components of a system (e.g., modules,representation defines the components of a system (e.g., modules,
objects, filters) and the manner in which those components areobjects, filters) and the manner in which those components are
packaged and interact with one another. For example, objects arepackaged and interact with one another. For example, objects are
packaged to encapsulate both data and the processing thatpackaged to encapsulate both data and the processing that
manipulates the data and interact via the invocation of methods .manipulates the data and interact via the invocation of methods .
Extra-functional properties.Extra-functional properties. The architectural design descriptionThe architectural design description
should address how the design architecture achieves requirementsshould address how the design architecture achieves requirements
for performance, capacity, reliability, security, adaptability, andfor performance, capacity, reliability, security, adaptability, and
other system characteristics.other system characteristics.
Families of related systems.Families of related systems. The architectural design should drawThe architectural design should draw
upon repeatable patterns that are commonly encountered in theupon repeatable patterns that are commonly encountered in the
design of families of similar systems. In essence, the design shoulddesign of families of similar systems. In essence, the design should
have the ability to reuse architectural building blocks.have the ability to reuse architectural building blocks.](https://image.slidesharecdn.com/designconcepts-151125171854-lva1-app6892/85/Design-concepts-and-principle-21-320.jpg)
Design concepts and principle,
- 1. 1 Design Concepts andDesign Concepts and PrinciplesPrinciples
- 2. 2 Design Concepts and PrinciplesDesign Concepts and Principles The designer's goal is to produce a model or representation of an entity that will later be built. There are two major phases to any design process: diversification and convergence. Diversification is the acquisition of a repertoire of alternatives, the raw material of design: components, component solutions, and knowledge, all contained in catalogs, textbooks, and the mind.
- 3. 3 Design Concepts and PrinciplesDesign Concepts and Principles During convergence, the designer chooses and combines appropriate elements from this repertoire to meet the design objectives, as stated in the requirements document and as agreed to by the customer. The second phase is the gradual elimination of all but one particular configuration of components, and thus the creation of the final product.
- 4. 4 Design Concepts and PrinciplesDesign Concepts and Principles What is it? Design is a meaningful engineering representation of something that is to be built. It can be traced to a customer’s requirements and at the same time assessed for quality against a set of predefined criteria for “good” design. In the software engineering context, design focuses on four major areas of concern: data, architecture, interfaces, and components.
- 5. 5 Design Concepts and PrinciplesDesign Concepts and Principles Who does it? Software engineers design computer based systems, but the skills required at each level of design work are different. At the data and architectural level, design focuses on patterns as they apply to the application to be built. At the interface level, human ergonomics often dictate our design approach. At the component level, a “programming approach” leads us to effective data and procedural designs.
- 6. 6 Design Concepts and PrinciplesDesign Concepts and Principles Why is it important? You wouldn’t attempt to build a house without a blueprint, would you? You’d risk confusion, errors, a floor plan that didn’t make sense, windows and doors in the wrong place . . . a mess. Computer software is considerably more complex than a house; hence, we need a blueprint— the design.
- 7. 7 Design Concepts and PrinciplesDesign Concepts and Principles What are the steps? Design begins with the requirements model. We work to transform this model into four levels of design detail: the data structure the system architecture, the interface representation, and the component level detail. During each design activity, we apply basic concepts and principles that lead to high quality
- 8. 8 Design Concepts and PrinciplesDesign Concepts and Principles What is the work product? Ultimately, a Design Specification is produced. The specification is composed of the design models that describe data, architecture, interfaces, and components. Each is a work product of the design process
- 9. 9 Design Concepts and PrinciplesDesign Concepts and Principles How do I ensure that I’ve done it right? At each stage, software design work products are reviewed for clarity, correctness, completness, and consistency with the requirements and with one another.
- 10. 10 Design PrinciplesDesign Principles The design process should not suffer from ‘tunnel vision.’The design process should not suffer from ‘tunnel vision.’ The design should be traceable to the analysis model.The design should be traceable to the analysis model. The design should not reinvent the wheel.The design should not reinvent the wheel. The design should “minimize the intellectual distance” [DAV95] between theThe design should “minimize the intellectual distance” [DAV95] between the software and the problem as it exists in the real world.software and the problem as it exists in the real world. The design should exhibit uniformity and integration.The design should exhibit uniformity and integration. The design should be structured to accommodate change.The design should be structured to accommodate change. The design should be structured to degrade gently, even when aberrantThe design should be structured to degrade gently, even when aberrant data, events, or operating conditions are encountered.data, events, or operating conditions are encountered. Design is not coding, coding is not design.Design is not coding, coding is not design. The design should be assessed for quality as it is being created, not after theThe design should be assessed for quality as it is being created, not after the fact.fact. The design should be reviewed to minimize conceptual (semantic) errors.The design should be reviewed to minimize conceptual (semantic) errors. From Davis [DAV95]
- 11. 11 FundamentalFundamental ConceptsConceptsabstractionabstraction—data, procedure, control—data, procedure, control refinementrefinement—elaboration of detail for all—elaboration of detail for all abstractionsabstractions modularitymodularity—compartmentalization of data and—compartmentalization of data and functionfunction architecturearchitecture—overall structure of the software—overall structure of the software Structural propertiesStructural properties Extra-structural propertiesExtra-structural properties Styles and patternsStyles and patterns procedureprocedure—the algorithms that achieve function—the algorithms that achieve function hidinghiding—controlled interfaces—controlled interfaces
- 12. 12 DataData AbstractionAbstractiondoor implemented as a data structure manufacturer model number type swing direction inserts lights type number weight opening mechanism
- 13. 13 ProceduralProcedural AbstractionAbstraction open implemented with a "knowledge" of the object that is associated with enter details of enter algorithm
- 14. 14 StepwiseStepwise RefinementRefinementopen walk to door; reach for knob; open door; walk through; close door. repeat until door opens turn knob clockwise; if knob doesn't turn, then take key out; find correct key; insert in lock; endif pull/push door move out of way; end repeat
- 15. 15 Modular DesignModular Design easier to build, easier to change, easier to fix ...
- 16. 16 ModularModular DesignDesign Let C(x) be a function that defines the perceived complexity of a problem x, and E(x) be a function that defines the effort (in time) required to solve a problem x. Fortwo problems, p1 and p2, if C(p1) > C(p2) it follows that E(p1) > E(p2) As a general case, this result is intuitively obvious. It does take more time to solve a difficult problem.
- 17. 17 ModularModular DesignDesign Another interesting characteristic has been uncovered through experimentation in human problem solving. That is, C(p1 + p2) > C(p1) + C(p2) Expression implies that the perceived complexity of a problem that combines p1 and p2 is greater than the perceived complexity when each problem is considered separately. it follows that E(p1 + p2) > E(p1) + E(p2) This leads to a "divide and conquer" conclusion—it's easier to solve a complex problem when you break it into manageable pieces. The result expressed in Expression has important implications with regard to modularity and software. It is, in fact, an argument for modularity.
- 18. 18 Modularity: Trade-offsModularity: Trade-offs What is the "right" number of modulesWhat is the "right" number of modules for a specific software design?for a specific software design? optimal numberoptimal number of modulesof modules cost ofcost of softwaresoftware number of modulesnumber of modules modulemodule integrationintegration costcost module development costmodule development cost
- 19. 19 Criteria that enable us to evaluate a design method Modular decomposability. If a design method provides a systematic mechanism for decomposing the problem into subproblems, it will reduce the complexity of the overall problem, thereby achieving an effective modular solution. Modular composability. If a design method enables existing (reusable) design components to be assembled into a new system, it will yield a modular solution that does not reinvent the wheel.
- 20. 20 Criteria that enable us to evaluate a design method Modular understandability. If a module can be understood as a standalone unit (without reference to other modules), it will be easier to build and easier to change. Modular continuity. If small changes to the system requirements result in changes to individual modules, rather than systemwide changes, the impact of change-induced side effects will be minimized. Modular protection. If an aberrant condition occurs within a module and its effects are constrained within that module, the impact of error-induced side effects will be minimized.
- 21. 21 ArchitecturArchitectur ee““The overall structure of the software andThe overall structure of the software and the ways in which that structure providesthe ways in which that structure provides conceptual integrity for a system.”conceptual integrity for a system.” [SHA95a][SHA95a]Structural properties.Structural properties. This aspect of the architectural designThis aspect of the architectural design representation defines the components of a system (e.g., modules,representation defines the components of a system (e.g., modules, objects, filters) and the manner in which those components areobjects, filters) and the manner in which those components are packaged and interact with one another. For example, objects arepackaged and interact with one another. For example, objects are packaged to encapsulate both data and the processing thatpackaged to encapsulate both data and the processing that manipulates the data and interact via the invocation of methods .manipulates the data and interact via the invocation of methods . Extra-functional properties.Extra-functional properties. The architectural design descriptionThe architectural design description should address how the design architecture achieves requirementsshould address how the design architecture achieves requirements for performance, capacity, reliability, security, adaptability, andfor performance, capacity, reliability, security, adaptability, and other system characteristics.other system characteristics. Families of related systems.Families of related systems. The architectural design should drawThe architectural design should draw upon repeatable patterns that are commonly encountered in theupon repeatable patterns that are commonly encountered in the design of families of similar systems. In essence, the design shoulddesign of families of similar systems. In essence, the design should have the ability to reuse architectural building blocks.have the ability to reuse architectural building blocks.
- 22. 22 Control Hierarchy depth and width provide an indication of the number of levels of control and overall span of control, respectively. Fan-out is a measure of the number of modules that are directly controlled by another module. Fan-in indicates how many modules directly control a given module
- 23. 23 Control Hierarchy The control hierarchy also represents two subtly different characteristics of the software architecture: visibility and connectivity. Visibility indicates the set of program components that may be invoked or used as data by a given component, even when this is accomplished indirectly. Connectivity indicates the set of components that are directly invoked or used as data by a given component
- 24. 24 Module YModule X Module Q Module R MODULE D MODULE E MODULE GMODULE CMODULE B MODULE F Module M Module N Module S Module TModule K Module L EXECUTIVEEXECUTIVE Depth Breadth Fan out Fan in MODULE A
- 25. 25 Structural Hierarchy the program structure can be partitioned both horizontally and vertically.
- 26. 26 Data StructureData Structure Data structure is a representation of the logical relationship among individual elements of data. Because the structure of information will invariably affect the final procedural design, data structure is as important as program structure to the representation of software architecture.
- 27. 27 Software ProcedureSoftware Procedure Program structure defines control hierarchy without regard to the sequence of processing and decisions. Software procedure focuses on the processing details of each module individually. Procedure must provide a precise specification of processing, including sequence of events, exact decision points, repetitive operations, and even data organization and structure.
- 28. 28 Information HidingInformation Hiding modulemodule controlledcontrolled interfaceinterface "secret""secret" •• algorithmalgorithm •• data structuredata structure •• details of external interfacedetails of external interface •• resource allocation policyresource allocation policy clientsclients a specific design decisiona specific design decision
- 29. 29 Why Information Hiding?Why Information Hiding? reduces the likelihood of “side effects”reduces the likelihood of “side effects” limits the global impact of local designlimits the global impact of local design decisionsdecisions emphasizes communication throughemphasizes communication through controlled interfacescontrolled interfaces discourages the use of global datadiscourages the use of global data leads to encapsulation—an attribute ofleads to encapsulation—an attribute of high quality designhigh quality design results in higher quality softwareresults in higher quality software
- 30. 30 FunctionalFunctional IndependenceIndependence COHESION - the degree to which a module performs one and only one function. COUPLING - the degree to which a module is "connected" to other modules in the system.