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Software Design: Architecture, Modules, and Metrics for Quality

This article discusses software design principles, architecture examples, module characterization, coupling and cohesion metrics, and object-oriented analysis and design. Learn how to design high-quality software using effective architectural patterns, modularization techniques, and measurement metrics.

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Software Design: Architecture, Modules, and Metrics for Quality

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  1. Contents • Introduction • Requirements Engineering • Project Management • Software Design • Detailed Design and Coding • Quality Assurance • Maintenance

  2. Software Design • Design activities • Architecture examples • Modularisation and Metrics

  3. Design Activities Transform the logical model ( RE) into a physical model, in sufficient detail to permit its physical realization. • Architectural design • Identify the systems components • Structure the system components • Assign functionality to components • Assign data to components • Plan for future changes • Define the structure of the implementation • Detailed design • Refine the (architectural) components • Choose specific data structures • Choose specific algorithms • Define the logic of the implementation

  4. Architecture Example: Pipes and filters

  5. Architecture Example: Layering

  6. Architecture Example: Repository

  7. Client-server architecture Film and picture library

  8. Design Principles • Abstraction • Encapsulation • Information Hiding • Structuring • Modularisation

  9. Characterisation of Modules • Logic entities which fulfil certain tasks • Simple entities, i.e. their tasks can be described clearly and briefly • Units containing data and/or operations • Provide resources usable by other modules • Their realisation is encapsulated • May use resources from other modules

  10. Classical metrics: LOC Cyclomatic number (McCabe) Control variable complexity (McClure) Software science (Halstead) “Wrong” understanding of module Late applicability More useful: Coupling Cohesion Fan-in/fan-out Graph-oriented metrics Weighted methods per class Depth/width of inheritance trees ... Measuring Module Quality Reliable and early data with significant impact on quality.

  11. Coupling • Measures the degree of independence between different modules • Content coupling • Common coupling • (External coupling) • Control coupling • Stamp coupling • Data coupling • Each module should communicate with as few as possible other module • Communicating modules should exchange as few as possible data • All communication must be explicit bad good

  12. How to Uncouple Modules • Data coupling • Exchange only necessary information • Do not pass data through several modules • Stamp coupling • Do not encapsulate unrelated data • Control coupling • Limit control information in interfaces • Common/External coupling • Pass data explicitly as parameters • Divide complex data into independent parts that can be exclusively used of different modules • Hide data • Content coupling USCH

  13. Cohesion • Measures “relatedness” of the resources encapsulated in one module • Coincidental cohesion • Logical cohesion • Temporal cohesion • Procedural cohesion • Communicational cohesion • Sequential cohesion • Functional/informational cohesion • Each element in a module should be a necessary and essential part of one and only task bad good

  14. Coupling / Cohesion Summary • The modules of a system should be highly cohesive and loosely coupled • Good modularisation

  15. M17 M19 M18 M11 M12 M13 M10 M15 M16 M14 M6 M8 M9 M2 M3 M1 M5 M7 M4 Fan-in vs Fan-out Depth Width M9: Fan-in = 2, fan-out = 4

  16. Fan-in vs Fan-out Rules • Minimise structures with high fan-out • Strive for fan-in as depth increases

  17. Object Oriented Analysis and Design Analysis Rectangle height width getArea resize • Each entity in the system is an object. • A method or operation is an action that can be performed directly by the object or can happen to the object. • Encapsulation: the methods form a protective boundary around an object. • Class hierarchies of objects encourage inheritance. • Polymorphism: same method for different objects, each with different behaviour. Design Rectangle {author = Joe} height: int width: int getArea(): int resize (int,int)

  18. Object Model

  19. Object Oriented Design Metrics • Weighted Methods Per Class (WMC) • Depth of Inheritance Tree (DIT) • Number of Children (NOC) • Coupling between Object Classes (CBO) • Response for a Class (RFC) • Lack of Cohesion in Methods (LCOM)

  20. Traceability Matrix • Relates requirements to design artefacts • Shows dependencies • Supports change management • Useful for other traceability purposes

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