Software Design Overview

# Software Design Overview Reference: Software Engineering, by Ian Sommerville, Ch. 12 & 13 ## Topics * What is Design? * Design Description * The Design Process * Architectural Design * Design Strategies * Design Quality * Component Cohesion * Component Coupling ## What is Design? * Where i...

# Software Design Overview Reference: Software Engineering, by Ian Sommerville, Ch. 12 & 13 ## Topics * What is Design? * Design Description * The Design Process * Architectural Design * Design Strategies * Design Quality * Component Cohesion * Component Coupling ## What is Design? * Where informal ideas are transformed to detailed implementation descriptions. * It is a creative process. * There is no design "cookbook". * It is learned by experience and study of existing systems. ## Design Description Three main design notations: * **Graphical notations** * Display relationships between components. * Relate the design to the real-world system. * **Program Description Languages (PDLs)** * Pseudocode * **Informal text** * For anything that can't be described formally (e.g. design rationale, non-functional considerations). ## The Design Process * **Architectural Design** * Subsystems and their relationships are identified and documented. * **Abstract specification** * Document an abstract specification of the services provided by and constraints on each subsystem. * **Interface Design** * Document each subsystem's interface. ## Architectural Design ### How a system is decomposed into subsystems that provide some related set of services. * **Domain-independent architectures** * Repository Model * Client-Server Model * Event-driven Model * Many others... * **Domain-specific architectures** ### Architectural Design - Repository Model * Systems which use large amounts of data are organized around a shared database or repository. * Suited to applications where data is generated by one subsystem and used by others. * Example: a management information system. A diagram shows a student information system with the following components: * Student Registration System * Grade Report Generator * Transcript Generator * Course Schedule Generator * Graduation Checkout System All of these components are connected to the *Student Information Repository*. ### Architectural Design - Client-Server Model * A distributed system model which shows how data and processing is distributed across a range of processors. * Servers offer services to other subsystems. * Clients call on the services offered by the servers. A diagram shows a film and picture library with the following components: * Client 1 * Client 2 * Client 3 * Client 4 All clients are connected to a shared *Wide-bandwidth network* and access the following servers: * Catalogue Server * Video Server * Picture Server * Hypertext Server *Catalogue server* provides access to *Catalogue*. *Video server* provides access to *Film clip files*. *Picture server* provides access to *Digitized photographs*. *Hypertext server* provides access to *Hypertext web*. ### Architectural Design - Event-Driven Model * Driven by externally generated events. * The timing is outside the control of the process that handles that event. * Examples: * Spreadsheets * GUI * Typically any real-time system. A diagram shows a generic event-driven system with: * Subsystem 1 * Subsystem 2 * Subsystem 3 * Subsystem 4 All these systems interact through a shared *Event and message handler*. ## Design Strategies * **Functional Design** * System is designed from a functional viewpoint, starting with a high-level view and refining this into a more detailed design. The system state is centralized and shared between the functions. * **Object-oriented Design** * System is viewed as a collection of objects rather than functions. The system state is decentralized. Each object manages its own information. ## Design Quality What is "good" design? No general agreement, but: * Should correctly implement specification * Must be understandable * Good naming conventions * Good internal and external documentation * Minimize complex algorithms * Must be able to adapt to modification or addition of new functionality * High component cohesion * Low component coupling