1 Software Design Overview Reference: Software Engineering, by Ian Sommerville, Ch. 12 & 13

2 Topics What is Design? Design Description The Design Process –Architectural Design Design Strategies Design Quality –Component Cohesion –Component Coupling

3 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

4 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)

5 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

6 The Design Process (con’t) Component Design –Break subsystems into components and document their interfaces Data Structure Design –Specify the data structures used in the system implementation Algorithm Design –Specify the implementation algorithms

7 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

8 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

9 Student Information Repository Course Schedule Generator Transcript Generator Graduation Checkout System Student Registration System Grade Report Generator A Student Information System

10 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

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12 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

13 Event and message handler Subsystem 1Subsystem 2Subsystem 3Subsystem 4 Generic Event-driven System

14 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.

15 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

16 Component Cohesion A measure of the closeness of the relationships between the component’s components Component should implement a single logical function/task (functional) or implement a single logical entity (OO) We want strong cohesion

17 Component Cohesion (con’t) 7 levels of cohesion (Constantine & Yourdan), weakest to strongest: Coincidental Cohesion –The parts of a component are not related but simply bundled into a single component Logical Association –Components that perform similar functions such as input, error handling and so on are put together in a single component

18 Component Cohesion (con’t) Temporal Association –All of the components that are activated at a single time, such as start up or shut down, are brought together Procedural Cohesion –The elements in a component make up a single control sequence Communicational Cohesion –All of the elements of a component operate on the same input data or produce the same output data

19 Component Cohesion (con’t) Sequential Cohesion –The output from one element in the component serves as input for some other element Functional Cohesion –Each part of the component is necessary for the execution of a single function/task

20 Component Cohesion (con’t) Cohesion applies to both functional and OO design approaches: Cohesive Function –Performs a single task Cohesive Object –A single entity is represented and all the operations on that entity are included with the object So, which promotes strong cohesion better -- functional or OO design?

21 Component Coupling A measure of the strength of the interconnections between components in a design Want components to be as independent as possible We want low coupling

22 Component Coupling (con’t) Functional Design –No/little global data –No hard-coded constants –Nothing that causes one function to require knowledge of another’s implementation OO Design –Inheritance by nature causes coupling between base and derived classes –Multiple inheritance greatly increases coupling

23 How components are coupled References from one component to another, such as invocation Amount of data passed from one component to another Amount of control one component has over another Degree of complexity of interface, e.g., one entry point vs. mutual entry points

24 Goal is to Minimize Coupling Enables us to change portion of system while disrupting rest of system little as possible Very low coupling might allow pull-out, plug-in replacement of only one component Loose coupling may require changing or replacing a few components High coupling may require widespread perturbations in system Low coupling reduces the number of components needing revision

25 Types of Coupling Content: one component directly modifies data or control flow of another Common: Organizing data into a common store Control: Control flags passed as parameters between components Stamp: Data structures passed Data: Only primitive data passed