Software Design

1. Software Design Requirements Design Code-Implementation Test Case Design Testing Builds Rel. Maintenance

2. Important “End-Results” of Design • “Well-Designed” software results in solution that is: • Easy to learn • Easy to use • Easy to code • Easy to maintain • Affordable (economical) • Dependable • Valuable (increase productivity, life saving, etc. – meets customer requirements ) Which ones do the users care? Any other property that you may want to add? --- easy to integrate or test?

3. Design of Software Product • Designers are interested not only in the code, but the complete product which includes: • Documentation and User Instructions • Code (functionality, data, security, reliability, etc.) • Interfaces (including user) • Packaging Scheme • Installation • Service (including maintenance) This seems like a lot of topics that software designers must cover !! So, there are specializations within the field of software design.

4. Design as an “Activity” (verb) Software “Product” Design is the activity of specifying - the nature and - composition of software products that satisfy the clients’ - needs and - desires subject to certain set of constraints What are the concepts behind “nature of” versus “composition of” ? What about “needs” versus “desire” ?

5. Understanding the Problem • Design occurs “after” requirements (which identifies the “problems”) • “after” is fuzzy time boundary – there is no clean boundary • Think of Design as both a) understanding & b) solving “problems” • Customer (user) identification of problem (requirements) sometimes borders on solution --- but is really a constraint. Thus we should thoroughly “understand” problems • For each problem there may be several solutions or designs that apply • Use some “time-honored” problem solving techniques • Modifyingthe problem slightly to • a) facilitate easier/cheaper solution or • b) to match previously solved problem • Trial and error (e.g. iterative prototyping is the fancy term) • Group brainstorming(I do not recommend this for design)

6. Two Basic Design Techniques • Abstraction • Focusing on the “relevant” properties of an entity by suppressing and ignoring some other properties and the details ----- trick is deciding on the “relevant.” • Modeling • Constructing a model “entity” to represent the real entity of interest through: • Associating :parts of the model to the parts of the real entity • Associating: the relationships among parts of the model to the relationships among parts of the entity of interest.

7. 2 aspects of Abstraction • A) Problem/Solution “Simplification”: concentrating on only the key property • (For example): In drawing a figure, we choose to first focus on shape of the figure (e.g. circle, oval, rectangle, diamond, etc.) before worrying about the color, the line width, solid/dotted line, etc. • B) Problem/Solution“Refinement: simplify first and then enhance the solution with more details • Top-down refinement (Decomposition): partition the problem into several key components first and then solve the details of each component • Bottom-up aggregation: solving pieces of a complex problem and then aggregate the solutions • Note: Bottom-up solution usually requires a top-down framework of key components.

8. Modeling • Modeling is based on problem/solution “simplification” aspect ofAbstraction. • Decide on the key components (abstraction) of the entity of interest. • Decide on the key relationships that exist among the components of the entity of interest. • Construct the components of the model to represent the components of the entity of interest. • Construct the relationships among the components of the model to reflect the relationships among the components of the original entity of interest. • In doing so: • There may be parts in the model that do not match the original entity • There may be relationships in the model that do not match that in the original entity • Note: If everything matched, then the model would just be a “copy,” not an abstraction of the original entity.

9. Modeling: “components” and “relations” Part B Part A modeling “Thing” that Needs modeling Part D Part C Part F

10. Another Definition of Design (from K. Lano) • Designis the activity of (i) “constructing” componentsand (ii) “organizing” their interactions in order to achieve the system that will satisfy the requirements. • Components: • Identifying the components • Specifying their functionalities • Specifying any constraints (performance, security, etc.) • Interactions: • Identify and Specify Component relations (inheritance, aggregation, etc.) • Identify and Specify Component dependencies (interfaces, joint responsibilities, sequences of interactions, etc.) Compare this with previous definition --- this is a better refinement

11. Develop a Software Model for A “Solar System” • Start the discussion of this in class • The remaining parts are given as an assignment. (see Assignment 1) For this assignment also think about what Is a “well constructed” model or design e.g. easy to understand easy to implement etc. See earlier slide

12. Advantages of modeling • Allows us to understand the problem or requirements better (model of the problem) • Allows us to create an “initial rough design” • “Model” of problem develops into “Model” of solution • Allows us to investigate and study both the problem and the solution in more detail • Allows us to have a documentation of the problem/solution descriptions (models)

13. Two Types of Design Models • Static model: • Represents the parts, relationships, and attributes that do not change (during execution of the software ) • Dynamic model: • Represents what happens during the execution of the software. In this course we will be using both static and dynamic models of software

14. Software Product Designer &Engineering Designer • A software product designer is concerned with: • Product features and capabilities • Visually appealing and easy to use user-interfaces • Fitting into users’ business processes and operability • Product packaging ** ( a closer tie to what is specified in the product requirements ) • A software engineering designer is concerned with: • Individual functionalities and performance • Internal structure of the software system • Individual programs and their interfaces & interactions • The data flow through the software system (including inputs and outputs) • System maintenance and evolution ** (a closer tie to the internal technical design ) (similarities and differences?)

15. Software Design Team • We Need both: • Software Product Designer • Software Engineering Designer • Different sets of skills needed: • Product Designer – knowledge of: user interface, communicating the requirements/product design, marketing, business processes, applications domain knowledge • Engineering Designer – knowledge of: architecture, programming, database and data structures, development platforms, networking, operating systems

16. Software Design product designer Requirements engineering designer Design Code-Implementation product designer Test Case Design Testing Builds Rel. Maintenance (product and engineering redesign)

17. Software Product Design.vs. Software Engineering Design • Product Design Activity – output is a software requirements specification document • Answers mostly “what” the user and customer needs and wants --- formulates the problem in an organized way : • Functionalities & features • Looks and interfaces • Constraints • Engineering Design Activity – output is a design specification document • Answers mostly “how” the software will be put together to respond to the users’ needs and wants --- formulates the solution in an organized way • High level software: • Components and • How they interact • Rationale behind your design decisions ( * my addition ) • Low level specifications of programs and data and their behavior organize: relating these * * * Information/data needs to be folded in also * * * Where is the analysis / innovation / creativity? ---- what do you think?

18. Is There a Design Method? • There is no ONE design method in the form of a “recipe” for design. • There are several methods that all speak to: • Design process: a set of steps that starts with “some inputs” and transform that input to some output and eventually results in a design specification (e.g. structured method, OO-method, etc.) • Design notation: a set of symbolic representations and how the symbols are to be used (e.g. UML, DFD, etc.) • Design “heuristics”: a set of guiding rules (e.g. low coupling and high cohesion) Note:There are cooking recipes! But most software designs are too large and complex compared to food dishes; so there are no design “recipes” . More importantly, the mental activities that perform the design (analysis/innovation), I believe, is not easily explainable or understood ---- thus it would be hard to “teach” it!

19. Brief History of Major Design “Methods” • Niklaus Wirth; E. Dijkstra (early 1970’s) : Stepwise Refinement • top-down approach that repeatedly decomposed procedures into smaller ones until we reach programming level. • Stevens, Myers, and Constantine (mid-1970’s): Structured Design: (also DeMarco; Ross; Mills; etc.) • Also top-down and procedural in nature • Included dataflow diagram and entity-relationship considerations • Started people on using CASE tools and associated notations provided by the tools • Object Oriented Design (late 1980’s- mid 1990’s) • Based on “Objects/ Classes” (inheritance, polymorphism, etc.) • Uses standard UML design notation (Booch, Jacobson, Rumbaugh) functional abstraction functional & data abstraction “object” abstraction