2. CSEB233: Fundamentals of Software Engineering Software Design

3. Objectives  Explain set of design principles, concepts, and practices  Describe four design models required for a complete design specification:  data design  architectural design  user interface design  component-level design  Use design support tools and evaluation  Explain the use of design specification document

4. Software Design Design Principles, Concepts, and Practices

5. What is Design?  Design creates a representation or model of the software  But unlike the analysis model, the design model provides details about:  software architecture  data structures  interfaces, and  components that are necessary to implement the system.  Why is it important?  The model can be assessed for quality and improved before code is generated  Tests are conducted, and more users are involved.

6. Analysis Models  Design Model The four design models Each of the elements of the analysis/requirements model provides information that is necessary to create the design models required for a complete design specification.

7. Characteristics of Good Design  Must implement all of the explicit requirements contained in the analysis model  Must accommodate all the implicit requirements desired by the customer  Must be a readable, understandable guide for those who generate code and for those who test and subsequently support the software  Should provide a complete picture of the software, addressing the data, functional, and behavioral domains from an implementation perspective (McGlaughlin, 1991)

8. Design Principles  The design process should not suffer from ‘tunnel vision’  The design should be traceable to the analysis model  The design should not reinvent the wheel  Reinventing the wheel means to duplicate a basic method that has previously been created or optimized by others  The design should ‘minimize the intellectual distance’ between the software and the problem as it exists in the real world  The design should exhibit uniformity and integration (Davis,1995 )

9. Design Principles  The design should be structured to accommodate change  The design should be structured to degrade gently, even when irregular data, events, or operating conditions are encountered  The design should be assessed for quality as it is being created, not after  The design should be reviewed to minimize conceptual (semantic) errors

10. Design Concepts  Fundamental design concepts that span both traditional and OO software development include:  Abstraction  Architecture  Patterns  Separation of concerns  Modularity  Information hiding  Functional Independence  Refinement  Aspects  Refactoring  OO Design concepts  Design classes

11. Abstraction  Designers should work to derive both procedural and data abstractions that serve the problem  Procedural abstraction  Sequence of instructions that have a specific and limited function  Data abstractions  Named collection of data that describes a data object

12. Abstraction door manufacturer model number type swing direction weight door manufacturer model number type swing direction weight Implemented as a data structure door details of enter algorithm door details of enter algorithm Implemented with a "knowledge" of the object that is associated with “ enter”

13. Abstraction  Data abstraction refers to, providing only essential features by hiding its background details b1 is an object calling input and output member functions. But that code is invisible to the object b1

14. Architecture  Is concerned with:  describing the fundamental organization of the system,  identifying its various components and their relationships to each other, and  the environment in order to meet the system's quality goals  It also describes the overall structure of the software:  organization of program modules,  the manner in which these modules interact, and  the structure of data used by the components

15. Patterns  Online pattern searching:  http://inventors.about.com/gi/dynamic/offsite. htm?site=http%3A%2F%Fpatents.uspto.gov %2Fpatft%2Findex.html http://inventors.about.com/gi/dynamic/offsite. htm?site=http%3A%2F%Fpatents.uspto.gov %2Fpatft%2Findex.html  Under Quick Search, try to search pattern no. 7669112.  You will get ‘automated spell analysis’ pattern.

16. Separation of Concerns  It is a rule of thumb to define how modules should be separated to each other:  different or unrelated concerns should be restricted to different modules  Any complex problem can be easily handled if it is sub-divided into pieces that can be solved independently  Why? So that a problem takes less time and effort to solve  Manifested in other design concepts: modularity, aspects, functional independence and refinement

17. Modularity  When software is divided into components (modules)  A design is modularized:  To ease the planning for implementation (coding)  To define and deliver software increments  To easily accommodate changes  To efficiently test and debug program, and  To conduct long-term maintenance without serious side effects

18. Modularity : Example Without Module With Module vs.

19. Information Hiding  Suggests that modules should be specified and designed so that information (data structures and algorithm) contain within a module is inaccessible to other modules that have no need for such information  Implies that effective modularity can be achieved by defining a set of independent modules that communicate with one another only necessary information

20. Information Hiding  Serves as an effective criterion for dividing any piece of equipment, software or hardware, into modules of functionality  Provides flexibility  This flexibility allows a programmer to modify functionality of a computer program during normal evolution as the computer program is changed to better fit the needs of users  When a computer program is well designed decomposing the source code solution into modules using the principle of information hiding, evolutionary changes are much easier because the changes typically are local rather than global changes

21. Information Hiding: Example  Suppose we have a Time class that counts the time of day:  The Display() member function prints the current time onscreen.  This member function is accessible to all  It is therefore declared public  In contrast, the data member ticks is declared private.  External users could not access it  Regardless of how Display() extracts the current timestamp from ticks, users can be sure that it will “do the right thing” and display the correct time onscreen.

22. Functional Independence  Achieved by developing independent modules – each module address a specific subset of requirements  Is assessed using cohesion and coupling.  Cohesion is an indication of the relative functional strength of a module  a cohesive module should (ideally) do just one thing  Coupling is an indication of the relative inter- dependence among modules  depends on the interface complexity between modules, the point at which entry or reference is made to a module, and what data pass across the interface.

23. Functional Independence  This class lacks cohesion

24. Functional Independence  The CashRegister involves two concepts: cash register and coin  Solution: Make two classes:

25. Functional Independence  Coupling: High and Low Coupling between Classes

26. Refinement open 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

27. Refactoring  “Refactoring is the process of changing a software system in such a way that it does not alter the external behavior of the code [design] yet improves its internal structure” (Fowler, 1999)  When software is refactored, the existing design is examined for  redundancy  unused design elements  inefficient or unnecessary algorithms  poorly constructed or inappropriate data structures  or any other design failure that can be corrected to yield a better design.

28. Software Design Design Models

29. Design Model Elements  Data/Class Design  Class diagrams transformed into the design class realization and the data structures required to implement the software  Architectural Design  Provides high-level overview of the system with detailed descriptions to be given by other design elements  Defines the relationship between major structural elements of the software, the architectural styles and design patterns that can be used to achieve the requirements of the system and the constraints that affect the way in which the architecture can be implemented

30. Design Models  Interface Design  Describes how the software communicates with systems that interoperate with it, and with human who use it  Component-Level Design  Defines the data structures, algorithms, interface characteristics, and communication mechanisms allocated to each software component or module

31. Data/Class Design Elements  Example: High-level DFD  Source: http://yourdon.com/strucanalysis/wiki/index.php?title=Chapter_9http://yourdon.com/strucanalysis/wiki/index.php?title=Chapter_9

32. Data/Class Design Elements  Example: ERD  Source: http://www.svgopen.org/2003/papers/SvgInterfaceElectricalSwitching/index.htmlhttp://www.svgopen.org/2003/papers/SvgInterfaceElectricalSwitching/index.html

33. Data/Class Design Elements  Example: Class Diagram  Source: http://www.agiledata.org/essays/objectOrientation101.htmlhttp://www.agiledata.org/essays/objectOrientation101.html

34. Architectural Design Elements  The architectural model is derived from three sources:  information about the application domain for the software to be built  specific requirements model elements such as data flow diagrams or analysis classes, their relationships and collaborations for the problem at hand, and  the availability of architectural patterns and styles

35. Architectural Design Elements  Example: Architecture diagram  Source: http://blog.tmcnet.com/blog/tom-keating/2004/10/index.asp?page=7http://blog.tmcnet.com/blog/tom-keating/2004/10/index.asp?page=7

36. Architectural Design Elements  Example: Architectural styles

37. Interface Design Elements  External interfaces to other systems, devices, networks or other producers or consumers of information

38. Interface Design Elements  The user interface (UI)

39. Component Design Elements  UML Component Diagram  Source: http://edn.embarcadero.com/article/31863#component-and-deployment-diagranshttp://edn.embarcadero.com/article/31863#component-and-deployment-diagrans

40. Software Design Design Support Tools

41.  All aspects of the software engineering can be supported by software tools:  from project management software through tools for business and functional analysis, system design, code storage, compilers, translation tools, test software, and so on.  However, tools that are concerned with analysis and design, and with using design information to create parts (or all) of the software product, are most frequently thought of as CASE tools.  List of CASE tools:  http://www.unl.csi.cuny.edu/faqs/software- enginering/tools.html http://www.unl.csi.cuny.edu/faqs/software- enginering/tools.html

42. CASE Tool: Example  IBM Rational Software Architect  Source: http://www.ibm.com/developerworks/rational/library/10/whats-new- in-rational-software-architect-8/index.htmlhttp://www.ibm.com/developerworks/rational/library/10/whats-new- in-rational-software-architect-8/index.html

43. CASE Tool: Example  Database Modeling: Enterprise Architect by Sparx Systems Pty Ltd.  Source: http://www.sparxsystems.com/images/screenshots/platforms/databasemodeling_tn.jpghttp://www.sparxsystems.com/images/screenshots/platforms/databasemodeling_tn.jpg

44. Design Evaluation  “A good software design minimizes the time required to create, modify, and maintain the software while achieving run-time performance” (Shore and Chromatic, 2007)  Design quality is people-sensitive.  For instance, design quality is dependent upon the programmers writing and maintaining the code.  Design quality is change-specific.  There are two general ways to make designs of higher quality with respect to this aspect:  generalizing from the tools like design patterns, and  using tests and refactoring as change-enablers. (Elssamadisy, 2007)

45. Design Evaluation  Modification and maintenance time are more important than creation time because  time spent in maintenance is much more than creation time of a software, and  in iterative development, modification happens during the initial creation of the software  Design quality is unpredictable  Because quality is really dependant on the team developing the software. As the team changes, or evolves, then the design quality also evolves  You really only know how good a design is by its standing the test of time and modifications

46. Design Evaluation  Points to ponder:  To maintain the quality of the design, we must maintain the theory of the design as the programming team evolves,  That design quality is tied to the people who are building and maintaining the software (Shore and Chromatic, 2007)

47. Software Design Design Specification

48.  IEEE Standard for Information Technology - Systems Design - Software Design Descriptions (IEEE 1016-2009) an improved version of the 1998 version  This standard specifies an organizational structure for a software design description (SDD)  An SDD is a document used to specify system architecture and application design in a software related project  Provides a generic template for an SDD

49. SDD: Examples  Refer to  http://www.cs.uofs.edu/~dmartin/exsesrm.ht m#_Toc514249734 http://www.cs.uofs.edu/~dmartin/exsesrm.ht m#_Toc514249734  Refer to SDDSample.doc

50. Summary  This module has:  Introduced set of design principles, concepts, and practices  Described four design models required for a complete design specification: data design, architectural design, user interface design, and component-level design  Introduced design support tools and evaluation  Introduced design specification document (SDD)

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