Sqa lecture08 usama
- 2. 2 Recap Software requirements and software quality assurance
- 3. 3 Today’s Lecture Software design Software design model and process Software design defects
- 4. 4 Design? Synonyms: plan, arrangement, lay out, map, scheme Antonyms: accident, fluke, chance, guess Design is an activity of creating a solution that satisfies a specific goal or need Design is the backbone of all products and services Considered an artistic and heuristic activity
- 5. 5 Software Design - 1 Software design is an artifact that represents a solution, showing its main features and behavior, and is the basis for implementing a program or collection of programs
- 6. 6 Software Design - 2 Design is a meaningful representation of something that is to be built. It can be traced to a customer’s requirements and at the same time assessed for quality against a set of pre-defined criteria of “good” design
- 7. 7 Software Design - 3 Software design is different from other forms of design because it is not constrained by physical objects, structures, or laws As a result, software design tends to be much more complex than other forms of design because it is conceptually unbounded, whereas the capabilities of the human mind are bounded
- 8. 8 Software Design - 4 Expert designers use a mostly breadth-first approach because it allows them to mentally simulate the execution of an evolving system to detect unwanted interaction, inconsistencies, weaknesses, and incompleteness of their designs
- 9. 9 Software Design - 5 Thus, design is driven by known solutions, which increases performance by allowing a user to dynamically shift goals and activities
- 10. 10 Software Design - 6 Good designers structure problem formulations by discovering missing information, such as problem goals and evaluation criteria, and resolving many open-ended constraints Hence the challenge of design necessitates the use of a methodical approach based on key principles and practices to effectively and efficiently produce high quality software designs
- 11. 11 Software Design - 7 However, designers must occasionally deviate from a defined method in response to newly acquired information of insights
- 12. 12 Software Design - 8 Software design principles identify strategic approaches to the production of quality software designs Software design practices identify tactical methods for producing quality software designs Software design procedures provide an organizational framework for designing software
- 13. 13 An Important Point Try to associate quality attributes with every aspect of software design
- 14. 14 Design and Quality Design is the place where quality is fostered in software engineering Design provides us with representation of software which can be assessed for quality Design is the only way that we can accurately translate a customer’s requirements into a finished software product or system
- 15. 15 Without Software Design We risk building an unstable system one that will fail when small changes are made one that may be difficult to test one whose quality cannot be assessed until late in the software process one that will be of no or very little use for similar projects (not reusable)
- 16. 16 Design Process and Model Software design is both a process and a model
- 17. 17 Design Process It is a sequence of steps that enables a designer to describe all aspects of the software to be built During the design process, the quality of the evolving design is assessed with a series of formal technical reviews or design walkthroughs Needs creative skills, past experience, sense of what makes “good” software, and an overall commitment to quality
- 18. 18 Design Model Equivalent to an architect’s plan for a house Represents the totality of the thing to be built Provides a variety of different views of the computer software
- 20. 20 Design Defects - 1 Defects introduced during preliminary design phase are usually not discovered until integration testing, which is too late in most cases Defects introduced during detailed design phase are usually discovered during unit testing
- 21. 21 Design Defects - 2 All four categories of defects are found in design models Errors of commission Errors of omission Errors of clarity and ambiguity Errors of speed and capacity
- 22. 22 Design Defects - 3 Most common defects are errors of omission, followed by errors of commission Errors of clarity and ambiguity are also common, and many performance related problems originate in design process also
- 23. 23 Design Defects - 4 Overall design ranks next to requirements as a source of very troublesome and expensive errors A combination of defect prevention and defect removal is needed for dealing with design defects
- 24. 24 Design Defects - 5 Formal design inspections are one of the most powerful and successful software quality approaches of all times Software professionals should incorporate inspections in their software development process
- 25. 25 Defects in Fundamental Design Topics Functions performed Function installation, invocation, control, and termination Data elements Data relationships Structure of the application Sequences or concurrency of execution Interfaces
- 26. 26 Functions Performed Errors in descriptions of functions the application will perform, are often errors of omission Often omitted functions are those which, are implied functions, rather than the explicitly demanded functions
- 27. 27 Function Installation, Invocation, Control, and Termination How to start-up a feature, control its behavior, and safely turn off a feature when finished are common in commercial and in-house software applications Fifty percent of the problems reported to commercial software vendors are of this class
- 28. 28 Data Elements Errors in describing the data used by the application are a major source of problems downstream during coding and testing A minor example of errors due to inadequate design of data elements can be seen in many programs that record addresses and telephone numbers Often insufficient space is reserved for names, etc.
- 29. 29 Data Relationships Errors in describing data relationships are very common and a source of much trouble later
- 30. 30 Structure of the Application Complex software structures with convoluted control flow tend to have higher error rates Poor structural design is fairly common, and is often due to haste or poor training and preparation Tools can measure cyclomatic and essential complexity Prevention is often better than attempting to simplify an already complex software structure
- 31. 31 Sequences or Concurrency of Execution Many errors of speed and capacity have their origin in failing to design for optimum performance Performance errors are a result of complex control flow, excessive branching, or too many sequential processing (use parallel processing) Minimize I/O operations
- 32. 32 Interfaces Chronic design problem Incompatible data types in message communication
- 33. 33 Errors in Eight Secondary Design Topics Security Reliability Maintainability Performance Human factors Hardware dependencies Software dependencies Packaging
- 34. 34 Addressing Design Problems Continuously evaluate your design model and design process Use design inspections or formal technical reviews, which have proven to be the most valuable mechanism to improve quality of software ever, and especially for design Develop software design by following design principles and guidelines
Editor's Notes
- #31: Software architecture exposes the structure of a system while hiding the implementation details. Architecture also focuses on how the elements and components within a system interact with one other. Software design delves deeper into the implementation details of the system. Design concerns include the selection of data structures and algorithms, or the implementation details of individual components. Cyclomatic: number of linearly independent paths through a program's source code.