Software engineering

1. SOFTWARE ENGINEERING
Introduction to ComputingCHAPTER # 7

2. Introduction to Computing 2Chapter # 7
Software Engineering
 Software engineering (SE) is an intellectual activity
and thus human-intensive
 Software is built to meet a certain functional goal
and satisfy certain qualities
 Software processes also must meet certain qualities
 Software qualities are sometimes referred to as
“ilities”

3. Introduction to Computing 3Chapter # 7
SE: A Unique Brand of Engineering
 SE is a unique brand of engineering
 Software is malleable
 Software construction is human-intensive
 Software is intangible and generally invisible
 Software problems are unprecedentedly complex
 Software directly depends upon the hardware
 It is at the top of the system engineering
 Software solutions require unusual rigor
 Software “state” means behaviors can depend on history
 Software has discontinuous operational nature

4. Introduction to Computing 4Chapter # 7
What does a Software Engineer Do?
 Software engineers should
 adopt a systematic and organised approach to all aspects
of software development
 use appropriate tools and techniques depending on
 the problem to be solved
 the development constraints and
 the resources available
 Understand and communicate processes for improved
software development within their organization
 Be effective team members and/or leaders
 Can be very technical or more managerial depending on
organizational need

5. Introduction to Computing 5Chapter # 7
Software Product
 Different from traditional types of products
 intangible
 difficult to describe and evaluate
 malleable
 human intensive
 involves only trivial “manufacturing” process

6. Introduction to Computing 6Chapter # 7
Software Process
 Prescribes all major activities
 Uses resources, within a set of constraints, to produce
intermediate and final products
 May be composed of sub-processes
 Each activity has entry and exit criteria
 Activities are organized in a sequence
 Has a set of guiding principles to explain goals
 Constraints may apply to activity, resource or product

7. Introduction to Computing 7Chapter # 7
Software Development Stages
1. Requirements Analysis & Specification
2. Design
a. Conceptual/System/Architectural Design
b. Detailed/Program Design
3. Implementation/Coding
4. Testing
a. Unit & Integration Testing
b. System Testing/Validation
5. System Delivery/Deployment
6. Maintenance

8. Introduction to Computing 8Chapter # 7
Software Lifecycle Model
 A software lifecycle model is a standardized format for
planning organizing, and running a new development project
 Hundreds of different kinds of models are known and used
 Many are minor variations on just a small number of basic
models

9. Introduction to Computing 9Chapter # 7
Software Lifecycle Model
 Few software lifecycle models are
 Waterfall Model
 Iterative & Incremental Model
 Prototyping Model
 Spiral Model
 Rapid application development
 Agile development
 Code and fix

10. Introduction to Computing 10Chapter # 7
Software Lifecycle Model (Waterfall)
Requirement
Design
Implementation
Verification
Maintenance

11. Introduction to Computing 11Chapter # 7
Software Qualities
 Critical Quality Attributes
 Correctness
 Maintainability
 Dependability
 Usability
 Reliability
 Other Attributes
 Completeness
 Compatibility
 Portability
 Internationalization
 Understandability
 Scalability
 Robustness
 Testability
 Reusability
 Customizability
 Efficiency

12. Introduction to Computing 12Chapter # 7
Classification of S/W Qualities "ilities"
 Internal vs. External
 External
 visible to the user
 Internal
 developers concern
 Product vs. Process
 Our goal is to develop software products
 The process is how we do it
 Internal qualities affect external qualities
 Process quality affects product quality

13. Introduction to Computing 13Chapter # 7
Internal vs. External Qualities
 External qualities are visible to the user
 reliability, usability, efficiency (maybe), robustness, scalability
 Internal qualities are the concern of developers
 they help developers achieve external qualities
 verifiability, maintainability, extensibility, evolve ability, adaptability,
portability, testability, reusability
 Internal qualities affect external qualities

14. Introduction to Computing 14Chapter # 7
Product vs. Process Qualities
 Product qualities concern the developed artifacts
 maintainability, performance, understandability
 Process qualities deal with the development activity
 products are developed through process
 maintainability, productivity, predictability
 Process quality affects product quality

15. Introduction to Computing 15Chapter # 7
Correctness
 Software is correct if it satisfies the functional requirements
specifications
 assuming that specification exists!
 established w.r.t. the requirements/specification
 If specifications are formal, since programs are formal
objects, correctness can be defined formally
 It can be proven as a theorem or disproved by counterexamples
(testing)
 Ideal quality
 The limit of correctness
 It is an absolute (yes/no) quality
 there is no concept of “degree of correctness”
 there is no concept of severity of deviation
 What if specifications are wrong?
 (e.g., they derive from incorrect requirements or errors in domain knowledge)

16. Introduction to Computing 16Chapter # 7
Reliability
 It is informally, user can rely on it
 It can be defined mathematically as “probability of
absence of failures for a certain time period”
 If specs are correct, all correct software is reliable,
but not vice-versa
 It is a statistical property

17. Introduction to Computing 17Chapter # 7
Robustness
 Software behaves “reasonably” even in unforeseen
circumstances (e.g., incorrect input, hardware
failure)
 “Reasonable” behavior in unforeseen circumstances
 A specified requirement is an issue of correctness
 whereas an unspecified requirement is an issue of
robustness

18. Introduction to Computing 18Chapter # 7
Performance
 Efficient use of resources
 memory, processing time, communication
 Can be verified
 complexity analysis
 measurement, analysis, and simulation
 performance evaluation (on a model, via simulation)
 Performance can affect scalability
 a solution that works on a small local network may not work on a
large intranet

19. Introduction to Computing 19Chapter # 7
Usability
 Ability of end-users to easily use software
 Expected users find the system easy to use
 Other term: user-friendliness
 Affected mostly by user interface
 e.g., visual vs. textual

20. Introduction to Computing 20Chapter # 7
Verifiability
 How easy it is to verify properties
 mostly an internal quality
 can be external as well (e.g., security critical application)
 It is performed by formal analysis or testing

21. Introduction to Computing 21Chapter # 7
Maintainability
 Maintainability: ease of maintenance
 Maintenance: changes after release
 Maintenance costs exceed 60% of total cost of software
 Can be decomposed as
 Reparability
 ability to correct defects in reasonable time
 Evolvability
 ability to adapt software to environment changes and to improve it in
reasonable time
 Three main categories of maintenance
 corrective: removing remaining errors (20%)
 adaptive: adjusting to environment changes (20%)
 perfective: quality improvements (>50%)

22. Introduction to Computing 22Chapter # 7
Evolvability
 Ability to add or modify functionality
 Addresses adaptive and perfective maintenance
 Problem: evolution of implementation is too easy
 Evolution should start at requirements or design

23. Introduction to Computing 23Chapter # 7
Reusability
 Ability to construct new software from existing pieces
 Existing product (or components) used (with minor modifications) to
build another product
 Also applies to process
 Reuse of standard parts measure of maturity of the field
 Occurs at all levels
 from people to process, from requirements to code

24. Introduction to Computing 24Chapter # 7
Portability
 Software can run on different hw platforms or sw
environments
 Remains relevant as new platforms and
environments are introduced (e.g. digital assistants)
 Relevant when downloading software in a
heterogeneous network environment

25. Introduction to Computing 25Chapter # 7
Understandability
 Ability of developers to easily understand produced
artifacts
 It is subjective
 Ease of understanding software
 Program modification requires program
understanding

26. Introduction to Computing 26Chapter # 7
Interoperability
 Ability of a system to coexist and cooperate with
other systems
 e.g., word processor and spreadsheet
 Ability to easily integrate into larger systems
 Common techniques include APIs, distributed
programming interfaces (CORBA, DCOM), plug-in
protocols, etc

27. Introduction to Computing 27Chapter # 7
Scalability
 Ability of a software system to grow in size while
maintaining its properties and qualities
 Assumes maintainability and evolvability
 Goal of component-based development

28. Introduction to Computing 28Chapter # 7
Typical Process Qualities
 Productivity
 denotes its efficiency and performance
 Timeliness
 ability to deliver a product on time
 Visibility
 all of its steps and current status are documented clearly

29. Introduction to Computing 29Chapter # 7
Timeliness: Issues
 Often the development process does not follow the
evolution of user requirements
 A mismatch occurs between user requirements and
status of the product

30. Introduction to Computing 30Chapter # 7
Application-Specific Qualities
 E.g., information systems
 Data integrity
 Security
 Data availability
 Transaction performance

31. Introduction to Computing 31Chapter # 7
Quality Measurement
 Many qualities are subjective
 No standard metrics defined for most qualities