Software Engineering Course

- Material: Software Engineering, SommerVille - Testing:

Final test: 50%

- Middle term: 20% - Group exercise : 30% -

Chapter 1 Introduction

2

Chapter 1- Introduction

Topics covered

 What is meant by software engineering.

 Professional software development

 A brief introduction to ethical issues that affect software

engineering.

 Software engineering ethics

3

 Software Processes

Professional software development

 Software engineering

 Software costs

 Software products

 Product specification

 Importance of software engineering

Importance of software engineering

A bridge from customer needs to programming implementation

Customer

Programmer

5

First law of software engineering Software engineer is willing to learn the problem domain (problem cannot be solved without understanding it first)

Importance of software engineering

Customer: Requires a computer system to achieve some business goals by user interaction or interaction with the environment in a specified manner

System-to-be System-to-be

Environment

Software-to-be Software-to-be

User

Software Engineer’s task: To understand how the system-to-be needs to interact with the user or the environment so that customer’s requirement is met and design the software-to-be

May be the same person

Programmer’s task: To implement the software-to-be designed by the software engineer

6

Frequently asked questions about software engineering

 What is software?

 What are the attributes of good software?

 What is software engineering?

 What are the fundamental software engineering

activities?

 What is the difference between software engineering and

computer science?

 What is the difference between software engineering and

Chapter 1 Introduction

7

system engineering?

Frequently asked questions about software engineering

 What are the key challenges facing software

engineering?

 What are the costs of software engineering?

 What are the best software engineering techniques and

methods?

 What differences has the web made to software

8

engineering?

Essential attributes of good software

 Maintainability

 Dependability and security

 Efficiency

9

 Acceptability

Software process activities

 Software specification

 Software development

 Software validation

10

 Software evolution.

Some characteristic

 The difference (Heterogeneity): central/distributed system,

computer or mobile devices…etc

 Business and social change

 Security and trust

 General issues that affect most software

11

 Software engineering diversity(variety)

Some characteristic

 Application types:

 Stand-alone applications

 Interactive transaction-based applications

 Embedded control systems

 Batch processing systems

 Entertainment systems

 Systems for modeling and simulation

 Data collection systems

12

 Systems of systems

Software engineering fundamentals

 Develop by using a managed and understood

development process.

 Dependability and performance are important for all

types of system.

 Software specification and requirements are important.

13

 Consider reuse software rather than write new software.

Software engineering and the web

 Platform for running application and organization

 Approach: Cloud computing

 Users do not buy software pay according to use.

 Software reuse is the dominant approach for

constructing web-based systems

 Web-based systems should be developed and delivered

incrementally.

 User interfaces are constrained by the capabilities of

14

web browsers.

Software engineering ethics

 Software engineering involves wider responsibilities than

simply the application of technical skills.

 Software engineers must behave in an honest and

ethically responsible way if they are to be respected as professionals.

 Ethical behaviour is more than simply upholding the law but involves following a set of principles that are morally correct.

Software Processes

 Process activities

 Coping with change

 An example of a modern software process.

16

 The Rational Unified Process

Software process descriptions

 specifying a data model

 designing a user interface,

 etc. and the ordering of these activities.

 Activities of:

 Products,

 Roles,

 Pre- and post-conditions

17

 Process descriptions may also include:

Software specification

 Feasibility study

• Is it technically and financially feasible to build the system?

 Requirements elicitation and analysis

• What do the system stakeholders require or expect from the system?

 Requirements specification

• Defining the requirements in detail

 Requirements validation

• Checking the validity of the requirements

18

 Requirements engineering process

19

The requirements engineering process

Software design and implementation

 The process of converting the system specification into

an executable system.

 Design a software structure that realises the specification;

 Software design

 Translate this structure into an executable program;

 Implementation

 The activities of design and implementation are closely

20

related and may be inter-leaved.

Design activities

 Architectural design

 Interface design

 Component design

21

 Database design

Software validation

 Verification and validation (V & V) is intended to show

that a system conforms to its specification and meets the requirements of the system customer.

 Involves checking and review processes and system

testing.

 System testing involves executing the system with test cases that are derived from the specification of the real data to be processed by the system.

22

 Testing is the most commonly used V & V activity.

23

Stages of testing

Testing stages

 Development or component testing

 System testing

24

 Acceptance testing

25

Testing phases in a plan-driven software process

Coping with change

 Change is inevitable in all large software projects.

 Change leads to rework so the costs of change include both rework (e.g. re-analysing requirements) as well as the costs of implementing new functionality

 Change avoidance, where the software process includes activities that can anticipate possible changes before significant rework is required.

26

 Change tolerance, where the process is designed so that changes can be accommodated at relatively low cost.

Software prototyping

 A prototype is an initial version of a system used to demonstrate concepts and try out design options.

 The requirements engineering process to help with requirements

elicitation and validation;

 In design processes to explore options and develop a UI design;

 In the testing process to run back-to-back tests.

27

 A prototype can be used in:

Benefits of prototyping

 Improved system usability.

 A closer match to users’ real needs.

 Improved design quality.

 Improved maintainability.

28

 Reduced development effort.

29

The process of prototype development

Prototype development

 May be based on rapid prototyping languages or tools

 Prototype should focus on areas of the product that are not well-

understood;

 Error checking and recovery may not be included in the

prototype;

 Focus on functional rather than non-functional requirements

such as reliability and security

30

 May involve leaving out functionality

Throw-away prototypes

 It may be impossible to tune the system to meet non-functional

requirements;

 Prototypes are normally undocumented;  The prototype structure is usually degraded through rapid

change;

 The prototype probably will not meet normal organisational

quality standards.

31

 Prototypes should be discarded after development as they are not a good basis for a production system:

Software process models

 The waterfall model

 Agile model

 Spiral model

 Incremental development

 Reuse-oriented software engineering

32

 In practice, most large systems are developed using a process that incorporates elements from all of these models.

Requirements

Design

Implementation

Testing

The waterfall model

Waterfall method

Deployment & Maintenance

33

Waterfall model phases

 Requirements analysis and definition

 System and software design

 Implementation and unit testing  Integration and system testing

 Operation and maintenance

 Phases in the waterfall model:

34

 The main drawback?

Waterfall model problems

 appropriate when the requirements are well-understood

 Few business systems have stable requirements.

 Difficult to respond to changing customer requirements

 Used for large systems engineering projects where a

 In those circumstances, the plan-driven nature of the waterfall

model helps coordinate the work.

35

system is developed at several sites.

Agile methods

 Focus on the code rather than the design

 Are based on an iterative approach to software development  Are intended to deliver working software quickly and evolve this

quickly to meet changing requirements.

 Dissatisfaction with the overheads involved in software design methods of the 1980s and 1990s led to the creation of agile methods. These methods:

36

 The aim of agile methods is to reduce overheads in the software process (e.g. by limiting documentation) and to be able to respond quickly to changing requirements without excessive rework.

Agile manifesto

 Individuals and interactions over processes and tools

Working software over comprehensive documentation Customer collaboration over contract negotiation Responding to change over following a plan

 We are uncovering better ways of developing 
software by doing it and helping others do it. Through this work we have come to value:

 That is, while there is value in the items on 
the right, we

37

value the items on the left more.

The principles of agile methods

Principle

Description

Customer involvement Customers should be closely

throughout

involved

the development process. Their role is provide and prioritize new system requirements and to evaluate the iterations of the system.

Incremental delivery

The software is developed in increments with the customer specifying the requirements to be included in each increment.

People not process

The skills of the development team should be recognized and exploited. Team members should be left to develop their own ways of working without prescriptive processes.

Embrace change

Expect the system requirements to change and so design the system to accommodate these changes.

Maintain simplicity

Focus on simplicity in both the software being developed and in the development process. Wherever possible, actively work to eliminate complexity from the system.

38

Agile method applicability

 Product development where a software company is

developing a small or medium-sized product for sale.

 Custom system development within an organization,

where there is a clear commitment from the customer to become involved in the development process and where there are not a lot of external rules and regulations that affect the software.

 Because of their focus on small, tightly-integrated teams,

39

there are problems in scaling agile methods to large systems.

Problems with agile methods

 It can be difficult to keep the interest of customers who

are involved in the process.

 Team members may be unsuited to the intense involvement that characterises agile methods.

 Prioritising changes can be difficult where there are

multiple stakeholders.

 Maintaining simplicity requires extra work.

 Contracts may be a problem as with other approaches to

40

iterative development.

Agile methods and software maintenance

 Most organizations spend more on maintaining existing

software than they do on new software development. So, if agile methods are to be successful, they have to support maintenance as well as original development.

 Are systems that are developed using an agile approach

maintainable, given the emphasis in the development process of minimizing formal documentation?

 Can agile methods be used effectively for evolving a system in

response to customer change requests?

 Two key issues:

 Problems may arise if original development team cannot

41

be maintained.

Plan-driven and agile development

 A plan-driven approach to software engineering is based around separate development stages with the outputs to be produced at each of these stages planned in advance.

 Not necessarily waterfall model – plan-driven, incremental

development is possible

 Iteration occurs within activities.

 Plan-driven development

 Specification, design, implementation and testing are inter- leaved and the outputs from the development process are decided through a process of negotiation during the software development process.

42

 Agile development

43

Plan-driven and agile specification

44

Incremental development

Incremental development benefits

 The cost of accommodating changing customer

requirements is reduced.

 It is easier to get customer feedback on the development

work that has been done.

 More rapid delivery and deployment of useful software to

.

45

the customer is possible.

Incremental development problems

 The process is not visible.

 System structure tends to degrade as new increments

46

are added.

Reuse-oriented software engineering

 Based on systematic reuse where systems are integrated from existing components or COTS (Commercial-off-the-shelf) systems.

 Component analysis;

 Requirements modification;

 System design with reuse;

 Development and integration.

 Process stages

 Reuse is now the standard approach for building many

 Reuse covered in more depth in later.

47

types of business system

48

Reuse-oriented software engineering

49

A general model of the design process

Incremental delivery

 Rather than deliver the system as a single delivery, the

development and delivery is broken down into increments with each increment delivering part of the required functionality.

 User requirements are prioritised and the highest priority

requirements are included in early increments.

50

 Once the development of an increment is started, the requirements are frozen though requirements for later increments can continue to evolve.

Incremental development and delivery

 Develop the system in increments and evaluate each increment before proceeding to the development of the next increment;

 Normal approach used in agile methods;

 Evaluation done by user/customer proxy.

 Incremental development

 Deploy an increment for use by end-users;

 More realistic evaluation about practical use of software;

 Difficult to implement for replacement systems as increments

have less functionality than the system being replaced.

51

 Incremental delivery

52

Incremental delivery

Incremental delivery advantages

 Customer value can be delivered with each increment so

system functionality is available earlier.

 Early increments act as a prototype to help elicit

requirements for later increments.

 Lower risk of overall project failure.

 The highest priority system services tend to receive the

53

most testing.

Incremental delivery problems

 Most systems require a set of basic facilities that are

 As requirements are not defined in detail until an increment is to be implemented, it can be hard to identify common facilities that are needed by all increments.

used by different parts of the system.

 The essence of iterative processes is that the

 However, this conflicts with the procurement model of many

organizations, where the complete system specification is part of the system development contract.

54

specification is developed in conjunction with the software.

Boehm’s spiral model

 Process is represented as a spiral rather than as a

sequence of activities with backtracking.

 Each loop in the spiral represents a phase in the

process.

 No fixed phases such as specification or design - loops in the spiral are chosen depending on what is required.

 Risks are explicitly assessed and resolved throughout

55

the process.

56

Boehm’s spiral model of the software process

Spiral model sectors

 Specific objectives for the phase are identified.

 Objective setting

 Risks are assessed and activities put in place to reduce the key

risks.

 Risk assessment and reduction

 A development model for the system is chosen which can be

any of the generic models.

 Development and validation

 The project is reviewed and the next phase of the spiral is

planned.

57

 Planning

Spiral model usage

 Spiral model has been very influential in helping people

think about iteration in software processes and introducing the risk-driven approach to development.

58

 In practice, however, the model is rarely used as published for practical software development.

The Rational Unified Process

 A modern generic process derived from the work on the

UML and associated process.

 Brings together aspects of the 3 generic process models

discussed previously.

 A dynamic perspective that shows phases over time;

 A static perspective that shows process activities;  A practive perspective that suggests good practice.

59

 Normally described from 3 perspectives

60

Phases in the Rational Unified Process

RUP phases

 Establish the business case for the system.

 Inception

 Develop an understanding of the problem domain and the

system architecture.

 Elaboration

 System design, programming and testing.

 Construction

 Deploy the system in its operating environment.

62

 Transition

RUP iteration

 Each phase is iterative with results developed incrementally.

 In-phase iteration

 As shown by the loop in the RUP model, the whole set of phases

may be enacted incrementally.

63

 Cross-phase iteration

Static workflows in the Rational Unified Process

Workflow

Description

Business modelling

The business processes are modelled using business use cases.

Requirements

Actors who interact with the system are identified and use cases are developed to model the system requirements.

Analysis and design

A design model is created and documented using architectural models, component models, object models and sequence models.

Implementation

implementation

into

The components in the system are implemented and structured sub-systems. Automatic code generation from design models helps accelerate this process.

64

Static workflows in the Rational Unified Process

Workflow

Description

Testing

Testing is an iterative process that is carried out in conjunction with implementation. System testing follows the completion of the implementation.

Deployment

A product release is created, distributed to users and installed in their workplace.

and

Configuration change management

This supporting workflow managed changes to the system (see Chapter 25).

Project management

This supporting workflow manages the system development (see Chapters 22 and 23).

Environment

This workflow is concerned with making appropriate software tools available to the software development team.

65

RUP good practice

 Plan increments based on customer priorities and deliver highest

priority increments first.

 Develop software iteratively

 Explicitly document customer requirements and keep track of

changes to these requirements.

 Manage requirements

 Organize the system architecture as a set of reusable

components.

66

 Use component-based architectures

RUP good practice

 Use graphical UML models to present static and dynamic views

of the software.

 Visually model software

 Ensure that the software meet’s organizational quality standards.

 Verify software quality

 Manage software changes using a change management system

and configuration management tools.

67

 Control changes to software

Model pros/cons

Review question – Chapter 1

 List the reasons for the “Software crisis”

 What is the Software crisis? Was Y2K a Software crisis?

 What is Software Engineering?

 What is the Software process? Why is it difficult to

improve it?

 Describe the characteristics of software contrasting it

with the characteristics of the hardware.

Chapter 1 Introduction

69

 Distinguish between generic and customized software products. Which one has large share of market and why?

Review question – Chapter 2

 List the advantages of using waterfall model instead of

adhoc build and fix model

 What are the advantages of developing the prototype of

the system?

 Compare iterative enhancement model and evolutionary

development model.

 What are the characteristics to be considered for the

selection of life cycle model?

 What is the role of user participation in the selection of

Chapter 1 Introduction

70

life cycle model?