©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 1 Slide 1 An Introduction to Software Engineering

©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 1 Slide 1

An Introduction to Software Engineering


Objectives

l To introduce software engineering and to explain its importance

l To set out the answers to key questions about software engineering

l To introduce ethical and professional issues and to explain why they are of concern to software engineers


Software engineering

l The economies of ALL developed nations are dependent on software. (ex: National infrastructures and utilities, most electrical products…)

l More and more systems are software controlled.

l Software engineering is concerned with theories, methods and tools for professional software development.

l Expenditure on software represents a significant fraction of GNP (Gross National Product) in all developed countries.


Software costs

l Software costs often dominate computer system costs. The costs of software on a PC are often greater than the hardware cost.

l Software costs more to maintain than it does to develop. For systems with a long life, maintenance costs may be several times development costs.

l Software engineering is concerned with cost-effective software development.


FAQs about software engineering

l What is software?l What is software engineering?l What is the difference between software engineering and computer

science?l What is the difference between software engineering and system

engineering?l What is a software process?l What is a software process model?l What are the costs of software engineering?l What are software engineering methods?l What is CASE (Computer-Aided Software Engineering)l What are the attributes of good software?l What are the key challenges facing software engineering?


What is software?

l Software : Computer programs and associated documentation such as requirements, design models and user manuals.

l Software products may be developed for a particular customer or may be developed for a general market.

l Software products may be• Generic - developed to be sold to a range of different customers e.g. PC

software such as Excel or Word.• Customized (bespoke) - developed for a single customer according to their

specification.

l More and more software companies are starting with a generic system and customizing it to the needs of a particular customer. (ex: SAP)


Computer Science vs. Engineeringl Computer Scientist

• Assumes techniques and tools have to be developed. • Proves theorems about algorithms, designs languages, defines knowledge

representation schemes• Has infinite time…

l Engineer• Develops a solution for a problem formulated by a client• Uses computers & languages, techniques and tools

l Software Engineer• Works in multiple application domains• Has only 3 months...• …while changes occurs in the problem formulation (requirements) and also in

the available technology.


Software Engineering: A Problem Solving Activity

l Analysis: • Understand the nature of the problem and break

the problem into piecesl Synthesis:

• Put the pieces together into a large structure

For problem solving we use techniques, methodologies and tools.


What is software engineering?

l Software engineering is an engineering discipline that is concerned with all aspects of software production.

l Software engineers should adopt a systematic and organised approach to their work and use appropriate tools and techniques depending on the problem to be solved, the development constraints and the resources available.


Techniques, Methodologies and Tools

l Techniques:• Formal procedures for producing results using some

well-defined notationl Methodologies:

• Collection of techniques applied across software development and unified by a philosophical approach

l Tools: • Instruments or automated systems to accomplish a

technique• CASE = Computer Aided Software Engineering


What is the difference between software engineering and computer science?

l Computer science is concerned with theory and fundamentals; software engineering is concerned with the practicalities of developing and delivering useful software.

l Computer science theories are still insufficient to act as a complete underpinning for software engineering (unlike e.g. physics and electrical engineering).


What is the difference between software engineering and system engineering?

l System engineering is concerned with all aspects of computer-based systems development including hardware, software and process engineering. Software engineering is part of this process concerned with developing the software infrastructure, control, applications and databases in the system.

l System engineers are involved in system specification, architectural design, integration and deployment.


What is a software process?

l Software process : A set of activities whose goal is the development or evolution of software.

l Generic activities in all software processes are:• Software specification – customers and engineers define it with its

constraints• Software development – where software is designed and programmed• Software validation – where software is checked to ensure that it is

what customer requires• Software evolution – where software is modified to adapt it to changing

customer and market requirements. Different types of systems need different development processes. For example, real time

software in an aircraft has to be completely specified before development begins, whereas, in e-commerce systems, the specification and the program are usually developed together.


What is a software process model?

l Software process model : A simplified representation of a software process, presented from a specific perspective.

l Examples of process perspectives are• Workflow perspective – shows the sequence of activities along with their inputs,

outputs and dependencies (human activities)• Data-flow perspective – shows how the input to the process , such as

specification, is transformed to an output, such as design, e.g. information flow (human or computer activities)

• Role/action perspective – represents roles of the people involved in the software process and the activities.

l Generic process models are• Waterfall• Iterative development• Component-based software engineering


Why is software development difficult?

l The problem domain (also called application domain) is difficultl The solution domain is difficultl The development process is difficult to managel Software offers extreme flexibilityl Software is a discrete system

• Continuous systems have no hidden surprises • Discrete systems can have hidden surprises! (Parnas)

David Lorge Parnas is an early pioneer in software engineering who developed the concepts of modularity and information hiding in systems which are the foundation of object oriented methodologies.


What are the costs of software engineering?

l Roughly 60% of costs are development costs, 40% are testing costs. For custom software, evolution costs often exceed development costs.

l Costs vary depending on the type of system being developed and the requirements of system attributes such as performance and system reliability.

l Distribution of costs depends on the development model that is used.


Activity cost distribution


Product development costs


What is CASE (Computer-Aided Software Engineering) ?

l CASE : Software systems that are intended to provide automated support for software process activities.

l CASE systems are often used for method support.

l Upper-CASE• Tools to support the early process activities of requirements and design;

l Lower-CASE• Tools to support later activities such as programming, debugging and testing.


What are the attributes of good software?

l The software should deliver the required functionality and performance to the user and should be maintainable, dependable and usable.

l Maintainability• Software should be written in such a way that it may evolve to meet the

changing needs of customers.

l Dependability• Software must be trustworthy (reliable, secure and safe).

l Efficiency• Software should not make wasteful use of system resources, such as memory

or processor cycles. Efficiency: Responsiveness, processing time, memory utilization, etc.

l Usability• Software must be usable without undue effort. It should have an appropriate

user interface and adequate documentation.


What are the key challenges facing software engineering?

l Software engineering in 21st century faces three key challenges:

l Heterogeneity• Challenge of developing techniques for building dependable software that is

flexible enough to cope with the heterogeneity.

l Delivery• Challenge of shortening delivery times for large and complex systems without

compromising system quality.

l Trust• Challenge of developing techniques that demonstrate that software can be

trusted by its users.


ACM/IEEE Code of Ethics

l The professional societies in the US have cooperated to produce a code of ethical practice.

l Members of these organisations sign up to the code of practice when they join.

l The Code contains eight principles related to the behaviour of and decisions made by professional software engineers, including practitioners, educators, managers, supervisors and policy makers, as well as trainees and students of the profession.


Key points

l Software engineering is an engineering discipline that is concerned with all aspects of software production.

l Software products consist of developed programs and associated documentation. Essential product attributes are maintainability, dependability, efficiency and usability.

l The software process consists of activities that are involved in developing software products. Basic activities are software specification, development, validation and evolution.

l Methods are organised ways of producing software. They include suggestions for the process to be followed, the notations to be used, rules governing the system descriptions which are produced and design guidelines.


Key points

l CASE tools are software systems which are designed to support routine activities in the software process such as editing design diagrams, checking diagram consistency and keeping track of program tests which have been run.

l Software engineers have responsibilities to the engineering profession and society. They should not simply be concerned with technical issues.

l Professional societies publish codes of conduct which set out the standards of behaviour expected of their members.

Documents

©Ian Sommerville 2006Software Engineering, 8th edition. Chapter 1 Slide 1 An Introduction to Software Engineering