Software Engineering Chapter 2

Software Engineering, 7th edition的英文课件作者Ian Sommerville

Socio-technical Systems

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 2

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Software Engineering, 7th edition的英文课件作者Ian Sommerville

Objectives●

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To explain what a socio-technical system is and the distinction between this and a computer-based system To introduce the concept of emergent system properties such as reliability and security To explain system engineering and system procurement processes To explain why the organisational context of a system affects its design and use To discuss legacy systems and why these are critical to many businesses

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 2

Slide 2

Software Engineering, 7th edition的英文课件作者Ian Sommerville

Topics covered●●●●

Emergent system properties Systems engineering Organizations, people and computer systems Legacy systems

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 2

Slide 3

Software Engineering, 7th edition的英文课件作者Ian Sommerville

What is a system?●

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A purposeful collection of inter-related components working together to achieve some common objective. A system may include software, mechanical, electrical and electronic hardware and be operated by people. System components are dependent on other system components The properties and behaviour of system components are inextricably inter-mingled

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 2

Slide 4

Software Engineering, 7th edition的英文课件作者Ian Sommerville

System categories●

Technical computer-based systems Systems that include hardware and software but where the operators and operational processes are not normally considered to be part of the system. The system is not self-aware. Systems that include technical systems but also operational processes and people who use and interact with the technical system. Socio-technical systems are governed by organisational policies and rules.

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Socio-technical systems

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 2

Slide 5

Software Engineering, 7th edition的英文课件作者Ian Sommerville

Socio-technical system characteristics●

Emergent properties Properties of the system of a whole that depend on the system components and their relationships. They do not always produce the same output when presented with the same input because the systems’s behaviour is partially dependent on human operators. The extent to which the system supports organisational objectives does not just depend on the system itself.

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Non-deterministic

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Complex relationships with organisational objectives

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 2

Slide 6

Software Engineering, 7th edition的英文课件作者Ian Sommerville

Emergent properties●

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Properties of the system as a whole rather than properties that can be derived from the properties of components of a system Emergent properties are a consequence of the relationships between system components They can therefore only be assessed and measured once the components have been integrated into a system

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 2

Slide 7

Software Engineering, 7th edition的英文课件作者Ian Sommerville

Property

Volume

Reliability

SecurityDescriptionThe volume of a system (the total space occupied) varies depending on how thecomponent assemblies are arranged and connected.System reliability depends on component reliability but unexpected interactions cancause new types of failure and therefore affect the reliability of the system.The security of the system (its ability to resist attack) is a complex property that

cannot be easily measured. Attacks may be devised that were not anticipated by the

system designers and so may defeat built-in safeguards.

This property reflects how easy it is to fix a problem with the system once it has been

discovered. It depends on being able to diagnose the problem, access the components

that are faulty and modify or replace these components.

This property reflects how easy it is to use the system. It depends on the technical

system components, its operators and its operating environment.RepairabilityUsability

Software Engineering, 7th edition的英文课件作者Ian Sommerville

Types of emergent property●

Functional properties These appear when all the parts of a system work together to achieve some objective. For example, a bicycle has the functional property of being a transportation device once it has been assembled from its components. Examples are reliability, performance, safety, and security. These relate to the behaviour of the system in its operational environment. They are often critical for computer-based systems as failure to achieve some minimal defined level in these properties may make the system unusable.

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Non-functional emergent properties

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 2

Slide 9

Software Engineering, 7th edition的英文课件作者Ian Sommerville

System reliability engineering●

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Because of component inter-dependencies, faults can be propagated through the system. System failures often occur because of unforeseen inter-relationships between components. It is probably impossible to anticipate all possible component relationships. Software reliability measures may give a false picture of the system reliability.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 2

Slide 10

Software Engineering, 7th edition的英文课件作者Ian Sommerville

Influences on reliability●

Hardware reliability What is the probability of a hardware component failing and how long does it take to repair that component?

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Software reliability How likely is it that a software component will produce an incorrect output. Software failure is usually distinct from hardware failure in that software does not wear out.

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Operator reliability How likely is it that the operator of a system will make an error?

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 2

Slide 11

Software Engineering, 7th edition的英文课件作者Ian Sommerville

Reliability relationships●

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Hardware failure can generate spurious signals that are outside the range of inputs expected by the software. Software errors can cause alarms to be activated which cause operator stress and lead to operator errors. The environment in which a system is installed can affec

t its reliability.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 2

Slide 12

Software Engineering, 7th edition的英文课件作者Ian Sommerville

The‘shall-not’ properties●

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Properties such as performance and reliability can be measured. However, some properties are properties that the system should not exhibit Safety - the system should not behave in an unsafe way; Security - the system should not permit unauthorised use.

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Measuring or assessing these properties is very hard.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 2

Slide 13

Software Engineering, 7th edition的英文课件作者Ian Sommerville

Systems engineering●

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Specifying, designing, implementing, validating, deploying and maintaining socio-technical systems. Concerned with the services provided by the system, constraints on its construction and operation and the ways in which it is used.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 2

Slide 14

Software Engineering, 7th edition的英文课件作者Ian Sommerville

The system engineering process●

Usually follows a‘waterfall’ model because of the need for parallel development of different parts of the system Little scope for iteration between phases because hardware changes are very expensive. Software may have to compensate for hardware problems.

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Inevitably involves engineers from different disciplines who must work together Much scope for misunderstanding here. Different disciplines use a different vocabulary and much negotiation is required. Engineers may have personal agendas to fulfil.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 2

Slide 15

Software Engineering, 7th edition的英文课件作者Ian Sommerville

The systems engineering process

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 2

Slide 16

Software Engineering, 7th edition的英文课件作者Ian Sommerville

Inter-disciplinary involvement

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 2

Slide 17

Software Engineering, 7th edition的英文课件作者Ian Sommerville

System requirements definition●

Three types of requirement defined at this stage Abstract functional requirements. System functions are defined in an abstract way; System properties. Non-functional requirements for the system in general are defined; Undesirable characteristics. Unacceptable system behaviour is specified.

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Should also define overall organisational objectives for the system.

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 2

Slide 18

Software Engineering, 7th edition的英文课件作者Ian Sommerville

System objectives●

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Should define why a system is being procured for a particular environment. Functional objectives To provide a fire and intruder alarm system for the building which will provide internal and external warning of fire or unauthorized intrusion. To ensure that the normal functioning of work carried out in the building is not seriously disrupted by events such as fire and unauthorized intrusion.

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Organisational objectives

©Ian Sommerville 2004

Software Engineering, 7th edition. Chapter 2

Slide 19

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