Software Evolution - Software Engineering - Lecture Notes, Study notes for Software Engineering

Software Evolution - Software Engineering - Lecture Notes, Study notes for Software Engineering

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This course includes topics like software processes, requirements analysis and specification, design, prototyping, implementation, validation and verification, UML-based modeling, integrated development environments, and...
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Figures – Chapter 9

SE Fundamentals 6 – Software Evolution

Topics covered

 Evolution processes

 Change processes for software systems

 Software maintenance

 Making changes to operational software systems

 Legacy system management

 Making decisions about software change

Software change

 Software change is inevitable:

 New requirements emerge when the software is used

 The business environment changes

 Errors must be repaired

 New computers and equipment are added to the system

 The performance or reliability of the system may have to be


 A key problem for all organizations is implementing and

managing change to their existing software systems

Importance of evolution

 Organizations have huge investments in their software systems - they are critical business assets

 To maintain the value of these assets to the business, they must be changed and updated

 The majority of the software budget in large companies is devoted to changing and evolving existing software rather than developing new software

A spiral model of development and evolution

Evolution and servicing

Evolution and servicing

 Evolution

 The stage in a software system’s life cycle where it is in

operational use and is evolving as new requirements are

proposed and implemented in the system

 Servicing

 At this stage, the software remains useful but the only changes

made are those required to keep it operational, i.e. bug fixes and

changes to reflect changes in the software’s environment. No

new functionality is added

 Phase-out

 The software may still be used but no further changes are made

to it

Evolution processes

 Software evolution processes depend on

 The type of software being maintained

 The development processes used

 The skills and experience of the people involved

 Proposals for change are the driver for system evolution

 Should be linked with components that are affected by the

change, thus allowing the cost and impact of the change to be


 Change identification and evolution continues throughout

the system lifetime

Change identification and evolution processes

The software evolution process

Change implementation

Change implementation

 Iteration of the development process where the revisions

to the system are designed, implemented and tested

 A critical difference is that the first stage of change

implementation may involve program understanding,

especially if the original system developers are not

responsible for the change implementation

 During the program understanding phase, you have to

understand how the program is structured, how it

delivers functionality and how the proposed change

might affect the program

Urgent change requests

 Urgent changes may have to be implemented without

going through all stages of the software engineering


 If a serious system fault has to be repaired to allow normal

operation to continue

 If changes to the system’s environment (e.g., an OS upgrade)

have unexpected effects

 If there are business changes that require a very rapid response

(e.g. the release of a competing product)

The emergency repair process

Modifying a program after it has been put into use

 The term is mostly used for changing custom software.

Generic software products are said to evolve to create

new versions.

Maintenance does not normally involve major changes to

the system’s architecture

 Changes are implemented by modifying existing

components and adding new components to the system

Software maintenance

Maintenance to repair software faults

 Changing a system to correct deficiencies in the way meets its


Maintenance to adapt software to a different operating


 Changing a system so that it operates in a different environment

(computer, OS, etc.) from its initial implementation

Maintenance to add to or modify the system’s


 Modifying the system to satisfy new requirements

Types of maintenance

Figure 9.8 Maintenance effort distribution

 Usually greater than development costs (2* to

100* depending on the application)

 Affected by both technical and non-technical factors

 Increases as software is maintained. Maintenance

corrupts the software structure so makes further

maintenance more difficult.

 Ageing software can have high support costs (e.g., old

languages, compilers etc.).

Maintenance costs

Figure 9.9 Development and maintenance costs

 Team stability

 Maintenance costs are reduced if the same staff are involved with them for some time

 Contractual responsibility

 The developers of a system may have no contractual responsibility for maintenance so there is no incentive to design for future change

 Staff skills

 Maintenance staff are often inexperienced and have limited domain knowledge

 Program age and structure

 As programs age, their structure is degraded and they become harder to understand and change

Maintenance cost factors

Maintenance prediction

Maintenance prediction is concerned with assessing

which parts of the system may cause problems and have

high maintenance costs

 Change acceptance depends on the maintainability of the

components affected by the change

 Implementing changes degrades the system and reduces its


 Maintenance costs depend on the number of changes and costs

of change depend on maintainability

Maintenance prediction

Change prediction

 Predicting the number of changes requires and

understanding of the relationships between a system

and its environment

 Tightly coupled systems require changes whenever the

environment is changed

 Factors influencing this relationship are

 Number and complexity of system interfaces

 Number of inherently volatile system requirements

 The business processes where the system is used

Complexity metrics

 Predictions of maintainability can be made by assessing

the complexity of system components

 Studies have shown that most maintenance effort is

spent on a relatively small number of system


 Complexity depends on

 Complexity of control structures

 Complexity of data structures

 Object, method (procedure) and module size

Process metrics

 Process metrics may be used to assess maintainability

 Number of requests for corrective maintenance

 Average time required for impact analysis

 Average time taken to implement a change request

 Number of outstanding change requests

 If any or all of these is increasing, this may indicate a

decline in maintainability

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