Chapter 9 – Software Evolution

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Topics covered

 Evolution processes
 Software maintenance

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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 is added to the system;
▪ The performance or reliability of the system may have to be
improved.
 A key problem for all organizations is implementing and
managing change to their existing software systems.

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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.

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A spiral model of development and evolution

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Evolution and servicing

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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.

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 Evolution processes

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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
estimated.
 Change identification and evolution continues throughout
the system lifetime.

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Change identification and evolution processes

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The software evolution process

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Change implementation

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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.

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Urgent change requests

 Urgent changes may have to be implemented without

going through all stages of the software engineering
process
▪ 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).

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The emergency repair process

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Agile methods and evolution

 Agile methods are based on incremental development so

the transition from development to evolution is a
seamless one.
▪ Evolution is simply a continuation of the development process
based on frequent system releases.
 Automated regression testing is particularly valuable
when changes are made to a system.
 Changes may be expressed as additional user stories.

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Handover problems

 Where the development team have used an agile

approach but the evolution team is unfamiliar with agile
methods and prefer a plan-based approach.
▪ The evolution team may expect detailed documentation to
support evolution and this is not produced in agile processes.
 Where a plan-based approach has been used for
development but the evolution team prefer to use agile
methods.
▪ The evolution team may have to start from scratch developing
automated tests and the code in the system may not have been
refactored and simplified as is expected in agile development.

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 Software maintenance

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Software maintenance

 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.

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Types of maintenance

 Fault repairs
▪ Changing a system to fix bugs/vulnerabilities and correct
deficiencies in the way meets its requirements.
 Environmental adaptation
▪ Maintenance to adapt software to a different operating
environment
▪ Changing a system so that it operates in a different environment
(computer, OS, etc.) from its initial implementation.
 Functionality addition and modification
▪ Modifying the system to satisfy new requirements.

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Maintenance effort distribution

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Maintenance costs

 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.).

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Maintenance costs

 It is usually more expensive to add new features to a

system during maintenance than it is to add the same
features during development
▪ A new team has to understand the programs being maintained
▪ Separating maintenance and development means there is no
incentive for the development team to write maintainable
software
▪ Program maintenance work is unpopular
• Maintenance staff are often inexperienced and have limited domain
knowledge.
▪ As programs age, their structure degrades and they become
harder to change

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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
maintainability;
▪ Maintenance costs depend on the number of changes and costs
of change depend on maintainability.

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Maintenance prediction

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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.

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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
components.
 Complexity depends on
▪ Complexity of control structures;
▪ Complexity of data structures;
▪ Object, method (procedure) and module size.

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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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Refactoring

 Refactoring is the process of making improvements to a

program to slow down degradation through change.
 You can think of refactoring as ‘preventative
maintenance’ that reduces the problems of future
change.
 Refactoring involves modifying a program to improve its
structure, reduce its complexity or make it easier to
understand.
 When you refactor a program, you should not add
functionality but rather concentrate on program
improvement.
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‘Bad smells’ in program code

 Duplicate code
▪ The same or very similar code may be included at different
places in a program. This can be removed and implemented as a
single method or function that is called as required.
 Long methods
▪ If a method is too long, it should be redesigned as a number of
shorter methods.
 Switch (case) statements
▪ These often involve duplication, where the switch depends on
the type of a value. The switch statements may be scattered
around a program. In object-oriented languages, you can often
use polymorphism to achieve the same thing.

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‘Bad smells’ in program code

 Data clumping
▪ Data clumps occur when the same group of data items (fields in
classes, parameters in methods) re-occur in several places in a
program. These can often be replaced with an object that
encapsulates all of the data.
 Speculative generality
▪ This occurs when developers include generality in a program in
case it is required in the future. This can often simply be
removed.

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Key points

 Software development and evolution can be thought of

as an integrated, iterative process that can be
represented using a spiral model.
 For custom systems, the costs of software maintenance
usually exceed the software development costs.
 The process of software evolution is driven by requests
for changes and includes change impact analysis,
release planning and change implementation.
 Legacy systems are older software systems, developed
using obsolete software and hardware technologies, that
remain useful for a business.
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