Chapter 9 – Software Evolution

Lecture 1

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Topics covered

 Evolution processes
 Change processes for software systems
 Program evolution dynamics
 Understanding software evolution
 Software maintenance
 Making changes to operational software systems
 Legacy system management
 Making decisions about software change

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Software change

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Importance of evolution

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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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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

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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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Program evolution dynamics

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Change is inevitable

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Lehman’s laws

Law Description
Continuing change A program that is used in a real-world environment must necessarily
change, or else become progressively less useful in that
environment.
Increasing As an evolving program changes, its structure tends to become more
complexity complex. Extra resources must be devoted to preserving and
simplifying the structure.
Large program Program evolution is a self-regulating process. System attributes
evolution such as size, time between releases, and the number of reported
errors is approximately invariant for each system release.
Organizational Over a program’s lifetime, its rate of development is approximately
stability constant and independent of the resources devoted to system
development.

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Lehman’s laws

Law Description
Conservation of familiarity Over the lifetime of a system, the incremental change in each
release is approximately constant.
Continuing growth The functionality offered by systems has to continually
increase to maintain user satisfaction.
Declining quality The quality of systems will decline unless they are modified to
reflect changes in their operational environment.
Feedback system Evolution processes incorporate multiagent, multiloop
feedback systems and you have to treat them as feedback
systems to achieve significant product improvement.

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Applicability of Lehman’s laws

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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.
 Lehman’s laws, such as the notion that change is
continuous, describe a number of insights derived from
long-term studies of system evolution.
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Chapter 9 – Software Evolution

Lecture 2

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Software maintenance

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Types of maintenance

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Figure 9.8 Maintenance effort distribution

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Maintenance costs

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Figure 9.9 Development and maintenance costs

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Maintenance cost factors

 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.
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Maintenance prediction

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Maintenance prediction

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Change prediction

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Complexity metrics

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Process metrics

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System re-engineering

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Advantages of reengineering

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The reengineering process

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Reengineering process activities

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Figure 9.12 Reengineering approaches

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Reengineering cost factors

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Preventative maintenance by 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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Refactoring and reengineering

 Re-engineering takes place after a system has been

maintained for some time and maintenance costs are
increasing. You use automated tools to process and re-
engineer a legacy system to create a new system that is
more maintainable.
 Refactoring is a continuous process of improvement
throughout the development and evolution process. It is
intended to avoid the structure and code degradation
that increases the costs and difficulties of maintaining a
system.

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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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Legacy system management

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Figure 9.13 An example of a legacy system
assessment

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Legacy system categories

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Business value assessment

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Issues in business value assessment

 The use of the system

 If systems are only used occasionally or by a small number of
people, they may have a low business value.
 The business processes that are supported
 A system may have a low business value if it forces the use of
inefficient business processes.
 System dependability
 If a system is not dependable and the problems directly affect
business customers, the system has a low business value.
 The system outputs
 If the business depends on system outputs, then the system has
a high business value.
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System quality assessment

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Business process assessment

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Factors used in environment assessment

Factor Questions
Supplier stability Is the supplier still in existence? Is the supplier financially stable and
likely to continue in existence? If the supplier is no longer in business,
does someone else maintain the systems?
Failure rate Does the hardware have a high rate of reported failures? Does the
support software crash and force system restarts?
Age How old is the hardware and software? The older the hardware and
support software, the more obsolete it will be. It may still function
correctly but there could be significant economic and business
benefits to moving to a more modern system.
Performance Is the performance of the system adequate? Do performance
problems have a significant effect on system users?

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Factors used in environment assessment

Factor Questions
Support requirements What local support is required by the hardware and
software? If there are high costs associated with this
support, it may be worth considering system replacement.
Maintenance costs What are the costs of hardware maintenance and support
software licences? Older hardware may have higher
maintenance costs than modern systems. Support software
may have high annual licensing costs.
Interoperability Are there problems interfacing the system to other systems?
Can compilers, for example, be used with current versions
of the operating system? Is hardware emulation required?

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Factors used in application assessment

Factor Questions
Understandability How difficult is it to understand the source code of the current
system? How complex are the control structures that are used?
Do variables have meaningful names that reflect their function?
Documentation What system documentation is available? Is the documentation
complete, consistent, and current?
Data Is there an explicit data model for the system? To what extent is
data duplicated across files? Is the data used by the system up to
date and consistent?
Performance Is the performance of the application adequate? Do performance
problems have a significant effect on system users?

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Factors used in application assessment

Factor Questions
Programming language Are modern compilers available for the programming
language used to develop the system? Is the programming
language still used for new system development?
Configuration Are all versions of all parts of the system managed by a
management configuration management system? Is there an explicit
description of the versions of components that are used in
the current system?
Test data Does test data for the system exist? Is there a record of
regression tests carried out when new features have been
added to the system?
Personnel skills Are there people available who have the skills to maintain the
application? Are there people available who have experience
with the system?

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System measurement

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Key points

 There are 3 types of software maintenance, namely bug

fixing, modifying software to work in a new environment,
and implementing new or changed requirements.
 Software re-engineering is concerned with re-structuring
and re-documenting software to make it easier to
understand and change.
 Refactoring, making program changes that preserve
functionality, is a form of preventative maintenance.
 The business value of a legacy system and the quality of
the application should be assessed to help decide if a
system should be replaced, transformed or maintained.
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