Chapter 9 – Software Evolution

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Topics Covered

1. Evolution processes
2. Legacy systems
3. Software maintenance

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 1-Software Change

 Software change is inevitable

▪ New requirements emerge when the software is used;
▪ The business environment changes;
▪ New computers and equipment is added to the system;
▪ The performance or reliability of the system may have to be
improved.
▪ Software have to update or upgrade
▪ Errors must be repaired;
 A key problem for all organizations is implementing and
managing change to their existing software systems.

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A spiral Model of Development & Evolution

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Evolution and Servicing (Self)

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Evolution Processes (Self)

 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
(Self)

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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 change of 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 (self)

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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 (self)

 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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3-Legacy systems (self)

 Legacy systems are older systems that rely on

languages and technology that are no longer used for
new systems development.
 Legacy software may be dependent on older hardware,
such as mainframe computers and may have associated
legacy processes and procedures.
 Legacy systems are not just software systems but are
broader socio-technical systems that include hardware,
software, libraries and other supporting software and
business processes.

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Legacy system components (self)

 System hardware Legacy systems may have been

written for hardware that is no longer available.
 Support software The legacy system may rely on a
range of support software, which may be obsolete or
unsupported.
 Application software The application system that provides
the business services is usually made up of a number of
application programs.
 Application data These are data that are processed by the
application system. They may be inconsistent, duplicated or
held in different databases.
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Legacy system components (self)

 Business processes These are processes that are used

in the business to achieve some business objective.
 Business processes may be designed around a legacy
system and constrained by the functionality that it
provides.
 Business policies and rules These are definitions of how
the business should be carried out and constraints on
the business. Use of the legacy application system may
be embedded in these policies and rules.

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Legacy system layers (self)

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Legacy system replacement (self)

 Legacy system replacement is risky and expensive so

businesses continue to use these systems
 System replacement is risky for a number of reasons
▪ Lack of complete system specification
▪ Tight integration of system and business processes
▪ Undocumented business rules embedded in the legacy system
▪ New software development may be late and/or over budget

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Legacy system categories

 Low quality, low business value

▪ These systems should be scrapped.
 Low-quality, high-business value
▪ These make an important business contribution but are
expensive to maintain. Should be re-engineered or replaced if a
suitable system is available.
 High-quality, low-business value
▪ Replace with COTS, scrap completely or maintain.
 High-quality, high business value
▪ Continue in operation using normal system maintenance.

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Business value assessment (self)

 Assessment should take different viewpoints into

account
▪ System end-users;
▪ Business customers;
▪ Line managers;
▪ IT managers;
▪ Senior managers.
 Interview different stakeholders and collate results.

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System quality assessment (self)

 Business process assessment

▪ How well does the business process support the current goals of
the business?
 Environment assessment
▪ How effective is the system’s environment and how expensive is
it to maintain?
 Application assessment
▪ What is the quality of the application software system?

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Factors used in environment assessment (self)

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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3- Software maintenance (self)

 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 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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Software reengineering

 Restructuring or rewriting part or all of a legacy

system without changing its functionality.
 Applicable where some but not all sub-systems of a larger
system require frequent maintenance.
 Reengineering involves adding effort to make them easier to
maintain. The system may be re- structured and re-
documented.

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Advantages of reengineering

 Reduced risk
▪ There is a high risk in new software development. There may be
development problems, staffing problems and specification
problems.
 Reduced cost
▪ The cost of re-engineering is often significantly less than the
costs of developing new software.

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The reengineering process

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Reengineering process activities

 Source code translation

▪ Convert code to a new language.
 Reverse engineering
▪ Analyse the program to understand it;
 Program structure improvement
▪ Restructure automatically for understandability;
 Program modularisation
▪ Reorganise the program structure;
 Data reengineering
▪ Clean-up and restructure system data.

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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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Key points

 It is often cheaper and less risky to maintain a legacy

system than to develop a replacement system using
modern technology.
 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.
 There are 3 types of software maintenance, namely bug
fixing, modifying software to work in a new environment,
and implementing new or changed requirements.

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Key points

 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.

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 Exercise

1. How are rapid change is implemented for difference scenarios?

2. Which software development model is the most suitable for evolution?
3. Elaborate Re-engineering process in your own words.