SOFTWARE EVOLUTION
PRESENTATION BY -
Abhishek (103)
Ankit (107)
Harshit (115)
Hemang (116)
Nishit (312)
Objectives
• To explain why change is inevitable if software
systems are to remain useful
• To discuss software maintenance and
maintenance cost factors
• To describe the processes involved in software
evolution
• To discuss an approach to assessing evolution
strategies for legacy systems
Topics covered
• Program evolution dynamics
• Software maintenance
• Evolution processes
• Legacy system evolution
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 organisations is implementing and
managing change to their existing software systems.
Importance of evolution
• Organisations 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 evolving existing
software rather than developing new software.
Spiral model of evolution
Specification Implemention
Star t
Release 1
Operation Validation
Release 2
Release 3
Program evolution dynamics
• Program evolution dynamics is the study of the
processes of system change.
• After major empirical studies, Lehman and
Belady proposed that there were a number of
‘laws’ which applied to all systems as they
evolved.
• There are sensible observations rather than laws.
They are applicable to large systems developed
by large organisations. Perhaps less applicable in
other cases.
Lehman’s laws
Law Description
Continuing change A program that is used in a real-world environment necessarily
must change or become progressively less useful in that
environment.
Increasing complexity As an evolving program changes, its structure tends to become
more complex. Extra resources must be devoted to preserving
and simplifying the structure.
Large program evolution Program evolution is a self-regulating process. System
attributes such as size, time between releases and the number of
reported errors is approximately invariant for each system
release.
Organisational stability Over a programÕs lifetime, its rate of development is
approximately constant and independent of the resources
devoted to system development.
Conservation of Over the lifetime of a system, the incremental change in each
familiarity 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 appear to be declining unless they
are adapted to changes in their operational environment.
Feedback system Evolution processes incorporate multi-agent, multi-loop
feedback systems and you have to treat them as feedback
systems to achieve significant product improvement.
Applicability of Lehman’s laws
• Lehman’s laws seem to be generally applicable to
large, tailored systems developed by large
organisations.
▫ Confirmed in more recent work by Lehman on the FEAST
project (see further reading on book website).
• It is not clear how they should be modified for
▫ Shrink-wrapped software products;
▫ Systems that incorporate a significant number of COTS
components;
▫ Small organisations;
▫ Medium sized systems.
Software maintenance
• Modifying a program after it has been put into
use.
• 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.
Maintenance is inevitable
• The system requirements are likely to change
while the system is being developed because
the environment is changing. Therefore a
delivered system won't meet its requirements!
• Systems are tightly coupled with their environment.
When a system is installed in an
environment it changes that environment and
therefore changes the system requirements.
• Systems MUST be maintained therefore if they
are to remain useful in an environment.
Types of maintenance
• Maintenance to repair software faults
▫ Changing a system to correct deficiencies in the way meets its
requirements.
• 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.
• Maintenance to add to or modify the system’s
functionality
▫ Modifying the system to satisfy new requirements.
Distribution of maintenance effort
Fault repair
(17%)
Functionality
Software addition or
adaptation
modification
(18%)
(65%)
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.).
Development/maintenance costs
System 1
System 2
45 0 $
0 50 1 00 15 0 200 2 50 3 00 35 0 400 500
Development costs Maintenance costs
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.
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.
Maintenance prediction
What par ts of the system
will be the most expensive
What par ts of the system are to maintain?
most likely to be affected by
change requests?
Predicting
maintainability
What will be the lifetime
maintenance costs of this
Predicting system Predicting system?
changes maintenance
costs
What will be the costs of
How many change maintaining this system
requests can be over the next year?
expected?
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
components.
• Complexity depends on
▫ Complexity of control structures;
▫ Complexity of data structures;
▫ Object, method (procedure) and module size.
Process metrics
• Process measurements 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.
Evolution processes
• 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. Change identification and evolution
continue throughout the system lifetime.
Change identification and evolution
Change identification
process
New system Change proposals
Software evolution
process
The system evolution process
Change Impact Release Change System
requests analysis planning implementa tion release
Platform System
Fault repair
adaptation enhancement
Change implementation
Proposed Requirements Requir ements Software
changes analysis upda ting de velopment
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;
▫ 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).
Emergency repair
Change Analys e Modify Deliver modified
requests sour ce code sour ce code system
System re-engineering
• Re-structuring or re-writing 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.
• Re-engineering involves adding effort to make
them easier to maintain. The system may be re-
structured and re-documented.
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.
Forward and re-engineering
System Design and New
specification implementation system
Forward eng ineering
Existing Understanding and Re-eng ineer ed
softw are system transf orma tion system
Softw are re-eng ineering
The re-engineering process
Original Prog ram Modularised Original data
prog ram documentation prog ram
Reverse
eng ineering
Data
Source code Prog ram re-eng ineering
translation modularisation
Prog ram
structure
improvement
Structured Re-eng ineered
prog ram data
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.
Re-engineering approaches
Automa ted pr og ram Pro gram and da ta
restructuring restructuring
Automa ted sour ce Automa ted r estructuring Restructuring plus
code con version with man ual changes architectur al changes
Increased cost
Reengineering cost factors
• The quality of the software to be reengineered.
• The tool support available for reengineering.
• The extent of the data conversion which is
required.
• The availability of expert staff for reengineering.
▫ This can be a problem with old systems based on
technology that is no longer widely used.
Legacy system evolution
• Organisations that rely on legacy systems must choose a
strategy for evolving these systems
▫ Scrap the system completely and modify business processes so
that it is no longer required;
▫ Continue maintaining the system;
▫ Transform the system by re-engineering to improve its
maintainability;
▫ Replace the system with a new system.
• The strategy chosen should depend on the system quality
and its business value.
System quality and business value
High business value
High business value
Low quality
High quality
9
10 8
6
7
Low business value Low business value
Low quality High quality
2 5
1 3 4
System quality
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.
Business value assessment
• Assessment should take different viewpoints
into account
▫ System end-users;
▫ Business customers;
▫ Line managers;
▫ IT managers;
▫ Senior managers.
• Interview different stakeholders and collate
results.
System quality assessment
• 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?
Business process assessment
• Use a viewpoint-oriented approach and seek answers
from system stakeholders
▫ Is there a defined process model and is it followed?
▫ Do different parts of the organisation use different processes for
the same function?
▫ How has the process been adapted?
▫ What are the relationships with other business processes and are
these necessary?
▫ Is the process effectively supported by the legacy application
software?
• Example - a travel ordering system may have a low
business value because of the widespread use of web-
based ordering.
QUESTIONS…..
