Operation, Maintenance and System Evolution
Lecture 11
1Chapter 9 Software evolution
Topics covered
 Evolution processes
 Change processes for software systems
 Lehman’s laws
 Understandingsoftwareevolution dynamics
 Software maintenance
 Making changes to operational software systems
 Legacy system management
 Making decisions about software change
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Evolution Processes
Lecture 11/Part 1
3Chapter 9 Software 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.
 For custom systems, the costs of software maintenance
usually exceed the software development costs.
 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
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 Evolution
 New functionality added, faults repaired.
 Servicing
 Faults repaired, no new functionality added.
 Phase-out
 The software still in use 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
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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More on 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;
 If changes to the system’s environment require instant action;
 If there are business changes that require a very rapid response
(e.g. release of a competing product).
 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 prefers 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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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.
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Lehman’s laws
Lecture 11/Part 2
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 Change 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 latest requirements!
 Systems MUST be changed if they are to remain useful.
 Program evolution dynamics is the study of the
processes of system change.
 After several major empirical studies, Lehman and
Belady proposed that there were a number of
observational ‘laws’ which applied to systems under
evolution.
Program evolution dynamics
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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
complexity
As an evolving program changes, its structure tends to become more
complex. Extra resources must be devoted to preserving and
simplifying the structure.
Self regulation 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.
Organizational
stability
Over a program’s lifetime, its rate of development is
approximately 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 shall be approximately constant, to allow system
users to maintain mastery of its usage.
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
 Lehman’s laws seem to be generally applicable to large,
tailored systems developed by large organisations.
 Confirmed in early 2000’s by work by Lehman on the FEAST
project.
 It is not clear how they should be modified for
 Generic software products;
 Systems that incorporate a significant number of COTS
components;
 Small organisations;
 Small and medium sized systems.
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Key points
 Program evolution dynamics is the study of the
processes of system change.
 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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Software Maintenance
Lecture 11/Part 3
22Chapter 9 Software evolution
 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
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 Corrective: Maintenance to repair software faults
 Changing a system to correct deficiencies in the way meets its
requirements.
 Adaptive: 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.
 Evolutionary: Maintenance to add to or modify the
system’s functionality
 Modifying the system to satisfy new requirements.
Types of maintenance
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Maintenance effort distribution
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 Usually greater than development costs (1* to 20*
depending on the application).
 Experience with custom information systems shows that around
20% of development costs needs to be allocated to maintenance
every year (within the first five years).
 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
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Development and maintenance costs
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 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
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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 degrade 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 an
understanding of the relationships between the
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.
32Chapter 9 Software evolution
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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System reengineering
 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.
 Reengineering involves adding effort to make them
easier to maintain. The system may be re-structured and
re-documented.
 How does reengineering relate to refactoring?
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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 reengineering is often significantly less than the
costs of developing new software.
35Chapter 9 Software evolution
Reengineering 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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The reengineering process
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Reengineering approaches
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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.
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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.
40Chapter 9 Software evolution
Refactoring and reengineering
 Reengineering takes place after a system has been
maintained for some time and maintenance costs are
increasing. You use automated tools to process and
reengineer 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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Examples of ‘Bad Smells’ in program code
 Duplicate code
 Can be implemented as a single method or function that is called
as required.
 Long methods
 Can be redesigned as a number of shorter methods.
 Switch (case) statements
 Whenever the switch depends on the type of a value, consider
using polymorphism to achieve the same thing.
 Data clumping
 Data clumps (repeating groups of data items) can often be
encapsulated into an object.
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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
 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 reengineering 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.
44Chapter 9 Software evolution
Legacy System Management
Lecture 11/Part 4
45Chapter 9 Software evolution
Legacy system management
 Organisations that rely on legacy systems must choose
a strategy for evolving these systems
 Scrap: Scrap the system completely and modify business
processes so that it is no longer required;
 Maintain: Continue maintaining the system;
 Reengineer: Transform the system by reengineering to improve
its maintainability;
 Replace: Replace the system with a new system.
 The strategy chosen should depend on the system
quality and its business value.
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An example of a legacy system assessment
47Chapter 9 Software evolution
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 reengineered
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.
48Chapter 9 Software evolution
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.
49Chapter 9 Software evolution
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
 Business process assessment
 How well does the business process support the current goals of
the business?
 Application assessment
 What is the quality of the application software system?
How expensive it is to maintain?
 Environment assessment
 How effective is the system’s environment?
How expensive it is to maintain?
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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 webbased
ordering.
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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
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
management
Are all versions of all parts of the system managed by a
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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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? Is the supplier replaceable?
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.
Performance Is the performance of the system adequate? Do performance
problems have a significant effect on system users?
Support
requirements
What local support is required by the hardware and software? If high,
it may be worth considering system replacement.
Maintenance
costs
What are the costs of hardware maintenance and support software
licenses (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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System measurement
 You may collect quantitative data to make an
assessment of the quality of the application system
 The number of system change requests;
 The number of different user interfaces used by the system;
 The volume of data used by the system.
56Chapter 9 Software evolution
Key points
 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.
 The business-value assessment should take different
stakeholder viewpoints into account.
57Chapter 9 Software evolution