Software evolution
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
Continuing change Description
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.).
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 are
What par ts of the system
will be the most expensive
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.
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
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.
