Introduction to Software
Engineering |
Lecture 9 Software
Evolution |
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
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 all organizations is implementing and managing change
to their existing software systems.
Importance of evolution
Organizations 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 changing and evolving existing software rather than developing
new software.
A spiral model of development and
evolution
Evolution and servicing
Evolution
The stage in a software
systems 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 softwares
environment. No new functionality is added.
Phase-out
The software may still be
used 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.
Change identification and evolution
processes
The software evolution process
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.
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 systems 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).
The emergency repair process
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.
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.
Program evolution dynamics
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 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.
It
is not clear if these are applicable to other types of software system.
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
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 changed if they are to remain useful in an environment.
Lehmans
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. |
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. |
Organizational stability |
Over a programs lifetime, its rate of
development is approximately constant and independent of the resources
devoted to system development. |
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. |
Applicability of Lehmans laws
Lehmans laws seem to be generally applicable to large, tailored systems
developed by large organisations.
Confirmed in early 2000s by work by Lehman on the FEAST project.
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.
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 systems architecture.
Changes
are implemented by modifying existing components and adding new components to
the system.
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 systems functionality
Modifying the system to satisfy new requirements.
Maintenance effort distribution
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 and 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.
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 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.
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.
The reengineering process
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.
Reengineering approaches
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.
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.
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.
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.
·
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.
Legacy system management
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.
An example of a legacy system
assessment
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.
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.
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 systems 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.
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? |
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? |
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? |
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? |
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.
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.