Software Evolution
Software change
Software change is inevitable because of the following :-
1. New requirements emerge when the software is used.
2. The business environment changes.
3. Errors must be repaired.
4. New computers and equipment is added to the system;
5. 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 system’s 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 software’s
environment. No new functionality is added.
Phase-out
The software may still be used but no further changes are
made to it.
Evolution and servicing
Evolution processes
Software evolution processes depend on:-
1. The type of software being maintained;
2. The development processes used;
3. 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.
Change implementation
Urgent change requests
Change requests sometimes relate to system
problems that have to be tackled urgently.
These urgent changes can arise for three
reasons:
1. If a serious system fault has to be repaired to allow
normal operation to continue;
2. If changes to the system’s environment (e.g. an OS
upgrade) have unexpected effects;
3. If there are business changes that require a very
rapid response (e.g. the release of a competing
product)
Urgent change requests
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 system’s architecture.
Changes are implemented by modifying existing
components and adding new components to
the system.
Types of maintenance
There are three different types of software maintenance:
1. Maintenance to repair software faults
Changing a system to correct deficiencies in the way meets
its requirements.
2. 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.
3. Maintenance to add to or modify the system’s functionality
Modifying the system to satisfy new requirements.
Development and maintenance costs
Maintenance cost factors
It is usually more expensive to add functionality after a system
is in operation than it is to implement the same functionality
during development.
1. Team stability
Maintenance costs are reduced if the same staff are involved with
them for some time.
2. Staff skills
Maintenance staff are often inexperienced and have limited domain
knowledge.
3. Program age and structure
As programs age, their structure is degraded and they become harder
to understand and change.
Maintenance prediction
Managers hate surprises, especially if these result in
unexpectedly high costs.
You should therefore try to predict what system changes
might be proposed and what parts of the system are likely to
be the most difficult to maintain.
You should also try to estimate the overall maintenance costs
for a system in a given time period.
Maintenance prediction
Process metrics
After a system has been put into service, you may
be able to use process data to help predict
maintainability.
Examples of process metrics that can be used for
assessing maintainability are as follows:
1. Number of requests for corrective maintenance;
2. Average time required for impact analysis;
3. Average time taken to implement a change request;
4. Number of outstanding change requests.
If any or all of these is increasing, this may indicate
a decline in maintainability.
