Bse 3105 lecture 2- software change

Software change
 Managing the processes of
software system change
1

Objectives
 To explain different strategies for
changing software systems
 Software maintenance
 Architectural evolution
 Software re-engineering
 To explain the principles of software
maintenance
2

Software change
 Software change is inevitable
 New requirements emerge when the software is
used
 The business environment changes
 Errors must be repaired
 New equipment must be accommodated
 The performance or reliability may have to be
improved
 A key problem for organisations is implementing and
managing change to their legacy systems
3

Software change strategies
 Software maintenance
 Changes are made in response to changed requirements
but the fundamental software structure is stable
 Architectural transformation
 The architecture of the system is modified generally from a
centralized architecture to a distributed architecture
 Software re-engineering
 No new functionality is added to the system but it is
restructured and reorganized to facilitate future changes
 These strategies may be applied separately or together
4

Program evolution
dynamics
 Program evolution dynamics is the study of the
processes of system change
 After major empirical study, 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
5

Lehman’s laws of Software
Evolution
 Lehman suggested eight laws of S/w evolution[Draw
graphs for laws 1 and 2].
 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.
 Organizational stability: Over a program’s lifetime, its rate of
development is approximately constant and independent of
the resources devoted to system development.
6

Lehman’s laws of Software
Evolution-Cont’d
• Conservation of familiarity: Over the lifetime of a system, the
incremental change in each releases 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.
7

Applicability of Lehman’s
laws
 This has not yet been established
 They are generally applicable to large, tailored
systems developed by large organisations
 It is not clear how they should be modified for
 Shrink-wrapped software products
 Systems that incorporate a significant number of
COTS components
 Small organizations
 Medium sized systems
8

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
9

Maintenance is Expensive …
but is it necessary?
The solution to the problem of software maintenance is to build systems properly
in the first place, right?
 Perfection seems to be unattainable
 Even if perfection were attainable, software is affected by other changes like;
• Technological changes
– E.g. tapes vs. hard disks
• Changes to laws and government regulations
– E.g. data protection act, taxation ...
• Changes to the business environment
– E.g. need to deal with new, or much larger numbers of,
customers/sales/products/...
– E.g. e-commerce
• Changes to organizational structure
– E.g. company mergers[case of Airtel and Warid] 10

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
11

When is S/W maintenance
needed?
Maintenance to repair software faults
– Coding errors are usually relatively cheap to correct
– Design errors are more expensive as they may involve rewriting several program components
– Requirements errors are the most expensive to repair because of the extensive system
redesign that may be necessary
Maintenance to adapt the software to a new operating environment
– This type of maintenance is required when some aspect of the system’s environment such as
the hardware, the platform operating systems or other support software changes
– The application system must be modified to adapt it to cope with these environmental changes
Maintenance to add to or modify the system's functionality
– This type of maintenance is necessary when the system requirements change in response to
organizational or business change
– The scale of the changes required to the software is often much grater that the other types of
maintenance
12

Types of Software Changes
done in Maintenance
• Studies reveal 4 types of change
• Corrective change
– correcting faults in system behavior
– caused by errors in coding, design or requirements
• Adaptive change
– due to changes in operating environment
– e.g., different hardware or Operating System
• Perfective change
– due to changes in requirements
– Often triggered by organizational, business or user learning
• Preventive change
– e.g., dealing with legacy systems (legacy systems are these
older systems that remain vital to the organization)
• Exercise: define each of the above, and find at least two cases of software projects for
each of them where it has been used. Also find out which of the 4 is common. 13

Managing S/W Maintenance
•
• Corrective:
– Requires maintenance strategy preferably negotiated contract
between supplier and customer(s)
– Policies for reporting and fixing errors; auditing of process
• Perfective:
– Should be treated as development (e.g., requirements,
specification, design, testing, etc.)
– Iterative (or evolutionary) development approach best suited
– Risks: drift, shift, creep, ooze, bloat, etc.
– When does design or development stop?
• Adaptive and Preventive:
– Can anticipate, schedule, monitor and manage, etc. 14

Distribution of maintenance
effort
Question: What are the reasons behind the percentages?
15

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.)
17

Development/maintenance
costs
18

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
19

Evolutionary software
 Rather than think of separate
development and maintenance phases,
evolutionary software is software that is
designed so that it can continuously
evolve throughout its lifetime
20

The maintenance process
NB: The arrow from “system release” to “change request” means that a change
can cause a new change request.
21

Change requests
 Change requests are requests for system changes
from users, customers or management
 In principle, all change requests should be carefully
analyzed as part of the maintenance process and
then implemented
 In practice, some change requests must be
implemented urgently
 Fault repair
 Changes to the system’s environment
 Urgently required business changes
22

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
25

Change prediction
 Predicting the number of changes requires
understanding of the relationships between a system and
its environment
 Tightly coupled systems require changes whenever the
environment is changed. Recall that in tightly coupled systems, one
module can heavily rely on another module such that a change in one module means
a change in the other module. For example if they are sharing external variables.
 Factors influencing this relationship are
 Number and complexity of system interfaces. Like how
many modules will need to be changed when one module is changed?
 Number of inherently volatile system requirements
 The business processes where the system is used
27

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[possibly due to the maintainability of
those few components]
 Complexity depends on
 Complexity of control structures
 Complexity of data structures
 Procedure and module size
28

Process metrics
 Process measurements may be used to assess
maintainability
 Number of requests for corrective maintenance.
E.g. How many errors have been reported?
 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
29

Architectural evolution
 There is a need to convert many legacy systems
from a centralized architecture to a client-server
architecture
 Change drivers
 Hardware costs. Servers are cheaper than
mainframes
 User interface expectations. Users expect
graphical user interfaces[not just black and white
screens]
 Distributed access to systems. Users wish to
access the system from different, geographically
separated, computers 30

Distribution factors (i.e factors that affect the shift
from centralized to distributed architecture)
31

Key points
 Software change strategies include software
maintenance, architectural evolution and software re-engineering
 Lehman’s Laws are invariant [or true] relationships
that affect the evolution of a software system
 Maintenance types are
 Maintenance for repair
 Maintenance for a new operating environment
 Maintenance to implement new requirements
32

Key points
 The costs of software change usually exceed the costs
of software development
 Factors influencing maintenance costs include staff
stability, the nature of the development contract, skill
shortages and degraded system structure
 Architectural evolution is concerned with evolving
centralised to distributed architectures
33

Bse 3105 lecture 2- software change

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