1. Software Evolution Managing the processes of software system change
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 centralised to a distributed architecture
Software re-engineering
No new functionality is added to the system but it is restructured and reorganised to facilitate future changes
These strategies may be applied separately or together

4. Lehman’s laws

5. 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

6. Types of maintenance Corrective Adaptive Perfective
Maintenance to repair software faults
Adaptive
Maintenance to adapt software to a different operating environment
Perfective
Maintenance to add to or modify the system’s functionality

7. 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, making further maintenance more difficult
Ageing software can have high support costs (e.g. old languages, compilers etc.)
Think of your software as continuously evolving

8. 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

9. Architectural evolution
There is a need to convert many legacy systems from a centralised architecture to a distributed (e.g., client-server) architecture
Change drivers
Hardware costs. Servers are cheaper than mainframes
User interface expectations. Users expect graphical user interfaces
Distributed access to systems. Users wish to access the system from different, geographically separated, computers

10. Distribution factors

11. Legacy system structure
Ideally, for distribution, there should be a clear separation between the user interface, the system services and the system data management
In practice, these are usually intermingled in older legacy systems

12. Layered distribution model

13. Legacy system distribution

14. Distribution options
The more that is distributed from the server to the client, the higher the costs of architectural evolution
The simplest distribution model is UI distribution where only the user interface is implemented on the server
The most complex option is where the server simply provides data management and application services are implemented on the client

15. Distribution option spectrum

16. Software re-engineering
Reorganising and modifying existing software systems to make them more maintainable
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
When to re-engineer
When system changes are mostly confined to part of the system then re-engineer that part
When hardware or software support becomes obsolete
When tools to support re-structuring are available

17. Re-engineering advantages
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

18. Forward engineering and re-engineering

19. Re-engineering approaches

20. Source code translation
Involves converting the code from one language (or language version) to another e.g. FORTRAN to C
May be necessary because of:
Hardware platform update
Staff skill shortages
Organisational policy changes
Only realistic if an automatic translator is available

21. Program restructuring – Spaghetti logic

22. Program restructuring – Structured control logic

23. Program restructuring – Condition simplification
-- Complex condition
if not (A > B and (C < D or not ( E > F) ) )...
-- Simplified condition
if (A <= B and (C>= D or E > F)...

24. Program modularisation
The process of re-organising a program so that related program parts are collected together in a single module
Usually a manual process that is carried out by program inspection and re-organisation

25. Recovering data abstractions
Many legacy systems use shared tables and global data to save memory space
Causes problems because changes have a wide impact in the system
Shared global data may be converted to objects or ADTs
Analyse common data areas to identify logical abstractions
Create an ADT or object for these abstractions
Use a browser to find all data references and replace with reference to the data abstraction

26. Reverse engineering
Analysing software with a view to understanding its design and specification
May be part of a re-engineering process but may also be used to re-specify a system for re-implementation
Builds a program data base and generates information from this
Program understanding tools (browsers, cross-reference generators, etc.) may be used in this process

27. Why reverse engineer
Reverse engineering often precedes re-engineering but is sometimes worthwhile in its own right
The design and specification of a system may be reverse engineered so that they can be an input to the requirements specification process for the system’s replacement
The design and specification may be reverse engineered to support program maintenance

28. Key points
Software change strategies include software maintenance, architectural evolution and software re-engineering
Lehman’s Laws are invariant relationships that affect the evolution of a software system
Maintenance types are adaptive, perfective, and corrective

29. 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
Re-engineering reorganizes and modifies existing software systems to make them more maintainable