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

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

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

Lehman’s laws

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

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

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

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

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

Distribution factors

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

Layered distribution model

Legacy system distribution

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

Distribution option spectrum

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

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

Forward engineering and re-engineering

Re-engineering approaches

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

Program restructuring – Spaghetti logic

Program restructuring – Structured control logic

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)...

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

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

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

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

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

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