Chapter 9: Software Evolution

Software change

It 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.

Spiral model of development and evolution

This model

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

The driver for the system of evolution

Should be linked with components that are affected by the change

Allows estimation of cost and impact of change

System Lifetime

Change identification and evolution continues throughout this

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 or have gone

During the program understanding phase, you have to understand how the program is structured, how it delivers functionality and how the proposed change(s) might affect the program

The emergency repair process

This model

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 should be expressed as additional user stories.

Handover problems

Short definition: When changing from one development approach to another (agile, plan-based) can cause problems

Where the development team has used an agile approach, but they are 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.

Legacy systems

Legacy systems are older systems that rely on languages and technology that are no longer used for new systems development.

Legacy software may be dependent on older hardware, such as mainframe computers and may have associated legacy processes and procedures.

Legacy systems are not just software systems but are broader socio-technical systems that include hardware, software, libraries and other supporting software and business processes.

Legacy system layers

This model

Legacy system replacement

Legacy system replacement is risky and expensive so businesses continue to use these systems

System replacement is risky for a number of reasons

• Lack of complete system specification

• Tight integration of system and business processes

• Undocumented business rules embedded in the legacy system

• New software development may be late and/or over budget

Legacy system change

Legacy systems are expensive to change for several reasons:

• No consistent programming style

• Use of obsolete programming languages with few people available with these language skills

• Inadequate system documentation

• System structure degradation

• Program optimizations may make them hard to understand

• Data errors, duplication and inconsistency

Legacy system management

Organizations 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.

Legacy assessment example

This model

Legacy system categories

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 management

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 system’s 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

Supplier stability

Failure rate

Age

Performance

Support Requirements

Maintenance Costs

Interoperability

Understandability

Documentation

Data
Performance

Programming Language
Configuration Management

Test Data

Personnel skills

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 licenses? 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?

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 assess the quality of the application system

• The number of system change requests; The higher this accumulated value, the lower the quality of the system.

• The number of different user interfaces used by the system; The more interfaces, the more likely it is that there will be inconsistencies and redundancies in these interfaces.

• The volume of data used by the system. As the volume of data (number of files, size of database, etc.) processed by the system increases, so too do the inconsistencies and errors in that data.

• Cleaning up old data is a very expensive and time-consuming process

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

Fault repairs

• Changing a system to fix bugs/vulnerabilities and correct deficiencies in the way meets its requirements.

Environmental adaptation

• 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.

Functionality addition and modification

• Modifying the system to satisfy new requirements.

Maintenance Costs

It is usually more expensive to add new features to a system during maintenance than it is to add the same features during development

• A new team has to understand the programs being maintained

• Separating maintenance and development means there is no incentive for the development team to write maintainable software

• Program maintenance work is unpopular

  • Maintenance staff are often inexperienced in older languages and have limited domain knowledge.

• As programs age, their structure degrades and they become harder to change

Maintenance prediction

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 predictions

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.

Advantages of re-engineering

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.

Re-engineering process

This model

Re-engineering process 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.

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 re-engineering

Reengineering 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 programming 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.