CSC440 Chapter 9: Software Evolution (Software Engineering, Sommerville, 10th Edition)

Software Timeline (Life Cycle)

1. Software Development 2. Software Evolution 3. Software Servicing 4. Software Retirement (Phase-out)

Maintenance Costs: Why is it more expensive to add new features 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 and have limited domain knowledge. - As programs age, their structure degrades and they become harder to change

Maintenance Effort Distribution

- 25% Fault Repair - 20% Environmental Adaptation - 55% Functionality Addition and Modification (I adapted these numbers from the book as the book's numbers on the graph represented 101%....)

General Steps in the Software Evolution Process

1. Change Requests 2. Impact Analysis 3. Release Planning (Includes fault repair, platform adaptation, and system enhancement) 4. Change Implementation 5. System Release

System Quality Assessment Components

- 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?

"Bad Smells" in Code that can be fixed with Refactoring

- 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.

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 or 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.

Reasons for urgent change requests

- If a serious system fault has to be repaired to allow normal operation to continue - If changes to the system's environment (e.g. an OS upgrade) have unexpected effects - If there are business changes that require a very rapid response (e.g. the release of a competing product).

Key Points (cont.)

- It is often cheaper and less risky to maintain a legacy system than to develop a replacement system using modern technology. - The business value of a legacy system and the quality of the application should be assessed to help decide if a system should be replaced, transformed or maintained. - There are 3 types of software maintenance, namely bug fixing, modifying software to work in a new environment, and implementing new or changed requirements.

Legacy System Replacement Risks

- 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 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.

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.

Reasons for Software Change

- 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

Legacy System Change Costs (Why so expensive?)

- 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

Change Prediciton

- 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: 1) Number and complexity of system interfaces 2) Number of inherently volatile system requirements 3) 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: 1) Complexity of control structures 2) Complexity of data structures; 3) Object, method (procedure) and module size.

Process Metrics

- Process metrics may be used to assess maintainability: 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.

Refactoring vs Reengineering

- Re-engineering 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.

Advantages of Reengineering

- 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.

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.

Software Rengineering

- Restructuring or rewriting 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. - Reengineering involves adding effort to make them easier to maintain. The system may be re-structured and re-documented.

Legacy System Management Strategic Options

- 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

Key Points

- Software development and evolution can be thought of as an integrated, iterative process that can be represented using a spiral model. - For custom systems, the costs of software maintenance usually exceed the software development costs. - The process of software evolution is driven by requests for changes and includes change impact analysis, release planning and change implementation. - Legacy systems are older software systems, developed using obsolete software and hardware technologies, that remain useful for a business.

Reengineering Process Activities

- Source code translation: Convert code to a new language - Reverse engineering: Analyse the program to understand it - Program structure improvement: Restructure automatically for understandability - Program modularisation: Reorganise the program structure - Data reengineering: Clean-up and restructure system data.

Factors used in environment assessment

- Supplier Stability - Failure Rate - Age - Performance - Support Requirements - Maintenance Costs - Interoperability

Components of Legacy Systems

- System hardware: Legacy systems may have been written for hardware that is no longer available. - Support software: The legacy system may rely on a range of support software, which may be obsolete or unsupported. - Application software: The application system that provides the business services is usually made up of a number of application programs. - Application data: These are data that are processed by the application system. They may be inconsistent, duplicated or held in different databases. - Business processes: These are processes that are used in the business to achieve some business objective. - Business policies and rules: These are definitions of how the business should be carried out and constraints on the business. Use of the legacy application system may be embedded in these policies and rules.

System Measurement (Quantitative data collected to make an assessment of 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

Reengineering cost 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)

Software Evolution Processes depend on:

- The type of software being maintained - The development processes used - The skills and experience of the people involved

Issues in Business Value Assessment

- 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

Factors used in application assessment

- Understandability - Documentation - Data - Performance - Programming Language - Configuration Management - Test Data - Personnel Skills

Maintenance Costs

- Usually greater than development costs (2x to 100x 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.).

Handover problems when software is handed over to another development team

- Where the development team have used an agile approach but the evolution team is 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.

General Steps in Change Implementation

1. Proposed changes 2. Requirements analysis 3. Requirements updating 4. Software development

Software 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.

Legacy System Assessment Criteria

Business Value vs System Quality - High Business Value but Low Quality (make an important business contribution but are expensive to maintain. Should be re-engineered or replaced if a suitable system is available.) - Low Business Value and Low Quality (should be scraped) - Low Business Value but High Quality (replace with COTS, scrap completely or maintain.) - High Business Value and High Quality (continue in operation using normal system maintenance)

How should the business value of a legacy system be assessed?

Interview different stakeholders and collate the results! - System end-users - Business customers - Line managers - IT managers - Senior managers

Change Implementation

Iteration of the development process where the revisions to the system are designed, implemented and tested.

Maintenance Prediction

Maintenance prediction is concerned with assessing which parts of the system may cause problems and have high maintenance costs

Software Retirement (Phase-out)

The software may still be used but no further changes are made to it.

Software 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.