Introduction to Software Evolution and Maintenance Software Evolution Chapter 1: History and Challenges of Software Evolution Tom Mens Software Engineering, 7th Edition Chapter 21: Software Evolution Ian Sommerville Software Evolution

Outline Overview Software evolution processes The research landscape

Importance of evolution Organizations have huge investments in their software systems - they are critical business assets. To maintain the value of these assets to the business, they must be changed and updated. The majority of the software budget in large companies is devoted to evolving existing software rather than developing new software. Studies indicate that up to 75% of all software professionals are involved in some form of maintenance/evolution activity. Software Evolution

Software change Software change 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 organizations is implementing and managing change to their existing software systems. Software Evolution

Types of changes Repair software faults Changing a system to correct deficiencies in the way meets its requirements. 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. Add to or modify the system’s functionality Modifying the system to satisfy new requirements. Improve the program structure and system performance Rewriting all or parts of the system to make it more efficient and maintainable. In the narrower sense of the word, “evolution” refers to changes that add functionality to the system. Software Evolution

Software evolution and software maintenance No standard definitions. Broad definition of evolution: Generally, software evolution refers to the study and management of the process of making changes to software over time. In this definition, software evolution comprises: Development activities Maintenance activities Reengineering activities Narrow definition of evolution: Sometimes, software evolution is used to refer to the activity of adding new functionality to existing software. Maintenance refers to the activity of modifying software after it has been put to use in order to maintain its usefulness. Software Evolution

Types of changes Repair software faults Changing a system to correct deficiencies in the way meets its requirements. 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. Add to or modify the system’s functionality Modifying the system to satisfy new requirements. Improve the program structure and system performance Rewriting all or parts of the system to make it more efficient and maintainable. “Maintenance” “Evolution” In the narrower sense of the word, “evolution” refers to changes that add functionality to the system. “Reengineering” Software Evolution

History 1960s – 1970s Inclusion of maintenance in waterfall lifecycle after delivery of the software product. Perception that post-delivery activities only consisted of bug fixes and minor adjustments. Did not account for the need to add functionality due to new and changed requirements. 1970s Lehman postulated the initial laws of program evolution. Stressed the need for continuous evolution due to changes in the software’s operational environment. Late 1970s – 1980s Initial process models that handled change requests. 1990s General acceptance of software evolution. Development of new process models that accounted for evolution activities: evolutionary development, spiral model, agile software development. Software Evolution

Evolutionary development Concurrent activities Validation Final version Development Intermediate versions Specification Initial Outline description Software Evolution

Spiral model of the software process Risk anal ysis Pr oto- type 1 ototype 2 ototype 3 Oper a- tional pr oto ype Concept of a tion Sim ula tions , models , benchmar ks S/W r equir ements R ement v alida Design V&V oduct design Detailed Code Unit test Integ test Acceptance Service De elop , v erify ne xt-le el pr Ev alua te alterna ti es, identify , r esolv e risks Determine objectives, alternatives, and constraints Plan ne xt phase and test plan elopment plan ements plan Life-cy cle plan EVI EW Software Evolution

An iterative development process Validate increment Build system Specify system Design system architecture Define system deliverables System complete? Integrate system Delier final v YES NO Software Evolution

Outline Overview Software evolution processes The research landscape

Evolution processes Processes for evolving a software product depend on The type of software being maintained; The development processes used; The skills and experience of the people involved. Proposals for change are the drivers for system evolution. Change identification and evolution continue throughout the system lifetime. Sommerville, Ch. 21 Software Evolution

Change identification and evolution Sommerville, Ch. 21 Software Evolution

The system evolution process Release planning Change implementation System r elease Impact analysis equests Platform adaptation enhancement Fault repair Sommerville, Ch. 21 Software Evolution

Change implementation Proposed Requirements Requirements Software changes analysis updating development Sommerville, Ch. 21 Software Evolution

Legacy systems For many systems, the software evolution process is not as straightforward as described. Associated models and documentation of the software may be missing or hopelessly outdated. The new requirements may not be anticipated by the design of the software, making the resulting changes difficult to implement correctly. Legacy systems are old systems that have become significantly difficult to modify. Accumulation of changes have eroded the modularity of the original design. The documentation has not been maintained and has become obsolete. One or more pieces of its underlying technologies have become obsolete. Two complementary techniques are employed to support the continued evolution of legacy systems: Reverse engineering. Reengineering. Software Evolution

Obsolete system components Hardware - may be obsolete mainframe hardware. Support software - may rely on support software from suppliers who are no longer in business. Application software - may be written in obsolete programming languages. Application data - often incomplete and inconsistent. Business processes - may be constrained by software structure and functionality. Business policies and rules - may be implicit and embedded in the system software. Software Evolution

Forward and reverse engineering and reengineering “Forward Engineering is the traditional process of moving from high-level abstractions and logical, implementation-independent designs to the physical implementation of a system.” “Reverse Engineering is the process of analyzing a subject system to identify the system’s components and their interrelationships and create representations of the system in another form or at a higher level of abstraction.” “Reengineering ... is the examination and alteration of a subject system to reconstitute it in a new form and the subsequent implementation of the new form.” — Chikofsky and Cross [R.S. Arnold, Software Reengineering, IEEE CS Press, 1993] Software Evolution

Reverse engineering In many legacy systems, the only reliable information about the system is the source code. Reverse engineering reconstructs requirements, design models, test cases and user documentation consistent with the current state of the source code. Reverse engineering encompasses several activities: program comprehension, software visualization, static and dynamic slicing, etc. Reverse engineering is often the initial activity in a reengineering project. Software Evolution

System reengineering Rewriting parts or all of a legacy system to make it more evolvable, so that it can more easily accommodate future change requests. Some authors [e.g., Sommerville] define it more strictly as the process of restructuring legacy software without changing its functionality. Others include a forward engineering phase as part of reengineering. Reengineering is 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 restructured and redocumented. Sommerville, Ch. 21 Software Evolution

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. Sommerville, Ch. 21 Software Evolution

The reengineering process Reverse engineering Program documentation Data re-engineering Original data structure improvement modularization Structured program Re-engineered data Modularized Original Source code translation Sommerville, Ch. 21 Software Evolution

Reengineering process activities Source code translation Convert code to a new language. Reverse engineering Analyze the program to understand it; Program structure improvement Restructure automatically for understandability; Program modularization Reorganize the program structure; Data reengineering Clean-up and restructure system data. Sommerville, Ch. 21 Software Evolution

Outline Overview Software evolution processes The research landscape

Research landscape Two aspects of software evolution research Reverse engineering and reengineering techniques Techniques for dealing with change Process and change management Evolution of software artifacts Mens & Demeyer, Ch. 1 Software Evolution

Two aspects of software evolution “What and why” Focuses on software evolution as a scientific discipline. Studies the nature of the software evolution phenomenon to understand its driving factors. Key interests include the formulation and refinement of fundamental theories and laws of software evolution. “How” Focuses on software evolution as an engineering discipline. Studies how to support the daily tasks of the software developer or project manager. Key interests include the development and validation of tools and techniques to guide, implement and control software evolution. Mens & Demeyer, Ch. 1 Software Evolution

Techniques for dealing with change Program comprehension Understanding the existing program in order to change it. Change impact analysis Identification of the parts of the system that will be affected by a proposed change. Change propagation Making sure that all affected parts are changed correctly. Restructuring/Refactoring Improving the software structure or architecture without changing the behavior. Regression testing Efficiently verifying that the change preserved the behavior of functionalities that should not be impacted. Mens & Demeyer, Ch. 1 Software Evolution

Management Economics of software evolution Software metrics Developing economic models to support evolution-related management decisions. Comparing the cost of different strategies for changes. Assessing the cost-benefits of investing in reengineering. Software metrics Measuring the quality of a change. Measuring the degree of modularity. Configuration management Change management processes. Management of multiple versions. Merging versions together. Release management. Mens & Demeyer, Ch. 1 Software Evolution

Evolution of software artifacts Requirements evolution Managing requirements changes. Architecture evolution Reengineering the architectures of legacy systems. Migration to distributed architectures, e.g., service-oriented architectures. Maintenance issues with modern architectures. Design evolution Evolution of design models. Test case evolution Adding and modifying test cases to verify that the system behavior was changed as intended. Traceability management How to assure the consistency of the different artifacts. Mens & Demeyer, Ch. 1 Software Evolution

Other evolution issues Data evolution Migrating to a new database schema. Verifying that the information in the existing databases are preserved. Runtime evolution How to modify a system without stopping it. Encompasses runtime reconfiguration, dynamic adaptation, dynamic upgrading. Language evolution Dealing with changes in the programming language definition. Especially an issue in multi-language systems. Designing languages to make them more robust to evolution. Mens & Demeyer, Ch. 1 Software Evolution

Our research interests Empirical studies of software evolution Mining software repositories Semantic change analysis Software Evolution