M. M. Lehman

Programs, life cycles, and laws of software evolution

By classifying programs according to their relationship to the environment in which they are executed, the paper identifies the sources of evolutionary pressure on computer applications and programs and shows why this results in a process of never ending maintenance activity. The resultant life cycle processes are then briefly discussed. The paper then introduces laws of Program Evolution that have been formulated following quantitative studies of the evolution of a number of different systems. Finally an example is provided of the application of Evolution Dynamics models to program release planning.

Metrics and laws of software evolution-the nineties view

The process of E-type software development and evolution has proven most difficult to improve, possibly due to the fact that the process is a multi-input, multi-output system involving feedback at many levels. This observation, first recorded in the early 1970s during an extended study of OS/360 evolution, was recently captured in a FEAST (Feedback, Evolution And Software Technology) hypothesis: a hypothesis being studied in on-going two-year project, FEAST/1. Preliminary conclusions based on a study of a financial transaction system-Logicas Fastwire (FW)-are outlined and compared with those reached during the earlier OS/360 study. The new analysis supports, or better does not contradict, the laws of software evolution, suggesting that the 1970s approach to metric analysis of software evolution is still relevant today. It is hoped that FEAST/1 will provide a foundation for mastering the feedback aspects of the software evolution process, opening up new paths for process modelling and improvement.

A model of large program development

Discussed are observations made on the development of OS/360 and its subsequent enhancements and releases. Some modeling approaches to organizing these observations are also presented.

Laws of Software Evolution Revisited

Data obtained during a 1968 study of the software process [8] led to an investigation of the evolution of OS/360 [13] and and, over a period of twenty years, to formulation of eight Laws of Software Evolution. The FEAST project recently initiated (see sections 4–6 below) is expected to throw additional light on the phenomenology underlying these laws, to increase understanding of them, to explore their finer detail, to expose their wider relevance and implications and to develop means for their beneficial exploitation. This paper is intended to trigger wider interest in the laws and in the FEAST study of feedback and feedback control in the context of the software process and its improvement to ensure beneficial exploitation of their potential.

On understanding laws, evolution, and conservation in the large-program life cycle

The paper presents interpretations of some recently discovered laws of evolution and conservation in the largeprogram life cycle. Program development and maintenance processes are managed and implemented by people; thus in the long term they could be expected to be unpredictable, dependant on the judgments, whims, and actions of programming process participants (e.g., managers, programmers, and product users). Yet, observed, measured, and modeled regularities suggest laws that are closer to biological laws or those of modern physics than to those currently formulated in other areas subject to human influence (e.g., economics and sociology). After a brief discussion of the first four laws, to highlight underlying phenomena and natural attributes of the program evolution process, the paper concentrates on a fifth law and shows how, and why, this law represents a conservation phenomenon: the Conservation of Familiarity.

Implications of evolution metrics on software maintenance

In the context of a hypothesis attributing the slow progress in achieving major global software process improvement, in part, to overlooking the role of feedback in that process, the FEAST/1 project is studying the impact of feedback on software evolution. Amongst its activities the project is analysing metrics of the evolution of several industrial systems, ranging from a financial transaction system to a very large real time system. The similarities which have emerged from a comparison of evolution metrics from several systems, support conclusions reached in a 1970s study of OS/360 evolution. The latest results suggest some refinement of earlier conclusions but indicate that both the metrics and the conclusions derived from them must be taken into account in the planning and implementation of successful software maintenance. Papers discussing the FEAST/1 results may accessed via the FEAST web page [fwp98].

Software's future: managing evolution

In his essay, Ed Yourdon expresses, justifies, and leaves unresolved two well-founded questions: What is the future of software? What does the future hold for the software professional? His prognosis is evasive, incomplete, and unsatisfying: the future will be good for some, not so for others. Given Yourdons extensive experience in the real world of computer usage, as proven by the problems he has observed, it is easy to see why he feels that soft-wares future is uncertain. But he does not point to a solution to this uncertainty, nor does he indicate what can be done to achieve the best possible outcome for software professionals. More importantly, Yourdons analysis does not indicate what should be done to ensure the security, well being, and survival of society, which depends increasingly on software. For more than a decade now, there have been those in the software engineering community who have accepted that the need to continually change and evolve software is a fact-a fact addressed through activity that is planned, executed, and controlled by humans. Thus, the software development and maintenance processes, which I prefer to unify and call software evolution, are key to managing computerization. In my view it is key to our survival in this computer age.

Software evolution: background, theory, practice

This paper opens with a brief summary of some 30 years of study of the software evolution phenomenon. The results of those studies include the SPE program classification, a principle of software uncertainty and laws of E-type software evolution. The laws were termed so because they encapsulate phenomena largely independent of the people, the organisations and the domains involved in the evolution of the E-type systems studied. Recent studies have refined earlier conclusions, yielded practical guidelines for software evolution management and provide a basis for the formation of a theory of software evolution. Given the volume of published material and the extent of recent discussions on the topic (see, e.g., [Proc. ICSM, Montreal, 2002, p. 66]), this paper is restricted to an overview that exposes the significance of the evolution phenomenon and its study to the wider community, providing a basis for the future and, in particular, development of a theory of software evolution.

Software engineering, the software process and their support

Computers are being applied more and more widely, penetrating ever deeper into the very fabric of society. Mankind is becoming increasingly dependent on the availability of software and its continuing validity. To achieve this consistently and reliably, in an operational domain that is forever changing, requires disciplined execution of the software development and evolution process and its effective management. That is the goal of advanced software engineering [1]. This paper summarises basic concepts of software engineering and of the software development process. This leads to a principle of uncertainty, analysis of its implications for the software development process, an overview of computer-assisted software engineering (CASE) and brief comments on the societal relevance of these topics. For researchers in the field and practitioners familiar with individual concepts, issues and specific solutions, the paper provides a unifying framework, a basis for conceptual advance. Those without a significant practical software engineering background and experienced graduate students will extend general familiarity with fresh insights, new concepts and additional detail. Undergraduate and graduate students without significant experience may treat the paper as an introductory text.

On evidence supporting the FEAST hypothesis and the laws of software evolution

As part of its study of the impact of feedback in the global software process on software product evolution, the FEAST/1 project has examined metric data relating to various systems in different application areas. High level similarities in the growth trends of the systems studied support the FEAST hypothesis. Inter alia, the results provide evidence compatible with the laws of software evolution, subject only to minor adjustments of the latter.