Software Evolution and Maintenance
Software Evolution and Maintenance
BASIC CONCEPTS AND PRELIMINARIES
Another flaw in the human character is that everybody wants to build and nobody wants to do maintenance.
-Kurt Vonnegut, Jr.
1.1 EVOLUTION VERSUS MAINTENANCE
In 1965, Mark Halpern introduced the concept of software evolution to describe the growth characteristics of software . Later, the term "evolution" in the context of application software was widely used. The concept further attracted the attentions of researchers after Belady and Lehman published a set of principles determining evolution of software systems [2, 3]. The principles were very general in nature. In his landmark article entitled "The Maintenance 'Iceberg'," R. G. Canning compared software maintenance to an "iceberg" to emphasize the fact that software developers and maintenance personnel face a large number of problems . A few years later, in 1976, Swanson introduced the term "maintenance" by grouping the maintenance activities into three basic categories: corrective, adaptive, and perfective . In the early 1970s, IBM called them "maintenance engineers" or "maintainers" who had been making intentional modifications to running code that they had not developed themselves. The main reason for using nondevelopment personnel in maintenance work was to free up the software development engineers or programmers from support activities . In this book, we will use maintainer, maintenance engineer, developer, and programmer interchangeably.
Bennett and Rajlich  researched the term "software evolution" and found that there is no widely accepted definition of the term. In addition, some researchers and practitioners used the phrases "software evolution" and "software maintenance" interchangeably. However, key semantic differences exist between the two. The two are distinguished as follows:
The concept of software maintenance means preventing software from failing to deliver the intended functionalities by means of bug fixing.
The concept of software evolution means a continual change from a lesser, simpler, or worse state to a higher or better state (, p. 1).
Bennett and Xu  made further distinctions between the two as follows:
All support activities carried out after delivery of software are put under the category of maintenance .
All activities carried out to effect changes in requirements are put under the category of evolution .
In general, maintenance and evolution are generally differentiated as follows :
Maintenance of software systems primarily means fixing bugs but preserving their functionalities. Maintenance tasks are very much planned. For example, bug fixing must be done and it is a planned activity. In addition to the planned activities, unplanned activities are also necessitated. For example, a new usage of the system may emerge. Generally, maintenance does not involve making major changes to the architecture of the system. In other words, maintenance means keeping an installed system running with no change to its design .
Evolution of software systems means creating new but related designs from existing ones. The objectives include supporting new functionalities, making the system perform better, and making the system run on a different operating system. Basically, as time passes, the stakeholders develop more knowledge about the system. Therefore, the system evolves in several ways. As time passes, not only new usages emerge, but also the users become more knowledgeable. As Mehdi Jazayeri observed: "Over time what evolves is not the software but our knowledge about a particular type of software" (, p. 3).
While we are on the topic of maintenance, it is useful to glance at the maintenance of physical systems. Maintenance of physical systems often requires repla