Alistair Cockburn - Agile Software Development

Some of the background of the views presented here is to be found in certain observations of what actually happens to programs and the teams of programmers dealing with them, particularly in situations arising from unexpected and perhaps erroneous program executions or reactions, and on the occasion of modifications of programs. The difficulty of accommodating such observations in a production view of programming suggests that this view is misleading. The theory building view is presented as an alternative.

A more general background of the presentation is a conviction that it is important to have an appropriate understanding of what programming is. If our understanding is inappropriate we will misunderstand the difficulties that arise in the activity and our attempts to overcome them will give rise to conflicts and frustrations.

In the present discussion some of the crucial background experience will first be outlined. This is followed by an explanation of a theory of what programming is, denoted the Theory Building View. The subsequent sections enter into some of the consequences of the Theory Building View.

I shall use the word programming to denote the whole activity of design and implementation of programmed solutions. What I am concerned with is the activity of matching some significant part and aspect of an activity in the real world to the formal symbol manipulation that can be done by a program running on a computer. With such a notion it follows directly that the programming activity I am talking about must include the development in time corresponding to the changes taking place in the real world activity being matched by the program execution, in other words program modifications.

One way of stating the main point I want to make is that programming in this sense primarily must be the programmers' building up knowledge of a certain kind, knowledge taken to be basically the programmers' immediate possession, any documentation being an auxiliary, secondary product.

As a background of the further elaboration of this view given in the following sections, the remainder of the present section will describe some real experience of dealing with large programs that has seemed to me more and more significant as I have pondered over the problems. In either case the experience is my own or has been communicated to me by persons having first hand contact with the activity in question.

Case 1 concerns a compiler. It has been developed by a group A for a language L and worked very well on computer X. Now another group B has the task to write a compiler for a language L + M, a modest extension of L, for computer Y. Group B decides that the compiler for L developed by group A will be a good starting point for their design, and get a contract with group A that they will get support in the form of full documentation, including annotated program texts and much additional written design discussion, and also personal advice. The arrangement was effective and group B managed to develop the compiler they wanted. In the present context the significant issue is the importance of the personal advice from group A in the matters that concerned how to implement the extensions M to the language. During the design phase group B made suggestions for the manner in which the extensions should be accommodated and submitted them to group A for review. In several major cases it turned out that the solutions suggested by group B were found by group A to make no use of the facilities that were not only inherent in the structure of the existing compiler but were discussed at length in its documentation, and to be based instead on additions to that structure in the form of patches that effectively destroyed its power and simplicity. The members of group A were able to

spot these cases instantly and could propose simple an effective solutions, framed entirely within the existing structure. This is an example of how the full program text and additional documentation is insufficient in conveying to even the highly motivated group B the deeper insight into the design, that theory which is immediately present to the members of group A.

In the years following these events the compiler developed by group B was taken over by other programmers of the same organization, without guidance from group A. Information obtained by a member of group A about the compiler resulting from the further modification of it after about 10 years made it clear that at that later stage the original powerful structure was still visible, but made entirely ineffective by amorphous additions of many different kinds. Thus, again, the program text and its documentation has proved insufficient as a carrier of some of the most important design ideas.

Case 2 concerns the installation and fault diagnosis of a large real-time system for monitoring industrial production activities. The system is marketed by its producer, each delivery of the system being adapted individually to its specific environment of sensors and display devices. The size of the program delivered in each installation is of the order of 200,000 lines. The relevant experience from the way this kind of system is handled concerns the role and manner of work of the group of installation and fault finding programmers. The facts are, first that these programmers have been closely concerned with the system as a full time occupation over a period of several years, from the time the system was under design. Second, when diagnosing a fault these programmers rely almost exclusively on their ready knowledge of the system and the annotated program text, and are unable to conceive of any kind of additional documentation that would be useful to them. Third, other programmers' groups who are responsible for the operation of particular installations of the system, and thus receive documentation of the system and full guidance on its use from the producer's staff, regularly encounter difficulties that upon consultation with the producer's installation and fault finding programmer are traced to inadequate understanding of the existing documentation, but which can be cleared up easily by the installation and fault finding programmers.

The conclusion seems inescapable that at least with certain kinds of large programs, the continued adaption, modification, and correction of errors in them, is essentially dependent on a certain kind of knowledge possessed by a group of programmers who are closely and continuously connected with them.

Ryle's Notion of Theory

If it is granted that programming must involve, as the essential part , a building up of the programmers' knowledge, the next issue is to characterize that knowledge more closely. What will be considered here is the suggestion that the programmers' knowledge properly should be regarded as a theory, in the sense of Ryle [1949]. Very briefly, a person who has or possesses a theory in this sense knows how to do certain things and in addition can support the actual doing with explanations, justifications, and answers to queries, about the activity of concern. It may be noted that Ryle's notion of theory appears as an example of what K. Popper [Popper, and Eccles, 1977] calls unembodied World 3 objects and thus has a defensible philosophical standing. In the present section we shall describe Ryle's notion of theory in more detail.

Ryle [1949] develops his notion of theory as part of his analysis of the nature of intellectual activity, particularly the manner in which intellectual activity differs from, and goes beyond, activity that is merely intelligent. In intelligent behaviour the person displays, not any particular knowledge of facts, but the ability to do certain things, such as to make and appreciate jokes, to talk grammatically, or to fish. More particularly, the intelligent performance is characterized in part by the person's doing them well, according to certain criteria, but further displays the person's ability to apply the criteria so as to detect and correct lapses, to learn from the examples o others, and so forth. It may be noted that this notion of intelligence does not rely on any notion that the intelligent behaviour depends on the person's following or adhering to rules, prescriptions, or methods. On the contrary, the very act of adhering to rules can be done more or less intelligently; if the exercise of intelligence depended on following rules there would have to be rules about how to follow rules, and about how to follow the rules about following rules, etc. in an infinite regress, which is absurd.