Extendibility

Extendibility is the ease of adapting software products to changes of specification.Software is supposed to be soft, and indeed is in principle; nothing can be easier than to change a program if you have access to its source code. Just use your favorite text editor. The problem of extendibility is one of scale. For small programs change is usually not a difficult issue; but as software grows bigger, it becomes harder and harder to adapt.

         A large software system often looks to its maintainers as a giant house of cards in which pulling out any one element might cause the whole edifice to collapse. We need extendibility because at the basis of all software lies some human phenomenon and hence fickleness. The obvious case of business software,where passage of a law or a company’s acquisition may suddenly invalidate the assumptions on which a system rested, is not special; even in scientific computation, where we may expect the laws of physics to stay in place from one month to the next, our way of understanding and modeling physical systems will change.

             Traditional approaches to software engineering did not take enough account of change, relying instead on an ideal view of the software lifecycle where an initial analysis stage freezes the requirements, the rest of the process being devoted to designing and building a solution. This is understandable: the first task in the progress of the discipline was to develop sound techniques for stating and solving fixed problems, before we could worry about what to do if the problem changes while someone is busy solving it. 

               But now with the basic software engineering techniques in place it has become essential to recognize and address this central issue. Change is pervasive in software development:change of requirements, of our understanding of the requirements, of algorithms, of data representation, of implementation techniques. Support for change is a basic goal of object technology and a running theme through this book.