Formal Aids for the Growth of Software Systems

Abstract

The use of formal techniques has for a long time been focused on relatively small and complex applications. The hardware domain lends itself well to this and it has therefore been the target of some of the most significant applications of formal techniques. The software applications that have typically been considered were for small, safety-critical systems.

This restricted focus was understandable and necessary while formal techniques were evolving and practical considerations limited the size of the system that could be specified and verified. However, there are now compelling demands for the use of more precise techniques for a variety of large-scale applications, ranging from smart cards to financial systems.

So there are now new reasons to extend the use of formal methods for all phases of software development: from requirements and software modeling to coding and testing. Problems of scale still remain so it is important to focus the use of formal techniques in areas where their impact will be most important.

Different formal techniques can be used for solving different problems. For example, use of model-checking during requirements modeling can identify incomplete or inconsistent specifications, while use of transformational techniques can be very effective for software modeling and enable generation of code directly from models. Program analysis techniques can be used to generate tests that will greatly improve functional coverage during testing.

The use of formal techniques continues during software maintenance through the following kinds of activities:

a. Remedial: correction of errors discovered during use;

b. Adaptive: making changes to cater to changes in the operating environment;

c. Enhancing: adding new features or capabilities; and

d. Improving: making the software more robust and easier to maintain.

It is estimated that the cost of software maintenance amounts to as much as 90% of the life-cycle cost of a software system. While this calls for major improvements in maintenance techniques, changes in software development methods can also help to reduce the need for, and therefore the cost of, making remedial improvements (i.e. bug fixing).

In this talk, I will describe the use of formal techniques for different areas of the software life-cycle and relate this to evidence obtained through the analysis of a large number of actual software development and maintenance projects.