An architectural marvel when it opened 11 years ago, the new Denver International Airport’s high-tech jewel was to be its automated baggage handler. It would autonomously route luggage around 26 miles of conveyors for rapid, seamless delivery to planes and passengers. But software problems dogged the system, delaying the airport’s opening by 16 months and adding hundreds of millions of dollars in cost overruns. Despite years of tweaking, it never ran reliably. Last summer airport managers finally pulled the plug–reverting to traditional manually loaded baggage carts and tugs with human drivers. The mechanized handler’s designer, BAE Automated Systems, was liquidated, and United Airlines, its principal user, slipped into bankruptcy, in part because of the mess.
…Such massive failures occur because crucial design flaws are discovered too late. Only after programmers began building the code–the instructions a computer uses to execute a program–do they discover the inadequacy of their designs. Sometimes a fatal inconsistency or omission is at fault, but more often the overall design is vague and poorly thought out. As the code grows with the addition of piecemeal fixes, a detailed design structure indeed emerges–but it is a design full of special cases and loopholes, without coherent principles. As in a building, when the software’s foundation is unsound, the resulting structure is unstable.
…Now a new generation of software design tools is emerging. Their analysis engines are similar in principle to tools that engineers increasingly use to check computer hardware designs. A developer models a software design using a high-level (summary) coding notation and then applies a tool that explores billions of possible executions of the system, looking for unusual conditions that would cause it to behave in an unexpected way. This process catches subtle flaws in the design before it is even coded, but more important, it results in a design that is precise, robust and thoroughly exercised. One example of such a tool is Alloy, which my research group and I constructed. Alloy (which is freely available on the Web) has proved useful in applications as varied as avionics software, telephony, cryptographic systems and the design of machines used in cancer therapy.
Daniel Jackson has also recently written a book Software Abstractions: Logic, Language, and Analysis. The book’s website describes the book:
Daniel Jackson introduces a new approach to software design that draws on traditional formal methods but exploits automated tools to find flaws as early as possible. This approach–which Jackson calls “lightweight formal methods” or “agile modeling”–takes from formal specification the idea of a precise and expressive notation based on a tiny core of simple and robust concepts but replaces conventional analysis based on theorem proving with a fully automated analysis that gives designers immediate feedback. Jackson has developed Alloy, a language that captures the essence of software abstractions simply and succinctly, using a minimal toolkit of mathematical notions. The designer can use automated analysis not only to correct errors but also to make models that are more precise and elegant. This approach, Jackson says, can rescue designers from “the tarpit of implementation technologies” and return them to thinking deeply about underlying concepts.