Resumen
This article focuses on requirements engineering. It's hard to avoid defects in functional requirements. After a major failure, it's usually easy to see where one should have paid more attention and exercised more care, but it's not so obvious while one is developing the system. The difficulty is that requirements aren't about the software but about the problem world, which is always potentially complex, even for a system whose software is itself quite simple. The problem world interacts with the machine at the machine-world interface, where they meet. For the e-commerce system, they meet at the customers' browsers and at the ports where electronic data interchange messages are exchanged with suppliers and credit-card companies. Requirements engineering must focus on the problem world from two related perspectives. Outer requirements describe the effects the stakeholders would like to experience in the problem world. Inner requirements specify how the machine should behave at the machine-world interface to ensure that the whole system satisfies the outer requirements. Software design and programming make the machine behave this way, but choosing, designing, and specifying the behavior is the vital second part of requirements engineering.