Assertions to better specify the Amazon bug

Modern Web applications are mainly distributed systems that exploit the Internet as communication means and the Web as neutral interface to access services and data. The addition of services to Web applications poses problems that are usually tackled at the technology level, but that should be addressed during design to deliver better quality Web applications. A typical example of these problems is the Amazon bug, an annoying problem that awkward users could encounter while shopping at Amazon. In this paper, we do not want to discuss all design aspects, but only how provided services/operations should jointly be designed with the rest of the system. We propose a new reference model for Web applications: Operations require a more complex model where they are not simply appended to information and navigation elements, but they can cooperate with them. Besides the reference model, the paper proposes the use of assertions to constraint the behavior of designed operations. Assertions do not only predicate on how data should be modified, but must also take into account how presentation and navigation could be affected by the execution of the operation.