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There is nothing like a disagreement concerning an arcane technical matter to bring out the best (and worst) in software architects and developers. As every reader knows from experience, it can be hard to get to the bottom of what exactly is being debated. One reason for this lack of clarity is often that different people care about different aspects of the problem. In the absence of agreement concerning the problem, it can be difficult to reach an agreement about the solutions. In this article we discuss a technical matter that has spurred vigorous debate in recent years: How to define interactions among Web services to support operations on state (that is, data values associated with a service that persist across interactions, so that the result of one operation can depend on prior ones). 4 An airline reservation system and a scheduler of computational jobs are two examples of systems with this requirement. Both must provide their clients with access to information about ongoing activities: reservations and jobs, respectively. Clients typically want to name and/or identify state (refer to a specific reservation or job), access that state (get the status of a flight reservation or the execution progress of a job), modify part of that state (change the departure time of a flight or set the CPU requirements of a job), and destroy it (cancel a reservation or kill a job).
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