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XML appliances reside between senders and receivers of information. Using Web services-based service-oriented architecture (SOA), a caller of a service does so using specifically formulated XML documents. A commerce Web site, for example, built on SOA principles, might use a third-party credit card validation service and a centralized shipping service. Under an SOA, a business decision such as moving toward decentralized shipping distribution centers (e.g., country-based distribution points) should only require a change in message routing; sending requests to a remote shipping service instead of the local, centralized one.
By communicating via an intermediate device rather than directly, the senders and receivers become insulated from extraneous details and largely immune to localized changes. This creates a loose coupling between the communicating parties, which is desirable in an SOA. Applications comprised of collections of loosely coupled services are simpler to design, maintain and, most importantly,...