We present XPi, a core calculus for XML messaging. XPi features asynchronous communications, pattern matching, name and code mobility, integration of static and dynamic typing. Flexibility and expressiveness of this calculus are illustrated by a few examples, some concerning description and discovery of web services. In XPi, a type system disciplines XML message handling at the level of channels, patterns, and processes. A run-time safety theorem ensures that in well-typed systems no service will ever receive documents it cannot understand, and that the offered services, even if re-defined, will be consistent with the declared channel capacities.
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