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---+ Overview http://www.w3.org/XML/Schema XML Schema is used to assess the validity of XML documents. 2 parts: 1 local validity: is each element valid? 1 overall assessment: are each element and all of its descendants valid? Schemas are composed of 13 types of schema components in 3 groups (primary, secondary, helper). * Primary * simple type def * complex type def * attribute def * element def * Secondary * attribute group def * id-constraint def * model group def * notation declarations * Helper (these depend on context) * annotations * model groups * particles * wildcards * attribute uses *Definition Components* define internal schema components. *Declaration Components* are associated by name to info items being validated. A schema document has a *target namespace* (which can be absent). It's the namespace that the schema validates, i.e. it's the namespace that documents must use if they want to be validated by that schema. ---+ Type Definitions Type definitions can be simple or complex. They are hierarchical, and the root of the hierarchy is =anyType=, also called the "ur-type." All other types are extensions or restrictions of some other type. *Simple types* are constraints on strings, i.e. attribute values or text-only elements with no children. They're always a retriction of =anySimpleType=. Simple types can also be lists of other simple types or unions of other simple types. An item of a type that is a restriction of another type "is a" instance of both types. A *Complex type* is a set of attribute definitions and a content type. It can be a restriction of a complex type, or an extension of a simple or complex type, or a restriction of the ur-type. If it extends another complex type it does so by "adding additional content model particles at the end of the other definition's content model, or by having additional attribute declarations, or both."
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