The W3C XML Schema recommendation defines the structure and data types for XML documents, but lacks explicit support for time-varying XML documents or for a time-varying schema. In previous work we introduced τ XSchema which is an infrastructure and suite of tools to support the creation and validation of time-varying documents, without requiring any changes to XML Schema. In this paper we extend τ XSchema to support versioning of the schema itself. We introduce the concept of a bundle, which is an XML document that references a base (non-temporal) schema, temporal annotations describing how the document can change, and physical annotations describing where timestamps are placed. When the schema is versioned, the base schema and temporal and physical schemas can themselves be time-varying documents, each with their own (possibly versioned) schemas. We describe how the validator can be extended to validate documents in this seeming precarious situation of data that changes over time, while its schema and even its representation are also changing.

Abstract. The W3C XML Schema recommendation defines the structure and data types for XML documents. XML Schema lacks explicit support for timevarying XML documents. Users have to resort to ad hoc, non-standard mechanisms to create schemas for time-varying XML documents. This paper presents a data model and architecture, called τXSchema, for creating a temporal schema from a non-temporal (snapshot) schema, a temporal annotation, and a physical annotation. The annotations specify which portion(s) of an XML document can vary over time, how the document can change, and where timestamps should be placed. The advantage of using annotations to denote the time-varying aspects is that logical and physical data independence for temporal schemas can be achieved while remaining fully compatible with both existing XML Schema documents and the XML Schema recommendation.

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Abstract-If past versions of XML documents are retained, what of the various integrity constraints defined in XML Schema on those documents? This paper describes how to interpret such constraints as sequenced constraints, applicable at each point in time. We also consider how to add new variants that apply across time, so-called non-sequenced constraints. Our approach supports temporal documents that vary over both valid and transaction time, whose schema can vary over transaction time. We do this by replacing the schema with a (possibly time-varying) temporal schema and replacing the document with a temporal document, both of which are upward compatible with conventional XML and with conventional tools like XMLLINT, which we have extended to support the temporal constraints introduced here.

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