Observing Transaction-Time Semantics with TTXPath

Transaction time is the time of database transactions that create, modify, or destroy facts. It is used to record when facts exist in a database. Accounting for transaction time is essential to supporting audit queries that delve into past database states and differential queries that pinpoint differences between two states. In a web context, transaction time is a problematic concept because there are no transactions. Browsers and other consumers of web data can observe snapshots of resources like XML documents but are rarely active participants in their creation or destruction. This paper presents the TTXPath data model and query language. TTXPath extends XPath with support for transaction time. XPath is a specification language for locations in an XML document. It serves as the basis for XML query languages like XSLT and XQuery. XPath has no temporal semantics. To construct a TTXPath data model, snapshots of an XML document are obtained over time by an observer. The snapshots are then merged and transaction times are associated with each edge and node. The TTXPath query language extends XPath with a transaction-time axis to enable a query to access past or future states, and with constructs to extract and compare times. TTXPath maximally reuses XPath hence the changes needed to support transaction time are minimal and TTXPath is fully backwardscompatible with XPath.