XRPC: Interoperable and efficient distributed XQuery

We propose XRPC, a minimal XQuery extension that enables distributed yet efficient querying of heterogeneous XQuery data sources. XRPC enhances the existing concept of XQuery functions with the Remote Procedure Call (RPC) paradigm. By calling out of an XQuery for-loop to multiple destinations, and by calling functions that themselves perform XRPC calls, complex P2P communication patterns can be achieved. The XRPC extension is orthogonal to all XQuery features, including the XQuery Update Facility (XQUF). We provide formal semantics for XRPC that encompasses execution of both read-only and update queries. XRPCis also a network SOAP sub-protocol, that integrates seamlessly with web services and Service Oriented Architectures (SOA), and AJAX-based GUIs. A crucial feature of the protocol is bulk RPC, that allows remote execution of many different calls to the same procedure, using possibly a single network round-trip. The efficiency potential of XRPC is demonstrated via an open-source implementation in MonetDB/XQuery. We show, however, that XRPC is not system-specific: every XQuery data source can service XRPC calls using a wrapper. Since XQuery is a pure functional language, we can leverage techniques developed for functional query decomposition to rewrite data shipping queries into XRPC-based function shipping queries. Powerful distributed database techniques (such as semi-join optimizations) directly map on bulk RPC, opening up interesting future work opportunities.