Loop-lifted XQuery RPC with deterministic updates

XRPC is a minimal XQuery extension that enables distributed query execution, combining the Remote Procedure Call (RPC) paradigm with the existing concept of XQuery functions. By calling out of a for-loop to multiple destinations, and by calling functions that themselves perform XRPC calls, complex P2P communication patterns can be achieved. We further propose the use of SOAP as the protocol for XRPC, which allows seamless integration with web services and Service Oriented Architectures (SOA). XRPC is implemented in the open source MonetDB/XQuery system. We show that the technique of loop-lifting, that executes all expressions inside a for-loop in a single bulk operator -- pervasively applied in MonetDB/XQuery to obtain efficient relational query plans -- also benefits XRPC. Loop-lifting enables us to send bulk RPC requests, dramatically reducing the number of SOAP messages, and thus the performance impact of network latency. The XRPC extension is orthogonal to all XQuery language features, including the XQuery Update Facility (XUF). The XUF W3C Draft proposal does not define the order in which multiple update actions to the same node must be applied. We instead choose to make this order deterministic, and show how distributed updates can be made deterministic using a small protocol extension.