1. A method for validating that a client's request has been routed to an appropriate server hosting a specific stateful web service instance in a system comprising a plurality of stateful web service instances, the specific stateful web service instance having required state data for processing the client's request, the method comprising; receiving a request for processing by a web service instance; extracting any identity data from the request, the identity data uniquely identifying the required state data and the identity of the target server that hosts the state data; determining whether the target server identity matches that of the server that has received the client's request; and responsive to a negative determination, reporting a routing failure.