A method, apparatus and computer program 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 is one which has required state data for processing the client's request. A request is received for processing by a web service instance. Any identity data is extracted from the request. The identity data uniquely identifies the required state data and the identity of the target server that hosts that state data. It is then determined whether the target server identity matches that of the server that has received the client's request and responsive to a negative determination, a routing failure is reported.