Provides efficient cache systems in a network service that allows for different expressions of requests with the same meaning, such as in a web work service using XML. The invention can be implemented as a network service system comprising a server and a client connected via a network. In this system, the client transmits a processing request that includes a certain canonicalized XML document to the server, and the server returns a response to the processing request while the server processes the XML document included in the processing request and caches the content of the response by using the processed XML document as a key.