Web service infrastrutures usually are comprised of front-end Web servers that accept requests and process them, and backend servers that manage data and services. Current Web servers use various API sets to access backend services. This model does not support service differentiation, overload control, caching of contents generated by backend servers. We have proposed a framework for using service brokers to facilitate these features. Service brokers are software agents that are the access points to backend services in Web servers. Unlike the current API-based scheme where accesses to backend services are through stateless and isolated APIs, in service broker framework, they are undertaken by passing messages to service brokers who gather all the requests and intelligently process them. We have prototyped this framework and validated its function in providing request clustering and service differentiation in accessing backend services. In addition, the performance in terms of the processing time is enhanced by this approach.