Web based services are becoming increasingly prevalent. These web services are used by clients or other web services. A stage of development of web services has arrived at which performance guarantees and differentiation are expected by clients of these web services. On the other hand, these web services are hosted by web hosting/aggregator sites. The web service providers that build these web services also expect certain guarantees from such web hosting sites. Specifying, measuring and guaranteeing in real time such service level agreements is a non-trivial task. This paper describes an adaptive control mechanism based on real-time measured data for guaranteeing such service level agreements dynamically. 17 Pages