This paper proposes a collaborative runtime monitoring framework to enhance the dependability of the software developed in traditional Web Services architecture. The enabling mechanism is an Active Service Broker (ASB) architecture which allows the service broker not only to serve as a passive service repository, but also to involve itself in service interactions and thus to play an active role in service execution and monitoring. The ASB communicates remotely with the distributed monitoring agents deployed at the service providers ' sites. Sensors are instrumented in services at different levels, including the composition level, interface level, and component level. The model-based approach is discussed for automatic sensor generation and runtime enforcement based on service's process model and verification model. A prototype is implemented for the research of the user and science data center of the astronomic satellite system at Tsinghua University.