To increase the total QoS (Quality of Service) provided by a system, run-time adaptation of application behavior to execution context is a promising way. This paper focuses on modeling of distributed applications in order to manage their QoS through a middleware. The model captures the information required to schedule application activities in order to improve QoS and share out resources. Since only applications hold the required knowledge for alternative execution or graceful degradation, they adapt themselves under the control of the middleware. To ease the design of applications, a virtual execution environment implements the proposed middleware and emulates the execution of applications from their model.