Quality of service control by middleware

Assuring a negotiated quality of service for a client is an important matter of availability. In value added telecommunication servers quality of service control mainly means preferring high priority applications in situations of resource shortages at the expense of applications with lower priority. Middleware that shall host arbitrary, heterogeneous services, which in turn are designed to serve heterogeneous applications, needs centralized control mechanisms for assignment of limited resources to services and applications. It must be explicitly emphasized, that every kind of centralized resource control itself consumes resources, which consequently reduces the system's performance. The challenge of the middleware is to find a balance between needed resource control and optimized performance. On the other side intelligent clients shall deliberately select the transport medium to access the server that hosts their needed services, dependent on the priority and reliability requirements of the actual message. The present contribution discusses several alternatives how servers can carry out quality of service control, bearing in mind that some alternatives must be provided in parallel because different clients have different needs.