Parametric Performance Contracts for QML-specified Software Components

The performance of a software component heavily depends on the environment of the component. As a software component only justifies its investment when deployed in several environments, one can not specify the performance of a component as a constant (e.g., as a single value or distribution of values in its interface). Hence, classical component contracts allowing to state the component's performance as a post-condition, if the environment realises a specific performance stated in the precondition, do not help. This fixed pair of pre- and postcondition do not model that a component can have very different performance figures depending on its context. Instead of that, parametric contracts are needed for specifying the environmental dependency of the component's provided performance. In this paper we discuss the specification of dependencies of external calls for the performance metric response time. We present an approach using parametric contracts to compute the statistical distribution of response time as a discrete distribution in dependency of the distribution of response times of environmental services. We use the Quality of Service Modeling Language (QML) as a syntax for specifying distributions.