The architecture of a software system has a significant impact on its quality of service (QoS) as measured by several performance metrics such as execution time, availability, throughput, and security. This paper presents a framework that is part of a large project called SASSY (Self-Architecting Software Systems), whose goal is to allow domain experts to specify the system requirements using a visual activity-based language. The SASSY framework automatically generates a base architecture that corresponds to the requirements. Then SASSY generates a new architecture, derived from the base architecture, that optimizes a utility function for the entire system. The utility function is a multivariate function of several QoS metrics. The paper shows a complete example and illustrates how SASSY automatically adapts to changes in the environment's QoS features.