With the prevalence of X-as-a-Service (e.g., software as a service, platform as a service, infrastructure as a service, etc.) and users' growing demand on good services, QoS (Quality of Service) assurance is becoming increasingly important to service delivery. Traditional service delivery mainly focuses on function or information provisioning, and does not give high priority to quality assurance. In this paper, we tackle the QoS assurance problem in a systematic way, from model to system. We first decompose traditional services into three components - namely software application, data and resource, then define models for these three kinds of basic services, and propose a set of operations for service publishing and composition. To illustrate our approach, we present a prototype system, the Platform as a Service (PaaS) system, which is developed in support of our framework and shows how QoS can be ensured through real-time monitoring and dynamic scaling (up or down).