Traditional database servers try to accommodate requests as fast as possible, which can be described as a best effort approach. However, different access items may have different quality-of-service (QoS) requirements. Therefore, it is important to provide QoS strategies such as differentiated services to accommodate those differences. In this paper we propose the QoS-Broker, which is a middleware for delivering QoS over database servers and applications, a modular and extensible architecture to support contracts over varied targets from queries to transactions or sessions and any desired QoS policy. Our experimental results illustrate the middleware by applying priority-based differentiation over terminals and over transactions on the TPCC benchmark.