QoS provisioning with qContracts in web and multimedia servers

The advent of performance-critical services such as online brokerage and e-commerce, as well as QoS-sensitive services such as streaming multimedia, makes existing FIFO servers incapable of meeting application QoS requirements. Re-designing server code to support QoS provisioning, on the other hand, is costly and time-consuming. To remedy this problem, we propose a new QoS-provisioning approach that does not require modification of server and QoS code. We develop a middleware, called qContracts, that can be transparently interposed between the server process and the operating system to achieve performance differentiation and soft QoS guarantees. The middleware enables reuse of existing legacy software in QoS-sensitive contexts, and off-loads QoS management concerns from future real-time service programmers. As an example, we show how the Apache web server is endowed with QoS support using qContracts on UNIX. Experimental results show the efficacy of the middleware in achieving the contracted QoS, while imposing less than 1% overhead.