Service-centric systems exist in a very dynamic environment. This requires these systems to adapt at runtime in order to keep fulfilling their QoS. In order to create self-adaptive service systems, developers should not only design the service architecture, but also need to design the self-adaptability aspects in a structured way. A key aspect in creating these self-adaptive service systems is modeling runtime variability properties. In this paper, we propose DySOA (Dynamic Service-Oriented Architecture), an architecture that extends service-centric applications to make them self-adaptive. DySOA allows developers to explicitly model elements that deal with QoS evaluation and variable composition configurations. Having the DySOA elements explicit enables separation of concerns, making them adaptable at runtime and reusable in next versions. We demonstrate the use of DySOA with an example.