  QoS-aware applications can satisfy not only the functional requirements of
the customers, but also the QoS requirements. QoS-aware Web Service
orchestration translates the QoS requirements of the customers into those of
its component Web Services. In a system viewpoint, we discuss issues on
QoS-aware Web Service orchestration and design a typical QoS-aware Web Service
orchestration engine called QoS-WSOE. More importantly, we establish a formal
model of QoS-WSOE based on actor systems theory. Within the formal model, we
use a three-layered pyramidal structure to capture the requirements of the
customers with a concept named QoS-Aware WSO Service, characteristics of
QoS-WSOE with a concept named QoS-Aware WSO System, and structures and
behaviors of QoS-WSOE with a concept named QoS-Aware WSO Behavior. Conclusions
showing that a system with QoS-Aware WSO Behavior is a QoS-Aware WSO System and
further can provide QoS-Aware WSO Service are drawn.
