service-centric softwar e systems

N o v e m b e r / D e c e m b e r 2 0 0 7 I E E E S O F T W A R E 3 9 We define a flexible process as one that can change its behavior dynamically according to variable execution contexts; an adaptive process is one that can execute a service when conditions at runtime differ from those assumed during the service’s initial design. While researchers have proposed several adaptation mechanisms, none of the existing frameworks systematically couples adaptation design-time and runtime execution. Focusing only on design-time issues reduces an application’s ability to adapt its behavior at runtime. In a loosely coupled environment, in fact, the actual execution context might differ dramatically from the conditions hypothesized during application design. In addition, implementing advanced process-execution mechanisms, such as service substitution, is possible only when processes and services are carefully designed in advance. To address this, we developed PAWS (Processes with Adaptive Web Services), a framework for flexible and adaptive execution of managed service-based processes. Our framework coherently supports both process design and execution. It also integrates several research results developed at Politecnico di Milano that address different aspects of adaptation, coupling design-time and runtime mechanisms in a global environment. We have two primary goals for PAWS. First, we want it be self-optimizing. PAWS should select the best available services for executing the process and define the most appropriate quality-of-service (QoS) levels for delivering them. Second, PAWS should guarantee service provisioning, even in case of failures, through recovery actions and self-adaptation if the context changes. To meet these goals, PAWS provides methods and a toolset to support design-time specification of all information required for automatic runtime adaptation of focus