Safe ‘Composability’

I n the coming decade, distributed computing will establish itself as the default mode of operation for a wide variety of applications. With the proliferation of mobile devices and ubiquitous computing environments today, applications are beginning to place specialized requirements on the computing and communication infrastructure. Interactive applications, including personal multimedia services accessible through handheld devices, will become commonplace. These applications require continuous service quality despite frequent interruptions in network connectivity and degradation of device power levels. Additional requirements, including secure communication in mobile environments, further complicate system management. Reflective middleware promises dynamic customizability to satisfy application requirements, including quality-of-service (QoS), security, mobility, and fault tolerance, in the presence of changing system conditions. However, a number of technical challenges complicate deployment of safe plug-and-play middleware environments where protocols and services are installed and uninstalled on the fly to accommodate dynamicity. Distributed applications have varying requirements, often stated as QoS parameters defining the extent of violation of performance specifications, including responsiveness, reliability, availability, cost-effective utilization, and security. These requirements are satisfied through appropriate resource management policies, such as replication, migration, and checkpointing. Satisfying application QoS in a dynamic environment implies the dynamic customization of the underlying management mechanisms to meet QoS requirements. The safe “composability,” or simultaneous operation, of the executing middleware entities is a prime concern. Fundamental problems arise in the concurrent execution of multiple distributed resource management services and protocols; further complications arise in the dynamic customization of these services and protocols. The changing, often complex, interactions among distributed policies and services within the middleware layer can violate correctness, destroy validity, and alter the semantics of applications built above generic distributed management infrastructures. Much of the need to ensure safe interactions between middleware services has been ignored in existing commercial frameworks and distributed middleware infrastructures. Frameworks like the Object Management Group’s Common Object Request Broker (CORBA) and Microsoft’s Distributed Component Object Model (DCOM) represent a step toward compositional software architectures but do not deal with the interactions of multiple object services executing at the same time or with the implication of having to compose object services. This article explores principles and practices for developing safe customizable middleware, specifiLook to composable middleware frameworks to ensure safe middleware interactions for ubiquitous computing applications. Safe ‘Composability’ of Middleware Services Nalini Venkatasubramanian