Central notions: CORBA compliance but amplified by mechanisms useful for attaining semantic interoperability of objects, and for composing such objects into what appear to be seamlessly interoperating sets of distributed and parallel services. Our work currently focuses on selected semantic information useful for attaining high performance for such collections of objects.
The key mechanism for capturing object semantics is similar to the one used in real-time objects, called `attributes'. The COBS attribute infrastructure permits the association of attributes with object classes, instance, and with individual invocations, so that they may be used to configure not just objects but also the infrastructure used by them. The particular `vertical' configuration (across layers of abstraction capable of interpreting attributes) we are planning to perform is one in which application-level objects configure communication protocol characteristics, including quality of service and security parameters. Additional methods used for dynamic object configuration include caching (object caching at the level of the object system itself), object fragmentation (for objects implementing shared `memory', as well as for objects implementing event services), and object activity (i.e., the association of events with objects, resulting in active objects). In summary, object and invocation representations may differ significantly even across multiple instances of single object classes, and such representations may be changed at runtime using attributes.
Our current work supports both (1) large-grain, CORBA-compliant objects using the Fresco toolkit (and we are now exploring alternative implementations using SGI's OCS object infrastructure developed as part of their Orlando field trial), and (2) other objects with lower granularities implementing specific kinds of abstractions, including the realization of efficient distributed event services and distributed shared memory, accessible via runtime libraries.
CORBA-compliant language support for COBS objects, for attributes, and for the association of different object representations with classes is provided via IDL and by using `configuration objects', which may be associated with the objects being configured. Attributes, then, are a basic mechanism useful for `vertical' configuration, and their use with configuration objects permits end users to easily express desired configuration alternatives, policies, or algorithms. Attributes may also be used as additional parameters carried via invocations with which applications can express desired service levels, negotiate resource access with each other or via more general resource management methods, namely, perform what may be termed `horizontal' configuration actions.