A Meta - Level Architecture for Prototyping Object Systems

As applications become larger and more complex, it is frequently the case that system components require varying models of computation. The use of different computational models is not well supported by standard object-oriented mechanisms and systems. Typical mechanisms implicitly encapsulate meta-level (i.e., computational) semantics along with the base-level (i.e., domain) behaviour. Objects defined using one model cannot easily be executed under another and so cannot be reused. A major problem is the inclusion of base-level language constructs in the meta-level architecture design. Meta-levels typically only facilitate concepts which are similar to those in the original base-level language and so cannot describe widely differing models of execution. We present a meta-level architecture founded on the novel principle of fine-grained, operational decomposition of the meta-level into objects. Unlike others, our approach bases the design of the architecture on the operations which occur during object execution (e.g., send, lookup) rather than the structural nature of an object’s representation (e.g., class, method). This clearly separates the elements of the meta-level from those of the base-level language and so opens the meta-level to more radical change. The power of this approach is shown via several markedly different object models and their combination and non-intrusive application to user code. We detail how computational domains are completely altered with almost no modification of the original application code or its semantics. This capability is applied to real-world problems in object reuse, object behaviour investigation and in novel application design. Detailed examples relating to distributed computing and object communication are presented. It is shown that this approach to meta-level design is more open and flexible, and better supports the application of common software engineering practices (e.g., encapsulation and reuse) to the components of the meta-level — Properties desired by anyone designing complex systems.