here - Stefan-Marr.de

5.2. Design of the OMOP
metaclass-based model as foundation for the implementation of these concurrent
programming concepts, the used library would only be usable for a
single concurrent programming concept, because it typically has one fixed
metaclass.
Message-reification-based models can be more powerful than metaclassbased
models, but are best applied to languages designed with an everythingis-a-message-send
approach. This is however an impractical constraint for multilanguage
VMs.
Hence, a better foundation for a meta interface is a metaobject-based model
that fulfills all requirements identified in Sec. 3.4. For the problems considered
in this dissertation, a proxy approach adds undesired complexity. Specifically,
the constraint that every client needs to use the correct proxy imposes a high
burden on the correct construction of the system, since the actual object reference
can easily leak. Furthermore, the added flexibility is not necessary to
satisfy the identified requirements, and the added overhead of an additional
proxy per object might provoke an undesirable performance impact.
To conclude, a metaobject-based approach that allows one metaobject to describe
the semantics for a set of objects similar to the group-based approaches
provides an suitable foundation.
5.2. Design of the OMOP
Following the stated requirements (cf. Tab. 3.7) for the support of concurrent
programming concepts, a unifying substrate needs to support the intercession
of state access and execution, provide the notion of ownership at the
level of objects, 4 and enable control over when the desired guarantees are enforced.
As argued in the previous section, metaobject-based models for MOPs
provide a promising foundation to design a minimal meta interface for the
purpose of this dissertation. Note that the goal of this chapter is to design
a unifying substrate for a multi-language VM. The main focus is to improve
support for language semantics of concurrent programming concepts. Therefore,
aspects such as security, reliability, distribution, and fault-tolerance are
outside the scope for the design of this substrate.
To satisfy the requirement of representing ownership at the granularity
of objects, the major element of the OMOP is the notion of a concurrency
domain. This domain enables the definition of language behavior similar to
metaobjects or metaclasses in other MOPs. The language behavior it defines
4 Our notion of ownership does not refer to the concept of ownership types (cf. Sec. 5.6).
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