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5.6. Related Work
In ABCL/R2, meta-groups coordinate for the management of system resources,
including scheduling. To overcome limitations of the metaobject and
the meta-group approaches, both are combined in ABCL/R2. Similarly to
the OMOP, ABCL/R2 also restricts each object to be member of one specific
group, and thus has a strong similarity with the notion of domains. However,
since the purpose of these metaobject protocols is different, they do not
offer the same intercession handlers the OMOP provides. Hence, neither of
the discussed group-based approaches fulfills the requirements identified in
Sec. 3.4.
Terminology While the notion of meta-groups is very close to the notion
of domains, this dissertation differentiates between both terms to make it
more explicit that objects have a single owner, i. e., a domain. For groups,
this is not necessarily the case. Other differences in the MOPs stem from the
differences in the intended use case. The MOPs could be extended to include
the intercession points provided by the OMOP, however the OMOP design
minimizes the MOP to provide the necessary intercession points only.
Distinguishing Properties of the OMOP The main distinguishing characteristics
of the OMOP are the combination of the specific set of intercession
handlers, the notion of ownership, and the integration with the notion of concurrent
execution, i. e., the inclusion of threads as a relevant concern for the
metaobject protocol. This combination is to our knowledge novel and based
on the previously identified requirements (cf. Sec. 3.4).
For example, it would be possible to achieve similar results by using the
meta-group-based MOPs of ABCL/R2 or Mitchell et al. [1997] and combine
them with the proposed intercession handlers for state access, execution, and
primitives. Since Mitchell et al. [1997] also include the notion of threads in
the MOP, all key elements of the OMOP are present and it could satisfy the
requirements. However, on their own, without the proposed intercession handlers,
neither of the two MOPs would cover all the required aspects for a
multi-language environment, since for instance ABCL/R2 focuses on message
sends between objects.
Unrelated Work: Ownership Types Clarke et al. [1998] proposed the notion
of ownership types to tackle the problem of encapsulation in object-oriented
programming languages by enabling a developer to describe rules on how
and when pointers to objects can escape or be changed from an object that
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