here - Stefan-Marr.de

3. Which Concepts for Concurrent and Parallel Progr. does a VM need to Support?
of the different actor properties are. The analysis in this dissertation concentrates
on the issues that are relevant for multi-language VMs and disregards
problems that are relevant for distributed systems only. The relevant actor
properties for this discussion are insufficient guarantees with regard to isolation
of state as well as with regard to scheduling guarantees. While Karmani
et al. concentrated on actor-oriented frameworks, the analysis in this section
shows that these actor properties are relevant for a wider range of concurrency
abstractions.
In addition to these problems, the analysis in this section includes previously
discussed issues related to immutability and reflection [Marr et al., 2011a].
Supporting reliable immutability and enabling reflection while maintaining
concurrency-related language guarantees lead to complex implementations of
concurrency abstractions. This section demonstrates with examples that language
implementers face a number of significant problems when they try to
realize such concepts on top of today’s VMs. Therefore, these problems need
to be considered in order to determine the requirements for multi-language
VMs with extended support for concurrent programming.
3.3.2. Isolation
Isolation, often also referred to as encapsulation, is a valuable property that enables
local reasoning about the effects of operations since it clearly separates
two entities from each other and enforces the use of explicit interfaces for
interaction. It is of high relevance, because many concurrent programming
concepts rely on strong isolation between entities in order to restrict the set
of directly mutable entities and thus, limit the number of potential data races
in a concurrent system. Karmani et al. make a distinction between state encapsulation
and safe messaging to clarify the underlying problems.
State Encapsulation Integrating the actor model correctly with languages
that have mutable state, e. g., conventional object-oriented languages, is a challenge.
Sharing mutable state between actors violates the actor model, because
communication and interaction are supposed to be based on message-based
communication only. Therefore, mutable state has to be owned by a single
actor and any form of access needs to be restricted to its owner.
Semantic Aspects See Lst. 3.1 for the example Karmani et al. use to illustrate
the problem. The depicted Scala code is supposed to implement a basic
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