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2.4. Common Approaches to Concurrent and Parallel Programming
The observations for concurrent programming concepts are different. Conventional
approaches based on mutexes require sequential execution of activities
that work on a shared resource in order to enforce correctness. Programs
using these concepts typically consist of multiple activities that have different
purposes but require a common resource. In such systems, interactions are
resource-centric, without a common purpose, and require that the invariants
for the shared resources hold. Reader/writer locks for instance exclude only
conflicting operations from using the same resource. Software transactional
memory (cf. Sec. 2.4.3) goes further by removing the need for managing resources
manually. In contrast, event-loop concurrency models (cf. Sec. 2.4.4)
promote resources to active entities that are responsible for managing their
consistency and allow clients to interact via an asynchronous interface only.
Thus, the main commonality of these concepts is the protection of invariants
of shared resources to guarantee correctness, while permitting interaction of
independent activities at runtime.
The following section discusses these approaches in more detail.
2.3.3. Conclusion
To conclude, the definitions of concurrency and parallelism as found in the
literature can be inappropriate when it comes to categorizing concepts.
In contrast, the alternative definitions for concurrent programming and parallel
programming given here categorize concepts in two disjoint sets. Instead of
focusing on the execution model, these definitions focus on the aspect of programming
and relate to the intent and goal of a programming concept. Therefore,
concurrent programming concepts coordinate modifications of shared
resources, while parallel programming concepts coordinate parallel activities
to compute a common result.
2.4. Common Approaches to Concurrent and Parallel
Programming
This section gives an overview of common concepts in the field of concurrent
and parallel programming to provide a foundation for our later discussions.
Note that this section introduces Clojure agents, which are used in later chapters
as running examples and therefore discussed here in more detail.
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