Towards Composable Concurrency Abstractions

TL;DR

This paper investigates how different concurrency abstractions in programming languages, especially Clojure, interact and whether they can be combined safely to support complex, multi-faceted applications.

Contribution

It provides an in-depth analysis of the composability of concurrency models in Clojure and identifies properties that affect their integration.

Findings

Some concurrency abstractions compose without issues

Certain combinations lead to subtle bugs or inconsistencies

General properties influencing composability are identified

Abstract

In the past decades, many different programming models for managing concurrency in applications have been proposed, such as the actor model, Communicating Sequential Processes, and Software Transactional Memory. The ubiquity of multi-core processors has made harnessing concurrency even more important. We observe that modern languages, such as Scala, Clojure, or F#, provide not one, but multiple concurrency models that help developers manage concurrency. Large end-user applications are rarely built using just a single concurrency model. Programmers need to manage a responsive UI, deal with file or network I/O, asynchronous workflows, and shared resources. Different concurrency models facilitate different requirements. This raises the issue of how these concurrency models interact, and whether they are composable. After all, combining different concurrency models may lead to subtle bugs or inconsistencies. In this paper, we perform an in-depth study of the concurrency abstractions provided by the Clojure language. We study all pairwise combinations of the abstractions, noting which ones compose without issues, and which do not. We make an attempt to abstract from the specifics of Clojure, identifying the general properties of concurrency models that facilitate or hinder composition.