Structured concurrency: Difference between revisions

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{{Programming paradigms}}

'''Structured concurrency''' is a [[programming paradigm]] aimed at improving the clarity, quality, and development time of a [[computer program]] by using a structured approach to [[concurrent computing|concurrent programming]]. The core concept is that when control splits into concurrent tasks that they join up again, that subtaskany errorstask causeerror childrenis and siblingspropagated to bethe cancelledspawning beforescope propagating to(once the parenttask's children and sibling tasks have completed), and that this control flow is readily evident by the structure of the source code. If a "main task" splits into several concurrent sub-tasks then those sub-tasks must terminate before the main task can complete. To be effective, this model must be applied consistently throughout all levels of the program-- otherwise concurrent tasks may leak out, become orphaned, or fail to have runtime errors correctly propagated.

The naming is inspired by [[structured programming]], which created higher level control-flow statements out of the very basic [[goto]]. Similarly, structured concurrency creates higher level mechanisms for concurrency constructs out of the basic concurrent task creation constructs ([[Spawn (computing)|spawn]], [[Thread (computing)|thread]]s, [[Fiber (computer science)|fiber]]s).

The concept was formulated in 2016 by Martin Sústrik (creator of [[ZeroMQ]])<ref>{{cite web |last1=Sústrik |first1=Martin |title=Structured Concurrency |url=http://250bpm.com/blog:71 |date=7 February 2016 |accessdate=1 August 2019}}</ref>, and then further refined in 2018 by Nathaniel J. Smith, who implemented it in [https://trio.readthedocs.io/en/stable/ Trio].<ref>{{cite web |last1=Smith |first1=Nathaniel J. |title=Notes on structured concurrency, or: Go statement considered harmful |url=https://vorpus.org/blog/notes-on-structured-concurrency-or-go-statement-considered-harmful/ |date=25 April 2018 |accessdate=1 August 2019}}</ref> Meanwhile, Roman Elizarov independently came upon the same ideas while developing an experimental coroutine library for the Kotlin language.<ref>{{cite web |last1=Elizarov |first1=Roman |title=Structured concurrency |url=https://medium.com/@elizarov/structured-concurrency-722d765aa952 |date=12 September 2018 |accessdate=21 September 2019}}</ref><ref>{{cite AV media |people=Elizarov, Roman |date=July 2019 |title=Structured concurrency |medium=Videotape |language=en |url=https://youtube.com/watch?v=Mj5P47F6nJg&t=2538 |access-date=21 September 2019 |publisher=Hydra Distributed computing conference |minutes=42 |quote="We needed a name and we needed to finalize this whole concept [...] and we stumble onto this blog post [...] by Nathaniel J. Smith."}}</ref>

==Variations==

A major point of variation is how an error in one member of a concurrent set of tasks is handled. Simple implementations will merely wait until the children and siblings of the failing task run to completion before propagating the error to the parent scope. However, that could take an indefinite amount of time. The alternative is to employ a general cancellation mechanism (typically a cooperative scheme allowing program invariants to be honored) to terminate the children and sibling tasks in an orderly manner.

==See also==