Structured concurrency: Difference between revisions

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{{Short description|Programming paradigm for improving clarity and development time of a computer program}}

{{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]].

'''StructuredThe concurrency'''core concept is athe [[programmingencapsulation paradigm]]of aimedconcurrent atthreads improvingof theexecution clarity,(here quality,encompassing kernel and developmentuserland timethreads ofand a [[computer program]]processes) by usingway aof structuredcontrol approachflow toconstructs [[concurrentthat computing|concurrenthave programming]].clear Theentry coreand conceptexit ispoints and that whenensure controlall splitsspawned intothreads concurrenthave taskscompleted thatbefore theyexit. joinSuch upencapsulation again,allows thaterrors anyin taskconcurrent errorthreads isto be propagated to the spawningcontrol structure's parent scope (onceand managed by the task'snative childrenerror andhandling siblingmechanisms tasksof haveeach completed),particular andcomputer thatlanguage. this It allows control flow isto remain readily evident by the structure of the source code despite the presence of concurrency. To be effective, this model must be applied consistently throughout all levels of the program-- – otherwise concurrent tasksthreads may leak out, become orphaned, or fail to have runtime errors correctly propagated.

Structured concurrency is analogous to [[structured programming]], which uses control flow constructs that encapsulate sequential statements and subroutines.

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).

== History ==

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>

The [[fork–join model]] from the 1960s, embodied by multiprocessing tools like [[OpenMP]], is an early example of a system ensuring all threads have completed before exit. However, Smith argues that this model is not true structured concurrency as the programming language is unaware of the joining behavior, and is thus unable to enforce safety.<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 |access-date=1 August 2019}}</ref>

The concept was formulated in 2016 by Martin Sústrik (creatora developer of [[ZeroMQ]]) with his C library libdill, with [[Go_(programming_language)#Concurrency:_goroutines_and_channels|goroutines]] as a starting point.<ref>{{cite web |last1=Sústrik |first1=Martin |title=Structured Concurrency |url=http://250bpm.com/blog:71 |date=7 February 2016 |accessdateaccess-date=1 August 2019}}</ref>, andIt thenwas further refined in 20182017 by Nathaniel J. Smith, who implementedintroduced ita "nursery pattern" in his [https://trio.readthedocs.io/en/stable/[Python Trio(programming language)|Python]] implementation called Trio.<ref>{{cite web |last1=Smith |first1=Nathaniel J. |title=NotesAnnouncing on structured concurrency, or: Go statement considered harmfulTrio |url=https://vorpus.org/blog/notesannouncing-on-structured-concurrency-or-go-statement-considered-harmfultrio/ |date=2510 AprilMarch 20182017 |accessdateaccess-date=123 AugustSeptember 20192022}}</ref> Meanwhile, Roman Elizarov independently came upon the same ideas while developing an experimental [[coroutine]] library for the [[Kotlin (programming language.)|Kotlin language]],<ref>{{cite web |last1=Elizarov |first1=Roman |title=Structured concurrency |url=https://medium.com/@elizarov/structured-concurrency-722d765aa952 |date=12 September 2018 |accessdateaccess-date=21 September 2019}}</ref><ref>{{cite AV media |people=Elizarov, Roman |date=11 July 2019 |title=Structured concurrency |medium=VideotapeVideo |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> which later became a standard library.<ref>{{cite web | url=https://kotlinlang.org/docs/coroutines-basics.html#structured-concurrency | title=Coroutines basics: Structured concurrency | website=Kotlin | publisher=JetBrains | access-date=3 March 2022}}</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.

In 2021, [[Swift (programming language)|Swift]] adopted structured concurrency.<ref>{{cite web |first1=John |last1=McCall |first2=Joe |last2=Groff |first3=Doug |last3=Gregor |first4=Konrad |last4=Malawski |access-date=3 March 2022 |title=Swift Structured Concurrency Proposal |website=Apple's Swift Evolution repo |publisher=GitHub |url=https://github.com/apple/swift-evolution/blob/main/proposals/0304-structured-concurrency.md}}</ref> Later that year, a draft proposal was published to add structured concurrency to [[Java (programming language)|Java]].<ref>{{cite web |last1=Pressler |first1=Ron |website=[[OpenJDK]] |publisher=Oracle |access-date=3 March 2022 |title=JEP draft: Structured Concurrency (Incubator) |url=https://openjdk.java.net/jeps/8277129}}</ref>

==See also==

== Variations ==

A major point of variation is how an error in one member of a concurrent setthread of taskstree is handled. Simple implementations will merely wait until the children and siblings of the failing taskthread 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 tasksthreads in an orderlyexpedient manner.

== See also ==

* [[Structured programming]]

== References ==

=== Citations ===

{{Reflist}}

=== External links===

* [https://vorpus.org/blog/notes-on-structured-concurrency-or-go-statement-considered-harmful/ Notes on structured concurrency, or: Go statement considered harmful] by Nathaniel J. Smith

* [https://wiki.openjdk.java.net/display/loom/Structured+Concurrency Structured Concurrency], Alan Bateman, [[OpenJDK]] wiki

* [https://trio.discourse.group/c/structured-concurrency Structured concurrency forum], cross-computer-language discussion of structured concurrency with participation by Sústrik, Smith, and Elizarov

* [https://archive.fosdem.org/2019/schedule/event/structured_concurrency/ FOSDEM 2019: Structured Concurrency], lightning talk by Martin Sustrik with links to some implementations

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