Asynchrony (computer programming): Difference between revisions

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{{Short description|Computer programming technique}}

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'''Asynchrony''', in [[computer programming]], refers to the occurrence of events independent of the main [[control flow|program flow]] and ways to deal with such events. These may be "outside" events such as the arrival of [[Unix signal|signals]], or actions instigated by a program that take place [[concurrent computing|concurrently]] with program execution, without the program [[hang (computing)|blockinghanging]] to wait for results.<ref name="davies">{{cite book |title=Async in C# 5.0 |first=Alex |last=Davies |publisher=O'Reilly |year=2012 |pages=1–2 |isbn=9781449337124 |url=https://books.google.com/books?id=xT45qhFrVnUC}}</ref> [[Asynchronous I/O|Asynchronous input/output]] is an example of the latter case of asynchrony, and lets programs issue commands to storage or network devices that service these requests while the [[Central processing unit|processor]] continues executing the program. Doing so provides a degree of [[parallel computingConcurrency_(computer_science)|parallelismconcurrency]].{{r|davies}}

A common way for dealing with asynchrony in a [[application programming interface|programming interface]] is to provide [[subroutine]]s that return a [[futures and promises|future or promise]] that represents the ongoing operation, and a synchronizing operation that [[Blocking (computing)|blocks]] until the future or promise is completed. Some programming languages, such as [[Cilk]], have special syntax for expressing an asynchronous procedure call.<ref name="spp">{{cite book |first1=Michael |last1=McCool |first2=James |last2=Reinders |first3=Arch |last3=Robison |title=Structured Parallel Programming: Patterns for Efficient Computation |publisher=Elsevier |year=2013 |pages=30}}</ref>

Examples of asynchrony include the following:

* [[Asynchronous procedure call]], a method to run a procedure concurrently, a lightweight alternative to [[Thread_(computing)|Threadsthreads]].

* [[Ajax (programming)|Ajax]] is a set of [[client-side]] [[web development|web technologies]] used onby the client to create asynchronous I/O [[web application]]s .

* '''Asynchronous method dispatch''' (AMD), a [[data communication]] method used when there is a need for the server side to handle a large number of long lasting client requests.<ref>[https://web.archive.org/web/20130721031512/http://zeroc.com/doc/Ice-3.3.0/manual/Overview.3.2.html ICE usage of AMD].</ref> Using synchronous method dispatch (SMD), this scenario may turn the server into an unavailable busy state resulting in a connection failure response caused by a [[Computer network|network]] connection request [[Timeout (computing)|timeout]]. The servicing of a client request is immediately dispatched to an available [[thread (computer science)|thread]] from a [[Thread pool pattern|pool of threads]] and the client is put in a blocking state. Upon the completion of the task, the server is notified by a callback. The server unblocks the client and transmits the response back to the client. In case of thread [[Starvation_(computer_science)|starvation]], clients are blocked waiting for threads to become available.

==See also==

{{Portal|Computer programming}}

* [[Asynchronous system]]

* [[Asynchronous circuit]]

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