View source for Push technology

{{Short description|Method of network communication where requests are sent by the publisher}}
{{tone|date=January 2019}}
'''Push technology,''' also known as '''server push,''' refers to a method of communication on the [[Internet]] where the initial request for a transaction is initiated by the [[Server (computing)|server]], rather than the [[Client (computing)|client]]. This approach is different from the more commonly known "[[Pull technology|pull]]" method, where information transmission is requested by the receiver or client.<ref>{{Cite web |date=2012-11-18 |title=Push Technology |url=https://www.techopedia.com/definition/5732/push-technology |access-date=2023-07-23 |website=Techopedia |language=en-US}}</ref>

In push technology, clients can express their preferences for certain types of information or data, typically through a process known as the [[Publish–subscribe pattern|publish–subscribe]] model. In this model, a client "subscribes" to specific information channels hosted by a server. When new content becomes available on these channels, the server automatically sends, or "pushes," this information to the subscribed client.

Under certain conditions, such as restrictive security policies that block incoming [[HTTP]] requests, push technology is sometimes simulated using a technique called [[Polling (computer science)|polling.]] In these cases, the client periodically checks with the server to see if new information is available, rather than receiving automatic updates.

== General use ==
[[Synchronous conferencing]] and [[instant messaging]] are examples of push services. Chat messages and sometimes [[Computer file|files]] are pushed to the user as soon as they are received by the messaging service. Both decentralized [[peer-to-peer]] programs (such as [[WASTE]]) and centralized programs (such as [[Internet Relay Chat|IRC]] or [[Extensible Messaging and Presence Protocol|XMPP]]) allow pushing files, which means the sender initiates the data transfer rather than the recipient.

[[Email]] may also be a push system: [[SMTP]] is a push protocol (see [[Push e-mail]]). However, the last step—from mail server to desktop computer—typically uses a pull protocol like [[POP3]] or [[IMAP]]. Modern e-mail clients make this step seem instantaneous by repeatedly [[polling (computer science)|polling]] the mail server, frequently checking it for new mail. The IMAP protocol includes the [[IMAP IDLE|IDLE]] command, which allows the server to tell the client when new messages arrive. The original [[BlackBerry]] was the first popular example of push-email in a wireless context.{{cn|date=September 2023}}

Another example is the [[PointCast (dotcom)|PointCast Network]], which was widely covered in the 1990s. It delivered news and stock market data as a screensaver. Both [[Netscape]] and [[Microsoft]] integrated push technology through the [[Channel Definition Format]] (CDF) into their software at the height of the [[browser wars]], but it was never very popular. CDF faded away and was removed from the browsers of the time, replaced in the 2000s with [[RSS]] (a pull system.)

Other uses of push-enabled [[web application]]s include software updates distribution ("push updates"), market data distribution (stock tickers), online chat/messaging systems ([[webchat]]), auctions, online betting and gaming, sport results, monitoring consoles, and [[sensor network]] monitoring.

== Examples ==
===Web push===

The Web push proposal of the [[Internet Engineering Task Force]] is a simple protocol using [[HTTP/2|HTTP version 2]] to deliver real-time events, such as incoming calls or messages, which can be delivered (or "pushed") in a timely fashion. The protocol consolidates all [[Real-time computing|real-time]] events into a single session which ensures more efficient use of network and radio resources. A single service consolidates all events, distributing those events to applications as they arrive. This requires just one session, avoiding duplicated overhead costs.<ref>{{Cite web |title= Generic Event Delivery Using HTTP Push |author= M. Thomson, E. Damaggio and B. Raymor |date= October 22, 2016 |work= Internet Draft |publisher= Internet Engineering Task Force |url= https://tools.ietf.org/html/draft-ietf-webpush-protocol-12 |access-date= October 28, 2016 }}</ref>

Web Notifications are part of the [[W3C]] standard and define an [[API]] for end-user notifications. A notification allows alerting the user of an event, such as the delivery of an email, outside the context of a web page.<ref>{{Cite web |title= Web Notifications |url= https://www.w3.org/TR/notifications/}}</ref> As part of this standard, Push API is fully implemented in [[Google Chrome|Chrome]], [[Firefox]], and [[Microsoft Edge|Edge]], and partially implemented in [[Safari (web browser)|Safari]] {{As of|2023|February|lc=y}}.<ref>{{Cite web |title= Web Push API |url= https://www.w3.org/TR/push-api/}}</ref><ref>{{Cite web |date=2023-02-22 |title=Push API - Web APIs {{!}} MDN |url=https://developer.mozilla.org/en-US/docs/Web/API/Push_API |access-date=2023-05-16 |website=developer.mozilla.org |language=en-US}}</ref>

=== HTTP server push ===
HTTP server push (also known as HTTP streaming) is a mechanism for sending unsolicited (asynchronous) data from a [[web server]] to a [[web browser]]. HTTP server push can be achieved through any of several mechanisms.

As a part of [[HTML5]] the [[WebSocket|Web Socket]] API allows a web server and client to communicate over a [[Duplex (telecommunications)|full-duplex]] TCP connection.

Generally, the web server does not terminate a connection after response data has been served to a client. The web server leaves the connection open so that if an event occurs (for example, a change in internal data which needs to be reported to one or multiple clients), it can be sent out immediately; otherwise, the event would have to be queued until the client's next request is received. Most web servers offer this functionality via [[Common Gateway Interface|CGI]] (e.g., Non-Parsed Headers scripts on [[Apache HTTP Server]]). The underlying mechanism for this approach is [[chunked transfer encoding]].

Another mechanism is related to a special [[MIME]] type called <code>[[x-mixed-replace|multipart/x-mixed-replace]]</code>, which was introduced by [[Netscape]] in 1995. Web browsers interpret this as a document that changes whenever the server pushes a new version to the client.<ref name=":0">[http://www.oreilly.com/openbook/cgi/ch06_06.html CGI Programming on the World Wide Web] O'Reilly book explaining how to use Netscape server-push</ref> It is still supported by [[Firefox]], [[Opera (web browser)|Opera]], and [[Safari (web browser)|Safari]] today, but it is ignored by [[Internet Explorer]]<ref>[http://victor.transformadora.com/Oreilly/wdesign/xhtml/ch13_03.htm Server-Push Documents (HTML & XHTML: The Definitive Guide)] {{webarchive|url=https://web.archive.org/web/20080417004248/http://victor.transformadora.com/Oreilly/wdesign/xhtml/ch13_03.htm |date=2008-04-17 }} O'Reilly book explaining server-push</ref> and is only partially supported by [[Google Chrome|Chrome]].<ref>[https://bugs.chromium.org/p/chromium/issues/detail?id=249132 Remove support for multipart/x-mixed-replace main resources]</ref> It can be applied to [[HTML]] documents, and also for streaming images in [[webcam]] applications.

The [[WHATWG]] Web Applications 1.0 proposal<ref>{{cite web|url=http://www.whatwg.org/specs/web-apps/current-work/#scs-server-sent|title=Web Applications 1.0 specification}}</ref> includes a mechanism to push content to the client. On September 1, 2006, the Opera web browser implemented this new experimental system in a feature called "[[Server-sent events|Server-Sent Events]]".<ref>{{cite web|url=http://my.opera.com/WebApplications/blog/show.dml/438711|access-date=2007-03-23|date=2006-09-01|title=Event Streaming to Web Browsers}}</ref><ref>{{cite web|url=http://operawatch.com/news/2006/09/opera-takes-the-lead-with-ajax-support-among-browsers-more-efficient-streaming.html|access-date=2007-03-23|date=2006-09-01|title=Opera takes the lead with AJAX support among browsers: More efficient streaming|url-status=dead|archive-url=https://web.archive.org/web/20070318013327/http://operawatch.com/news/2006/09/opera-takes-the-lead-with-ajax-support-among-browsers-more-efficient-streaming.html|archive-date=2007-03-18}}</ref> It is now part of the [[HTML5]] standard.<ref>{{Cite web |date=31 March 2022 |title=HTML Standard – Server-sent events |url=https://html.spec.whatwg.org/multipage/server-sent-events.html |access-date=1 April 2022 |website=html.spec.whatwg.org}}</ref>

=== Pushlet ===
In this technique, the server takes advantage of [[HTTP persistent connection|persistent HTTP connections]], leaving the response perpetually "open" (i.e., the server never terminates the response), effectively fooling the browser to remain in "loading" mode after the initial page load could be considered complete. The server then periodically sends snippets of [[JavaScript]] to update the content of the page, thereby achieving push capability. By using this technique, the client doesn't need [[Java applet]]s or other plug-ins in order to keep an open connection to the server; the client is automatically notified about new events, pushed by the server.<ref>[http://www.pushlets.com/doc/whitepaper-toc.html Pushlets introduction]</ref><ref>{{cite web |last1=Van Den Broecke |first1=Just |date=2000-03-01 |df=dmy |url=https://www.infoworld.com/article/2076063/pushlets--send-events-from-servlets-to-dhtml-client-browsers.html |title=Pushlets: Send events from servlets to DHTML client browsers |work=[[JavaWorld]] |access-date=2020-07-13}}</ref> One serious drawback to this method, however, is the lack of control the server has over the browser timing out; a page refresh is always necessary if a timeout occurs on the browser end.

=== Long polling ===
Long polling is itself not a true push; long polling is a variation of the traditional polling technique, but it allows emulating a push mechanism under circumstances where a real push is not possible, such as sites with security policies that require rejection of incoming HTTP requests.

With long polling, the client requests to get more information from the server exactly as in normal polling, but with the expectation that the server may not respond immediately. If the server has no new information for the client when the poll is received, then instead of sending an empty response, the server holds the request open and waits for response information to become available. Once it does have new information, the server immediately sends an HTTP response to the client, completing the open HTTP request. Upon receipt of the server response, the client often immediately issues another server request. In this way the usual response latency (the time between when the information first becomes available and the next client request) otherwise associated with polling clients is eliminated.<ref>{{cite journal|url=https://tools.ietf.org/html/rfc6202#section-2.1|access-date=2016-05-14|title=RFC6202 - Known Issues and Best Practices for the Use of Long Polling and Streaming in Bidirectional HTTP|date=April 2011|last1=Saint-Andre|first1=Peter|last2=Loreto|first2=Salvatore|last3=Salsano|first3=Stefano|last4=Wilkins|first4=Greg|doi=10.17487/RFC6202 }}</ref>

For example, [[BOSH (protocol)|BOSH]] is a popular, long-lived HTTP technique used as a long-polling alternative to a continuous TCP connection when such a connection is difficult or impossible to employ directly (e.g., in a web browser);<ref>{{cite web|url=http://xmpp.org/extensions/xep-0124.html#intro|access-date=2012-06-26|title=XEP-0124: Bidirectional-streams Over Synchronous HTTP (BOSH)}}</ref> it is also an underlying technology in the [[XMPP]], which Apple uses for its iCloud push support.

=== Flash XML Socket relays ===
This technique, used by [[online chat|chat]] applications, makes use of the [[XMLSocket|XML Socket]] object in a single-pixel [[Adobe Flash]] movie. Under the control of [[JavaScript]], the client establishes a [[TCP connection]] to a [[unidirectional network|unidirectional]] relay on the server. The relay server does not read anything from this [[network socket|socket]]; instead, it immediately sends the client a [[unique identifier]]. Next, the client makes an [[HTTP request]] to the web server, including this identifier with it. The web application can then push messages addressed to the client to a local interface of the relay server, which relays them over the Flash socket. The advantage of this approach is that it appreciates the natural read-write asymmetry that is typical of many web applications, including chat, and as a consequence it offers high efficiency. Since it does not accept data on outgoing sockets, the relay server does not need to poll outgoing TCP connections ''at all'', making it possible to hold open tens of thousands of concurrent connections. In this model, the limit to scale is the TCP stack of the underlying server operating system.

=== Reliable Group Data Delivery (RGDD) ===
In services such as [[Cloud computing|Cloud Computing]], to increase reliability and availability of data, it is usually pushed (replicated) to several machines. For example, the Hadoop Distributed File System (HDFS) makes 2 extra copies of any object stored. RGDD focuses on efficiently casting an object from one location to many while saving bandwidth by sending minimal number of copies (only one in the best case) of the object over any link across the network. For example, Datacast<ref>{{Cite journal |title= Datacast: A Scalable and Efficient Reliable Group Data Delivery Service For Data Centers |journal= Microsoft Research|author= C. Guo|display-authors=etal|date= November 1, 2012 |publisher= ACM |url= https://www.microsoft.com/en-us/research/publication/datacast-a-scalable-and-efficient-reliable-group-data-delivery-service-for-data-centers-2/ |access-date= Jun 6, 2017 }}</ref> is a scheme for delivery to many nodes inside data centers that relies on regular and structured topologies and DCCast<ref>{{Cite web |title= DCCast: Efficient Point to Multipoint Transfers Across Datacenters |author= M. Noormohammadpour|display-authors=etal|date= July 10, 2017 |publisher= USENIX |url= https://www.researchgate.net/publication/316921061 |access-date= Jun 6, 2017 }}</ref> is a similar approach for delivery across data centers.

===Push notification===
{{See also|Mobile marketing#Push notifications}}
A push notification is a message that is "pushed" from a back-end server or application to a user interface, e.g. mobile applications<ref>{{Cite journal|last=Wohllebe|first=Atilla.|date=2020|title=Consumer Acceptance of App Push Notifications: Systematic Review on the Influence of Frequency|journal= International Journal of Interactive Mobile Technologies |volume=14|issue=13|pages=36–47|doi=10.3991/ijim.v14i13.14563|doi-access=free}}</ref> or desktop applications. [[Apple Inc.|Apple]] introduced push notifications for [[iPhone]] in 2009,<ref>{{Cite web|url=https://www.engadget.com/2008/06/09/iphone-push-notification-service-for-devs-announced/|title=iPhone push notification service for devs announced|website=Engadget|access-date=2016-10-18}}</ref> and in 2010 [[Google]] released "Google Cloud to Device Messaging" (superseded by [[Google Cloud Messaging]] and then by [[Firebase Cloud Messaging]]).<ref>{{Cite web|url=https://www.infoq.com/news/2012/08/GoogleCMReplacesC2Dm|title=Google Cloud Messaging for Android (GCM) Unveiled, to Replace C2DM Framework|website=InfoQ|access-date=2016-10-18}}</ref>
In November 2015, [[Microsoft]] announced that the [[Windows Notification Service]] would be expanded to make use of the Universal Windows Platform architecture, allowing for push data to be sent to [[Windows 10]], [[Windows 10 Mobile]], [[Xbox]], and other supported platforms using universal API calls and POST requests.<ref>{{Cite web|url=https://docs.microsoft.com/en-us/windows/uwp/controls-and-patterns/tiles-and-notifications-windows-push-notification-services--wns--overview|title=Windows Push Notification Services (WNS) overview|last=mijacobs|website=docs.microsoft.com|language=en-us|access-date=2017-10-20}}</ref>

Push notifications are mainly divided into two approaches, local notifications and remote notifications.<ref>{{Cite web|url=https://developer.apple.com/library/archive/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/|title=Local and Remote Notifications in Depth|website=developer.apple.com|access-date=2016-10-18}}</ref> For local notifications, the application schedules the notification with the local device's OS. For remote notifications, the application sets a timer in the application itself, provided it is able to continuously run in the background. When the event's scheduled time is reached, or the event's programmed condition is met, the message is displayed in the application's user interface.

Remote notifications are handled by a remote server. Under this scenario, the client application needs to be registered on the server with a unique key (e.g., a [[UUID]]). The server then fires the message against the unique key to deliver it to the client via an agreed client/server protocol such as [[HTTP]] or [[XMPP]], and the client displays the message received. When the push notification arrives, it can transmit short notifications and messages, set badges on application icons, blink or continuously light up the [[notification LED]], or play alert sounds to attract user's attention.<ref>{{Cite web|url=http://jatapp.com/blog/android-ios-push-notifications/|title=Android and iOS Push Notifications – Blog – JatApp|website=jatapp.com|language=en|access-date=2017-10-20}}</ref>{{Dead link | date = March 2024 | fix-attempted = yes }} Push notifications are usually used by applications to bring information to users' attention. The content of the messages can be classified in the following example categories:
* Chat messages from a messaging application such as [[Messenger (software)|Facebook Messenger]] sent by other users.<ref>{{Cite web|url=https://www.facebook.com/help/103859036372845?helpref=faq_content|title=How do I adjust my mobile push notifications from Facebook? {{!}} Facebook Help Center {{!}} Facebook|website=www.facebook.com|access-date=2016-10-18}}</ref>
* Vendor special offers: A vendor may want to advertise their offers to customers.
* Event reminders: Some applications may allow the customer to create a reminder or alert for a specific time.
* Subscribed topic changes: Users may want to get updates regarding the weather in their location, or monitor a web page to track changes, for instance.

Real-time push notifications may raise privacy issues since they can be used to bind virtual identities of social network pseudonyms to the real identities of the smartphone owners.<ref>{{Cite journal|last1=Loreti|first1=Pierpaolo|last2=Bracciale|first2=Lorenzo|last3=Caponi|first3=Alberto|date=2018|title=Push Attack: Binding Virtual and Real Identities Using Mobile Push Notifications|journal= Future Internet|volume=10|issue=2|pages=13|doi=10.3390/fi10020013|doi-access=free}}</ref> The use of unnecessary push notifications for promotional purposes has been criticized as an example of [[attention theft]].<ref name="IEEE">{{cite news |last1=McFedries |first1=Paul |title=Stop, Attention Thief! |url=https://spectrum.ieee.org/stop-attention-thief |access-date=9 August 2021 |work=[[IEEE Spectrum]] |publisher=[[Institute of Electrical and Electronics Engineers]] |date=22 May 2014 |language=en}}</ref>

== See also ==
* [[BlazeDS]]
* [[BOSH (protocol)]]
* [[Channel Definition Format]]
* [[Client–server model]]
* [[Comet (programming)]]
* [[File transfer]]
* [[GraniteDS]]
* [[HTTP/2]]
* [[Lightstreamer]]
* [[Notification LED]]
* [[Pull technology]]
* [[Push Access Protocol]]
* [[Push email]]
* [[SQL Server Notification Services]]
* [[Streaming media]]
* [[WebSocket]]
* [[WebSub]]

== References ==
 {{Reflist}}

== External links ==
* W3C Push Workshop. A 1997 workshop that discussed push technology and some early examples thereof
* [https://web.archive.org/web/20051201013553/http://ajaxpatterns.org/HTTP_Streaming HTTP Streaming with Ajax] A description of HTTP Streaming from the Ajax Patterns website
* [http://www.w3.org/TR/websockets/ The Web Socket API] candidate recommendation
* [http://www.w3.org/TR/eventsource/ HTML5 Server-Sent Events] draft specification

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