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Push technology or server push is a style of Internet-based communication where the request for a given transaction is initiated by the publisher or central
server Server may refer to: Computing *Server (computing), a computer program or a device that provides functionality for other programs or devices, called clients Role * Waiting staff, those who work at a restaurant or a bar attending customers and su ...
. It is contrasted with pull/get, where the request for the transmission of information is initiated by the receiver or client. Push services are often based on information preferences expressed in advance. It is called a publish/subscribe model. A client "subscribes" to various information "channels" provided by a server; whenever new content is available on one of those channels, the server pushes that information out to the client. Push is sometimes emulated with a polling technique, particularly under circumstances where a real push is not possible, such as sites with security policies that reject incoming HTTP/S requests.


General use

Synchronous conferencing and instant messaging are typical examples of push services. Chat messages and sometimes
files File or filing may refer to: Mechanical tools and processes * File (tool), a tool used to ''remove'' fine amounts of material from a workpiece **Filing (metalworking), a material removal process in manufacturing ** Nail file, a tool used to gent ...
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 IRC or 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 Push email is an email system that provides an always-on capability, in which new email is actively transferred ( pushed) as it arrives by the mail delivery agent (MDA) (commonly called mail server) to the mail user agent (MUA), also called the ema ...
). 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 the mail server, frequently checking it for new mail. The IMAP protocol includes the
IDLE Idle generally refers to idleness, a lack of motion or energy. Idle or ''idling'', may also refer to: Technology * Idle (engine), engine running without load ** Idle speed * Idle (CPU), CPU non-utilisation or low-priority mode ** Synchronous ...
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. Another example is the PointCast Network, which was widely covered in the 1990s. It delivered news and stock market data as a screensaver. Both
Netscape Netscape Communications Corporation (originally Mosaic Communications Corporation) was an American independent computer services company with headquarters in Mountain View, California and then Dulles, Virginia. Its Netscape web browser was onc ...
and Microsoft integrated push technology through the
Channel Definition Format Channel Definition Format (CDF) was an XML file format formerly used in conjunction with Microsoft's Active Channel, Active Desktop and Smart Offline Favorites technologies. The format was designed to "offer frequently updated collections of info ...
(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 RSS ( RDF Site Summary or Really Simple Syndication) is a web feed that allows users and applications to access updates to websites in a standardized, computer-readable format. Subscribing to RSS feeds can allow a user to keep track of many di ...
(a pull system.) Other uses of push-enabled web applications include software updates distribution ("push updates"), market data distribution (stock tickers), online chat/messaging systems (
webchat A web chat is a system that allows users to communicate in real-time using easily accessible web interfaces. It is a type of Internet online chat distinguished by its simplicity and accessibility to users who do not wish to take the time to insta ...
), auctions, online betting and gaming, sport results, monitoring consoles, and
sensor network Wireless sensor networks (WSNs) refer to networks of spatially dispersed and dedicated sensors that monitor and record the physical conditions of the environment and forward the collected data to a central location. WSNs can measure environmental c ...
monitoring.


Examples


Web push

The Web push proposal of the Internet Engineering Task Force is a simple protocol using 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 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. Web Notifications are part of
W3C The World Wide Web Consortium (W3C) is the main international standards organization for the World Wide Web. Founded in 1994 and led by Tim Berners-Lee, the consortium is made up of member organizations that maintain full-time staff working to ...
standard and defines an API for end-user notifications. A notification allows alerting the user outside the context of a web page of an occurrence, such as the delivery of email. As part of this standard Push API defined by W3C, is now being implemented by Chrome, Firefox, Edge, and Safari.


HTTP server push

HTTP server push (also known as HTTP streaming) is a mechanism for sending unsolicited (asynchronous) data from a
web server A web server is computer software and underlying hardware that accepts requests via HTTP (the network protocol created to distribute web content) or its secure variant HTTPS. A user agent, commonly a web browser or web crawler, initiate ...
to a web browser. HTTP server push can be achieved through any of several mechanisms. As a part of HTML5 the Web Socket API allows a web server and client to communicate over a 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 CGI (e.g., Non-Parsed Headers scripts on
Apache HTTP Server The Apache HTTP Server ( ) is a free and open-source cross-platform web server software, released under the terms of Apache License 2.0. Apache is developed and maintained by an open community of developers under the auspices of the Apache So ...
). The underlying mechanism for this approach is
chunked transfer encoding Chunked transfer encoding is a streaming data transfer mechanism available in Hypertext Transfer Protocol (HTTP) version 1.1, defined in RFC 9112 §7.1. In chunked transfer encoding, the data stream is divided into a series of non-overlapping "c ...
. Another mechanism is related to a special MIME type called multipart/x-mixed-replace, which was introduced by
Netscape Netscape Communications Corporation (originally Mosaic Communications Corporation) was an American independent computer services company with headquarters in Mountain View, California and then Dulles, Virginia. Its Netscape web browser was onc ...
in 1995. Web browsers interpret this as a document that changes whenever the server pushes a new version to the client.CGI Programming on the World Wide Web
O'Reilly book explaining how to use Netscape server-push
It is still supported by Firefox, Opera, and
Safari A safari (; ) is an overland journey to observe wild animals, especially in eastern or southern Africa. The so-called "Big Five" game animals of Africa – lion, leopard, rhinoceros, elephant, and Cape buffalo – particularly form an importa ...
today, but it is ignored by Internet Explorer and is only partially supported by
Google Chrome Google Chrome is a cross-platform web browser developed by Google. It was first released in 2008 for Microsoft Windows, built with free software components from Apple WebKit and Mozilla Firefox. Versions were later released for Linux, macOS ...
. It can be applied to HTML documents, and also for streaming images in webcam applications. The WHATWG Web Applications 1.0 proposal 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". It is now part of the HTML5 standard.


Pushlet

In this technique, the server takes advantage of 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 applets 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. 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/S Requests. With long polling, the client requests information from the server exactly as in normal polling, but with the expectation the server may not respond immediately. If the server has no new information for the client when the poll is received, 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/S response to the client, completing the open HTTP/S 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 at the next client request) otherwise associated with polling clients is eliminated. For example,
BOSH Bosh or BOSH may refer to: * BOSH (protocol), a transport protocol in computer networking * Bosh (band), a Christian rock band from Bournemouth, United Kingdom *Bosh (rapper), French rapper * BOSH (software), a project for release engineering, dep ...
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); 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
chat Chat or chats may refer to: Communication * Conversation, particularly casual * Online chat, text message communication over the Internet in real-time * Synchronous conferencing, a formal term for online chat * SMS chat, a form of text messagin ...
applications, makes use of the XML Socket object in a single-pixel Adobe Flash movie. Under the control of JavaScript, the client establishes a
TCP connection TCP may refer to: Science and technology * Transformer coupled plasma * Tool Center Point, see Robot end effector Computing * Transmission Control Protocol, a fundamental Internet standard * Telephony control protocol, a Bluetooth communication s ...
to a unidirectional relay on the server. The relay server does not read anything from this socket; instead, it immediately sends the client a unique identifier. Next, the client makes an HTTP request to the web server, including with it this identifier. 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, 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 is a scheme for delivery to many nodes inside data centers that relies on regular and structured topologies and DCCast is a similar approach for delivery across data centers.


Push notification

A push notification is a message that is "pushed" from back-end server or application to user interface, e.g. (but not limited to) mobile applications and desktop applications. Push notifications were first introduced by Apple in 2009. In 2010 Google released its own service, Google Cloud to Device Messaging (since superseded first by Google Cloud Messaging and then
Firebase Cloud Messaging Firebase Cloud Messaging (FCM), formerly known as Google Cloud Messaging (GCM), is a cross-platform cloud solution for messages and notifications for Android, iOS, and web applications, which as of June 2022 can be used at no cost. Firebase Cl ...
). November 2015, Microsoft announced that the
Windows Notification Service Windows Push Notification Service (commonly referred to as Windows Notification Service or WNS) is a notification service developed by Microsoft for all devices running Microsoft Windows platforms. It allows for developers to send push data ("to ...
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, as well as other supported platforms using universal API calls and POST requests. Push notifications are mainly divided into 2 approaches, local notifications and remote notifications. 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 A universally unique identifier (UUID) is a 128-bit label used for information in computer systems. The term globally unique identifier (GUID) is also used. When generated according to the standard methods, UUIDs are, for practical purposes, uni ...
). The server then fires the message against the unique key to deliver the message to the client application 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 A Notification LED is a small RGB or monochrome LED light usually present on the front-facing screen bezel (display side) of smartphones and feature phones whose purpose is to blink or pulse to notify the phone user of missed calls, incoming SMS ...
, or play alert sounds to attract user's attention. 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, e.g.: messages from Facebook messenger sent by other users. * Vendor special offers, e.g.: A vendor may want to advertise their social offers to the customers. * Event reminder, e.g.: Some application may allow the customer to create reminder or alert for a specific time. * Subscribed topics changes, e.g.: 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. The use of unnecessary push notifications for promotional purposes has been criticized as an example of attention theft.


See also

*
BlazeDS BlazeDS is a server-based Java remoting and web messaging technology that allows users to connect to back-end distributed data and push data to Apache Flex and Adobe AIR Rich Internet applications (RIA). Because of its open licensing, BlazeDS is n ...
*
BOSH Bosh or BOSH may refer to: * BOSH (protocol), a transport protocol in computer networking * Bosh (band), a Christian rock band from Bournemouth, United Kingdom *Bosh (rapper), French rapper * BOSH (software), a project for release engineering, dep ...
*
Channel Definition Format Channel Definition Format (CDF) was an XML file format formerly used in conjunction with Microsoft's Active Channel, Active Desktop and Smart Offline Favorites technologies. The format was designed to "offer frequently updated collections of info ...
* Client–server model * Comet * File transfer *
GraniteDS Flex frameworks are software libraries that assist developers in building rich web applications on the Apache Flex platform. List * Tide, part of the Granite Data Services platform. * Swiz * Parsley * Cairngorm * PureMVC DropAS3Fabrication* Mat ...
* Lightstreamer *
Notification LED A Notification LED is a small RGB or monochrome LED light usually present on the front-facing screen bezel (display side) of smartphones and feature phones whose purpose is to blink or pulse to notify the phone user of missed calls, incoming SMS ...
*
Pull technology Pull coding or client pull is a style of network communication where the initial request for data originates from the client, and then is responded to by the server. The reverse is known as push technology, where the server ''pushes'' data to c ...
*
Push Access Protocol {{Unreferenced, date=December 2009 Push Access Protocol (or PAP) is a protocol defined in WAP-164 of the Wireless Application Protocol (WAP) suite from the Open Mobile Alliance. PAP is used for communicating with the Push Proxy Gateway, which is us ...
* Push email * HTTP/2 *
SQL Server Notification Services SQL Server Notification Services is a platform developed by Microsoft for the development and deployment of notification applications based on SQL Server technology and the Microsoft .NET Framework. Notification Services offers a scalable server ...
*
Streaming media Streaming media is multimedia that is delivered and consumed in a continuous manner from a source, with little or no intermediate storage in network elements. ''Streaming'' refers to the delivery method of content, rather than the content it ...
* Web Socket * Web Sub


References


External links

* W3C Push Workshop. A 1997 workshop that discussed push technology and some early examples thereof
HTTP Streaming with Ajax
A description of HTTP Streaming from the Ajax Patterns website
The Web Socket API
candidate recommendation
HTML5 Server-Sent Events
draft specification {{DEFAULTSORT:Push Technology Ajax (programming) Internet terminology Mobile technology Web development