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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. It is contrasted with pull technology, or get, where the request for the transmission of information is initiated by the receiver or client.
Push services are often based on information and data preferences expressed in advance, called the 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" or "publishes" 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 requests.
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 the mail server, frequently checking it for new mail. The IMAP protocol includes the IMAP 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.
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 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 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.
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. As part of this standard, Push API is fully implemented in Google Chrome, Firefox, and Microsoft Edge, and partially implemented in Safari .
As a part of HTML5 the WebSocket 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 underlying mechanism for this approach is chunked transfer encoding.
Another mechanism is related to a special MIME type called multipart/x-mixed-replace, 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. 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 today, but it is ignored by Internet Explorer Server-Push Documents (HTML & XHTML: The Definitive Guide) O'Reilly book explaining server-push and is only partially supported by Google Chrome. Remove support for multipart/x-mixed-replace main resources 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.
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 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 at the next client request) otherwise associated with polling clients is eliminated.
For example, 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); it is also an underlying technology in the XMPP, which Apple uses for its iCloud push support.
Push notifications are mainly divided into two 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). 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. 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:
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.
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