Summary
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, 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. Synchronous conferencing and instant messaging are examples of push services. Chat messages and sometimes 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). 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 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.
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