Push technology

Push technology or server push describes a style of Internet based communication where the request for a given transaction originates with the publisher or central server. It is contrasted with pull technology, where the request for the transmission of information originates with the receiver or client.

General use

Push services are often based on information preferences expressed in advance. They can also be called a "subscription model". A client might "subscribe" to various information "channels". Whenever new content is available on one of those channels, the server would push that information out to the user.

Synchronous conferencing and instant messaging are typical examples of push services. Chat messages and files are pushed to the user as soon as they are received by the messaging service. Both decentralised peer-to-peer programs (such as WASTE) and centralised programs (such as IRC or Jabber) allow pushing files, this means the sender initiates the data transfer rather than the recipient.

E-mail is also a classic Internet push medium, for example SMTP is an email push protocol. However, the last step - from a user's mail server to his desktop - is typically a pull operation using a pull protocol like POP3 or IMAP. Modern e-mail clients make this last step appear to be a "push" operation by making repeated pull requests, for example by checking for new mail on an IMAP server every few minutes. 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 technology in a wireless context.

Another popular type of Internet push technology was PointCast Network which gained popularity in the 1990s, it delivered news and stock courses. Both Netscape and Microsoft integrated it into their software at the height of the browser wars, but it later faded away and was replaced in the 2000s with RSS (a pull technology).

Other uses are push enabled web applications including market data distribution (stock tickers), online chat/messaging systems (webchat), auctions, online betting and gaming, sport results, monitoring consoles and sensor network monitoring.

Technologies

HTTP server push

A sub category of push technology specifically related to the HTTP protocol. The term server push was originally coined by Netscape in 1995. A special MIME type called 'multipart/x-mixed-replace' was added, which Netscape 1.1 and subsequent web browsers would interpret as a document changing whenever the server felt like pushing a new version to the client. It is still supported by Mozilla, Firefox, Opera and Safari today, but traditionally ignored by Microsoft. It can be applied to HTML documents, making it quite popular for web chats, but also for streaming images in webcam applications.

The WHATWG is drafting a Web Applications 1.0 specification which pushes content to the client. On September 1st, 2006, the Opera web browser implemented this new experimental technology in a feature called "Server-Sent Events."

HTTP streaming

HTTP streaming is a mechanism for sending data from a Web server to a Web browser in response to an event. HTTP Streaming is achieved through several common mechanisms.

In one such mechanism the web server does not terminate the response to the client after data has been served. This differs from the typical HTTP cycle in which the response is closed immediately following data transmission.

The web server leaves the response open such that if an event is received, it can immediately be sent to the client. Otherwise the data would have to be queued until the client's next request is made to the web server. The act of repeatedly queing and re-requesting information is known as a Polling mechanism.

Java pushlet

A pushlet is a Java technique where data is pushed by server-side objects to a client browser containing HTML pages. By using this technique the client doesn't need Java applets or other plug-ins to retrieve the data from the server. The clients will be automatically notified by new events, pushed by the server .

Long polling

Long polling is a variation of the traditional polling technique and allows to emulate information push from a server to a client. With long polling, the client requests information from the server in a similar way to a normal poll. However, if the server does not have any information available for the client, instead of sending an empty response, the server holds the request and waits for an information to be available. Once the information becomes available (or after a suitable timeout), a complete response is sent to the client. The client will normally then immediately re-request information from the server, so that the server will almost always have an available waiting request that it can use to deliver data in response to an event.

Long polling is itself not a push technology, but can be used under circumstances where a real push is not possible.

Other technologies

The term Comet has been used to describe push technologies applied to Ajax web applications, it's an umbrella term for a combination of web technologies such as HTTP server push and long polling (see above).

See also

• BOSH
• Comet
• Client-server architecture
• Pull technology
• Push Access Protocol
• Push-Pull strategy
• Reverse Ajax
• Streaming media, streaming media systems

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
• HTML 5 Draft with Server-sent events A mechanism for allowing servers to dispatch events, 2008 draft