Push technology

Push technology, or server push, describes 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, where the request for the transmission of information is initiated by the receiver or client.

General use

Push services are often based on information preferences expressed in advance. This is called a publish/subscribe 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 sometimes 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 XMPP) allow pushing files, which means the sender initiates the data transfer rather than the recipient.

Email is also a push system: the SMTP protocol on which it is based 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 technology for email 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 market data. 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.

Push technology is used in Windows Update to push updates to a user's computer.

Technologies

HTTP server push

HTTP server push (also known as HTTP streaming) is a mechanism for sending data from a web server to a web browser. HTTP server push can be achieved through several mechanisms.

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 such that if an event is received, it can immediately be sent to one or multiple clients. Otherwise the data would have to be queued until the client's next request is received. Most web servers offer this functionality via CGI (e.g. NPH scripts on Apache).

Another mechanism is related to a special MIME type called multipart/x-mixed-replace, which was introduced by Netscape in 1995. Web browsers would interpret this as a document changing whenever the server felt like pushing a new version to the client. It is still supported by Firefox, Opera and Safari today, but traditionally ignored by Microsoft. It can be applied to HTML documents, but also for streaming images in webcam applications.

The WHATWG Web Applications 1.0 proposal included a mechanism to push content to the client. On September 1, 2006, the Opera web browser implemented this new experimental technology in a feature called "Server-Sent Events." It is now being standardized as part of HTML5. Another related part of HTML5 is the Web Sockets API, which allows a web server and client to communicate over a full-duplex TCP connection. Web Sockets are available on Google Chrome since 4.0.249.0, and there is a JavaScript library by Kaazing that emulates them.

Java pushlet

A pushlet is a technique originally developed for Java web applications, though the same techniques can be employed on other web frameworks as well. In this technique, the server takes advantage of persistent HTTP connections and leaves the response perpetually "open" (i.e. it never terminates the response), effectively fooling the browser into continuing in "loading" mode after the initial page load would normally be 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 to keep an open connection to the server. The clients will be automatically notified by 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 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 some 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 is used as an umbrella term for a combination of web technologies such as HTTP server push and long polling (see above).

XMPP is often used for push applications as well, especially the PubSub extensions. Apple uses this technology for its Mobile Me push support.

BOSH is an long-lived HTTP technique used in XMPP, but that can be used on the web. The specification (XEP-0124: Bidirectional-streams Over Synchronous HTTP (BOSH)) reads: This specification defines a transport protocol that emulates the semantics of a long-lived, bidirectional TCP connection between two entities (such as a client and a server) by efficiently using multiple synchronous HTTP request/response pairs without requiring the use of frequent polling or chunked responses.

See also

• Apple Push Notification Service
• BOSH
• Comet
• Client-server architecture
• File transfer
• Pull technology
• Push Access Protocol
• Push Email
• 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
• HTML5 Web Sockets draft specification
• HTML5 Server-Sent Events draft specification
• HTTP Server Push at UDAParts A commercial grade Comet implementation with source code available
• Google Web Toolkit's Server Push FAQ Explains Server Push and how you can achieve this with GWT
• Ajax Push Engine An open source web server for pushing real-time data to Rich Internet Applications using Comet
• GWT Comet Adapter Open source adapter for implementing server push in Google Web Toolkit
• XEP-0124: Bidirectional-streams Over Synchronous HTTP (BOSH) An dual HTTP long-lived connection