The use of Comet techniques in web development predates the use of the word Comet as a neologism for the collective techniques. Comet is known by several other names, including Ajax Push, Reverse Ajax, Two-way-web, HTTP Streaming, and HTTP server push among others.
Early Java applets
The ability to embed Java applets into browsers (starting with Netscape 2.0 in March 1996) made real-time communications possible, using a raw TCP socket to communicate between the browser and the server. This socket can remain open as long as the browser is at the document hosting the applet. Event notifications can be sent in any format — text or binary — and decoded by the applet.
First Comet applications
In April 2001 Chip Morningstar began developing a Java-based (J2SE) web server which used two HTTP sockets to keep open two communications channels between the custom HTTP server he designed and a client designed by Douglas Crockford; a functioning demo system existed as of June 2001. The server and client used a messaging format that the founders of State Software, Inc. assented to coin as JSON following Crockford's suggestion. The entire system, the client libraries, the messaging format known as JSON and the server, became the State Application Framework, parts of which were sold and used by Sun Microsystems, Amazon.com, EDS and Volkswagen.
In mid 2004, Selenium Remote Control (RC) was released as open source, and used a Comet technique in the browser to pick up requests from a commanding remote process (tests written in Java, Python, C# or Ruby). The browser would send the last result to a command, and pick up the next command in the same TCP/IP request. If there were no further command yet it would wait until there was one before replying. This was as close as the Selenium-RC team got to turning the browser into a socket listener. Selenium2 (WebDriver) implemented a true socket listener for each browser, so the Comet code and technique isn't used any more.
In March 2006, software engineer Alex Russell coined the term Comet in a post on his personal blog. The new term was a play on Ajax (Ajax and Comet both being common household cleaners in the USA).
Comet applications attempt to eliminate the limitations of the page-by-page web model and traditional polling by offering real-time interaction, using a persistent or long-lasting HTTP connection between the server and the client. Since browsers and proxies are not designed with server events in mind, several techniques to achieve this have been developed, each with different benefits and drawbacks. The biggest hurdle is the HTTP 1.1 specification, which states "this specification... encourages clients to be conservative when opening multiple connections". Therefore, holding one connection open for real-time events has a negative impact on browser usability: the browser may be blocked from sending a new request while waiting for the results of a previous request, e.g., a series of images. This can be worked around by creating a distinct hostname for real-time information, which is an alias for the same physical server. This strategy is an application of domain sharding.
Specific methods of implementing Comet fall into two major categories: streaming and long polling.
An application using streaming Comet opens a single persistent connection from the client browser to the server for all Comet events. These events are incrementally handled and interpreted on the client side every time the server sends a new event, with neither side closing the connection.
Specific techniques for accomplishing streaming Comet include the following:
A basic technique for dynamic web application is to use a hidden iframe HTML element (an inline frame, which allows a website to embed one HTML document inside another). This invisible iframe is sent as a chunked block, which implicitly declares it as infinitely long (sometimes called "forever frame"). As events occur, the iframe is gradually filled with
script tag is executed as it is received. Some browsers require a specific minimum document size before parsing and execution is started, which can be obtained by initially sending 1–2 kB of padding spaces.
One benefit of the iframe method is that it works in every common browser. Two downsides of this technique are the lack of a reliable error handling method, and the impossibility of tracking the state of the request calling process.
The XMLHttpRequest (XHR) object, the main tool used by Ajax applications for browser–server communication, can also be pressed into service for server–browser Comet messaging, in a few different ways.
In 1995, Netscape Navigator added a feature called "server push", which allowed servers to send new versions of an image or HTML page to that browser, as part of a multipart HTTP response (see History section, below), using the content type
multipart/x-mixed-replace. Since 2004, Gecko-based browsers such as Firefox accept multipart responses to XHR, which can therefore be used as a streaming Comet transport. On the server side, each message is encoded as a separate portion of the multipart response, and on the client, the callback function provided to the XHR
onreadystatechange function will be called as each message arrives. This functionality is included in Gecko-based browsers, there is discussion of adding it to WebKit. Internet Explorer 10 also supports this functionality.
onreadystatechange callback each time it receives new data.
Ajax with long polling
None of the above streaming transports work across all modern browsers without negative side-effects. This forces Comet developers to implement several complex streaming transports, switching between them depending on the browser. Consequently many Comet applications use long polling, which is easier to implement on the browser side, and works, at minimum, in every browser that supports XHR. As the name suggests, long polling requires the client to poll the server for an event (or set of events). The browser makes an Ajax-style request to the server, which is kept open until the server has new data to send to the browser, which is sent to the browser in a complete response. The browser initiates a new long polling request in order to obtain subsequent events. Specific technologies for accomplishing long-polling include the following:
XMLHttpRequest long polling
Script tag long polling
While any Comet transport can be made to work across subdomains, none of the above transports can be used across different second-level domains (SLDs), due to browser security policies designed to prevent cross-site scripting attacks. That is, if the main web page is served from one SLD, and the Comet server is located at another SLD (which does not have cross-origin resource sharing enabled), Comet events cannot be used to modify the HTML and DOM of the main page, using those transports. This problem can be sidestepped by creating a proxy server in front of one or both sources, making them appear to originate from the same domain. However, this is often undesirable for complexity or performance reasons.
Unlike iframes or XMLHttpRequest objects,
A long-polling Comet transport can be created by dynamically creating
Browser-native technologies are inherent in the term Comet. Attempts to improve non-polling HTTP communication have come from multiple sides:
EventSourceand a new MIME type
text/event-stream. Experimental implementation of this feature was introduced in Opera 9.
- The HTML 5 WebSocket API working draft specifies a method for creating a persistent connection with a server and receiving messages via an
- The BOSH protocol by the XMPP standards foundation. It emulates a bidirectional stream between browser and server by using two synchronous HTTP connections.
- The JSONRequest object, proposed by Douglas Crockford, would be an alternative to the XHR object.
- Use of plugins, such as Java applets or the proprietary Adobe Flash (using RTMP protocol for data streaming to Flash applications). These have the advantage of working identically across all browsers with the appropriate plugin installed and need not rely on HTTP connections, but the disadvantage of requiring the plugin to be installed
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- [dead link]
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