Web Design & Development Guide

Reverse Ajax

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Reverse Ajax, not unlike DHTML, LAMP, Ajax and SPA, is not a technology in itself, but a term that refers to the use of a group of technologies together. These technologies include:

• Ajax for handling the data on the client side in a smooth and interactive way, and passing data between server and client.
• A technology for pushing server data to a browser
  • Comet, a connection between a server and client is kept open, by slowly loading a page in a hidden frame.
  • Piggyback, extra data is added (piggybacked) onto a normal client-server interaction.
  • Polling, the client repetitively queries (poll) the server.

Reverse Ajax is different from Ajax, as Reverse Ajax is a suite of technologies for pushing data from a server to a client. These technologies are built upon an Ajax framework.

Notes

The Polling technique is not properly a Reverse Ajax solution, because in this case the server simply responds to a repeated direct ajax request. The result is the same that Reverse Ajax, but this technique is not too elegant to be used because it makes a lot of unnecessary traffic.

Let's take a simple example:

The broadcast message.

We have 2 clients and 1 server: client1 needs to send "hello" to all the other clients

With Polling:

• client1 sends the message "hello"
• server receives the message "hello"
• client2 polls the server
• client2 receives the message "hello"
• client1 polls the server <-- unnecessary traffic starts
• client1 receives the message "hello"
• client2 polls the server
• client2 receives the message "hello" <-- unnecessary traffic continues

and these last lines repeat forever.

With Comet:

• client1 sends the message "hello"
• server receives the message "hello"
• server sends the message "hello" to all clients

no unnecessary traffic.

With PiggyBack:

• client1 sends the message "hello"
• server receives the message "hello"
• client2 does any request to the server
• server adds to the response the message "hello"

no unnecessary traffic.

See also

• Ajax

External links

Articles

• Reverse Ajax
• Exploring Reverse Ajax
• Beyond Ajax
• Changing the Web Paradigm
• Comet Low Latency Data for the Browser

Implementations

• DWR Project homepage
• Full-duplex Ajax A module for Lighttpd web server
• RJAX A Java Open Source solution using an applet and a server embedded in the web application

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