Re: push data from server to client web browser

From: Thorbjørn Ravn Andersen
The AJAX technologies are all about getting the browser to
process server pushes.

The problem with a statement like that is that somebody will interpret it to mean that a servlet or cgi program can initiate a conversation with a browser, and send something to it, similar to the way a batch program can send an email message to thousands of different smtp servers around the globe.

I find that even experienced web developers get confused, and sometimes think they can write a servlet or cgi program that can initiate a conversation with a browser, and send something to it.

But that's not how servlets and cgi programs work. They can't initiate a conversation with a client. Browsers always initiate the conversation. Browsers always make the request. Browsers are always the ones doing the "pushing".

HTTP servers, web application servers (like Tomcat), servlets, and cgi programs can only respond (they implement a purely "pull" interface).

Even with AJAX, browsers are in control of the conversation. The browser makes an asynchronous request. A request may be initiated by a timer event and run in the background. The request doesn't necessarily require a user driven event such as a mouse click or keypress. But the browser is always the one initiating the request-response cycle. The server is only responding.

Nathan.