Send data to browser

Question

An example:

Say, I have an AJAX chat on a page where people can talk to each other. How is it possible to display (send) the message sent by person A to persons B, C and D while they have the chat opened?

I understand that technically it works a bit different: the chat(ajax) is reading from DB (or other source), say every second, to find out if there are new messages to display.

But I wonder if there is a method to send the new message to the rest of the people just when it is sent, and not to load the DB with 1000s of reads every second.

Please note that the AJAX chat example is just an example to explain what I want, and is not something I want to realize. I just need to know if there is a method to let all the opened browser at a specific page(ajax) that there is new content on the server that should be gathered.

{sorry for my English}

Answer 1

Since the server cannot respond to a client without a corresponding request, you need to keep state for each user's queued message. However, this is exactly what the database accomplishes. You cannot get around this by replacing the database with something that doesn't just accomplish the same thing in a different way. That said, there are surely optimizations you could do. Keep in mind, however, that you shouldn't prematurely optimize situations like this; databases are designed to handle extremely high traffic, and it's very possible (and in fact, likely), that the scenario described will be handled just fine by the database out of the box.

Answer 2

What you're describing is generally referred to as the 'Comet' concept. See the Wikipedia article for details, especially implementation options (long polling, etc.).

Answer 3

Another answer is to have the server push changes to connected clients, that way there is just one call to the database and then the server pushes the change to all the clients. This article indicates it is possible, however I have never tried this myself.

Answer 4

It's very basic, but if you want to stick with a standard AJAX solution, a simple means of reducing load on the server when polling would be to get the AJAX call to forward the last collected comment ID for that client - you then use that (with the appropriate escaping) in the lookup query on the server side to ensure you only return new comments.