Concurrent AJAX interval polling

Question

Aside from the obvious heavy load on the server, is it bad to have several concurrent AJAX polls going on at the same time? Or is there even a limit as to what a browser/server can handle?

Example (all AJAX polling)..

• Function A is on a 3 second interval pulling for new message for a global chat system (like Facebook/MySpace).

• Function B is on a 2 second interval for pulling updates and what actions to show the user (such as in an online poker application).

• Function C is on a 4 second interal, but also pulling for new message for a separate messaging system.

Answer 1

Browsers used to have a limit of 4 (or in really old cases 2) concurrent connections per domain. Most modern browsers have upped this to 6 or 8 (based on my own testing and some supporting reading).

So long-polling connections can eat up those slots in the browser and potentially prevent other things from downloading as quickly.

Otherwise, it could be a bandwidth hog for people on slow connections (dial-up or cellular networks).

There may be other drawbacks, but those are two big ones I see.

Answer 2

You should never need more than two active AJAX connections simultaneously. Either you do long-polling, e.g the server responds with headers but then send the entity body when there is data to deliver.

If you run a polling scheme as the one you describe above with A, B, C you can do it with one AJAX thread that alternates between the 3 requests, but with a timeout value set to the next request interval. You create an imaginary timeline where you distribute your requests to A, B and C using one single AJAX thread.