Comet and Simultaneous Ajax request

Question

Hi ,

I am trying to use a COMET solution using ASP.NET .

Trouble is i want to implement sending and notification part in the same page. On IE7, whenever i try to send a request ,it just gets queued up. After reading on internet and stackoverflow pages i found that i can only do 2 simultaneous asyn ajax requests per page.

So until i close my comet Ajax request,my 2nd request doesnt get completed ,doesnt even go out from the browser. And when i checked with Firefox i just one Ajax comet request running all time..so doesnt that leave me one more ajax request?

Also the solution uses IRequiressessionstate for Asynchronous HTTP Handler which i had removed.but still it creates problems on multiple instances of IE7.

I had one work around which is stated here http://support.microsoft.com/kb/282402
it means we can increase the request limit from registry by default is 2.
By changing "MaxConnectionsPer1_0Server" key
in hive "HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings"
we can increase the number of requests.

Basically i want to broadcast information to multiple clients connected to a server using Comet and the clients can also send messages to the Server.
Broadcasting works but the send request back to server doesnt work.

Im using IIS 6 and ASP.NET .

Are there any more workarounds or ways to send more requests?

References :

http://stackoverflow.com/questions/561046/how-many-concurrent-ajax-xmlhttprequest-requests-are-allowed-in-popular-browser

http://stackoverflow.com/questions/349381/ajax-php-sessions-and-simultaneous-requests

http://stackoverflow.com/questions/2412807/jquery-ajax-request-blocked-by-long-running-ajax-request

http://stackoverflow.com/questions/898190/jquery-making-simultaneous-ajax-requests-is-it-possible

Answer 1

The main idea in COMET is to keep one client-to-server request open, until a response is necessary.

If you design your code properly, then you don't need more than 2 requests to be open simultaneously. Here's how it works:

• client uses a central message send-receive loop to send out a request to the server
• server receives the request and keeps it open.
• at some point, the server responds to the client.
• the client (browser) receives the response, handles it in its central message loop.
• immediately the client sends out another request.
• repeat

The key is to centralize and asynchronize all communications in the client. So you will never need to have 2 open requests.

But to answer your question directly, no, there are no additional workarounds.

Raise the connection limit or reduce the number of connections you use.

Answer 2

You are limited to 2 connections, but typically that's all you need - 1 to send, 1 to receive, even in IE.

That said, you can totally do this; we do it all the time in WebSync. The solution lies in subdomains.

The thing to note is that IE (and other browsers, although they typically limit to 6 requests, not 2) limits requests per domain - but that limitation is for the entire domain excluding subdomains. So for example, you can have 2 requests open to "www.stackoverflow.com" and 2 more requests open to "static.stackoverflow.com", all at the same time.

Now, you've got to be somewhat careful with this approach, because if you make a request from the www subdomain to the static subdomain, that's considered a cross-domain request, so you're immediately limited to not using direct XHR calls, but at that point you have nevertheless bypassed the 2 connection limit; JSONP, HTML5, etc, are all your friend for bypassing the cross-domain limitations.

Edit

Managing with > 1 instance of IE comes back to the same problem. The limitation applies across all instances. So, if you have two browsers open, and they're both using comet, you're stuck with 2 long-polling connections open. If you've maximized your options, you're going to be connecting those long-polling requests to something like "comet.mysite.com", and your non-long-polling requests will go to "mysite.com". That's the best you'll get without going into wildcard DNS.

Check out some of our WebSync Demos; they work in 2 instances of IE without a problem. If you check out the source, you'll see that the DNS for the streaming connection is different from the main page; we use JSONP to bypass the cross-domain limitation.