what's the skinny on long polling with ajax and webapi...is it going to kill my server? and string comparisons

Question

I have a very simple long polling ajax call like this:

(function poll(){
    $.ajax({ url: "myserver", success: function(data){
        //do my stuff here

    }, dataType: "json", complete: poll, timeout: 30000 });
})();

I just picked this example up this afternoon and it seems to work great. I'm using it to build out some html on my page and it's nearly instantaneous as best I can tell. I'm a little worried though that this is going to keep worker threads open on my server and that if I have too big of a load on the server, it's going to stop entirely. Can someone shed some light on this theory? On the back end I have a webapi service (.net mvc 4) that calls a database, build the object, then passes the object back down. It also seems to me that in order for this to work, the server would have to be calling the database constantly...and that can't be good right???

My next question is what is the best way on the client to determine if I need to update the html on my page? Currently I"m using JSON.stringify() to turn my object into a string and comparing the string that comes down to the old string and if there's a delta, it re-writes the html on the page. right now there's not a whole lot in the object coming down, but it could potentially get very large and I think doing that string comparison could be pretty resource intensive on the client...especially if it's doing it nearly constantly.

bottom line for me is this: I"m not sure exactly sure how long polling works. I just googled it and found the above sample code and implemented it and, on the surface, it's awesome. I just fear that it's going to bog things down (on the server) and my way of comparing old results to new is going to bog thigns down (on the client).

any and all information you can provide is greatly appreciated.

TIA.

Answer 1

OK, my two cents:

1. As others said, SignalR is tried and tested code so I would really consider using that instead of writing my own.
2. SignalR does change some of the IIS settings to optimise IIS for this sort of work. So if you are looking to implement your own, have a look at IIS setting changes done in SignalR
3. I suppose you are doing long polling so that your server could implement some form of Server Push. Just bear in mind that this will turn your stateless HTTP machine into a stateful machine which is not good if you want to scale. Long polling behind a load ballancer is not nice :) For me this is the worst thing about server push.
4. ASP.NET uses ThreadPool for serving requests. A long poll will hog a ThreadPool thread. If you have too many of these threads you might end up in thread starvation (and tears). As a ballpark figure, 100 is not too many but +1000 is.
5. Even SignalR team say that the IIS box which is optimised for SingalR, probably not optimised for normal ASP.NET and they recommend to separate these boxes. So this means cost and overhead.

At the end of the day, I recommend to using long polling if you are solving a business problem (and not because it is just cool) because then that will pay its costs and overheads and headaches.