I am implementing a comet using AsyncHttpHandlers in my current asp.net application. According to my implementation client initially sends Notification Hook request to server (with its user id) on AsyncHttpHandler, and on server side I maintain a Global (Application level) dictionary of userid(key) and IAynsResult (value). So when ever a request is received to send notification to a user I just pick the matching IAsyncResult from the Global Dictionary and send response to the client user.
My concern is, is maintaing a Dictionary of Userid and IAsyncResult at Application level a good design? I feel it will put a lot of load on the server, at the time of high traffic. Is there any other way I can achieve the comet. or what will be the good design to achieve comet for high traffic scenarios.
That depends on the number of requests to the server, IAsyncResult request utilizes the process's ThreadPool which automatically manages the number of worker threads. These threads are assigned a task, run them to completion, then are returned to the ThreadPool for reuse.
The ThreadPool is used by other aspects of .NET, and provides a limited number of threads. If you overuse it, there is the possibility your tasks will be blocked waiting for others to complete!
So basically, running a comet server on an ASP.net needs either a strong hardware, or application distribution over several servers.
I would recommend HTML5 WebSocket, which is the W3C API for implementing sockets in HTML and is easier to setup on an ASP.net server: