How should I implement the C# server side portion of long-polling for ajax requests?

Question

I've got an architecture that involves browsers polling via ajax every 3 seconds for updates and I'd like to change that to long-polling.

I'd like to have 1, 2.. {n} clients long-polling, waiting for updates and have something happen on the server to signal the waiting clients to return. My first thought was to use an EventWaitHandle, and I can do this easily if I just want to support 1 client. I'd just have an AutoResetEvent WaitHandle that would WaitOne to block the client, maybe with a timeout, maybe not. Either way, an AutoResetEvent will only allow me to support 1 client (since it only wakes 1 waiting thread) and I want n clients.

I'm pretty sure I need to use a ManualResetEvent WaitHandle, but I'm not sure when to call Reset after I Set it (when waking the threads). Should I simply Thread.Sleep some arbitrary amount in between the Set and Reset?

In psuedo code, the waking logic would be

• get ManualResetEventWaitHandle
• call Set
• ensure all waiting clients have woken, while preventing new requests from blowing through
• call Reset now that all waiting clients have received their updates

Its that 3rd line that i'm having a hard time with. Currently I am tossing around the idea of having a LastTxID that the client / server maintain and potentially using 2 wait handles. However, before I went crazy with this implementation I wanted to get feedback here to see how they would implement the waking logic.

Edit: assume I've got the problems associated with having max concurrent users figured out, either by tweaking IIS or hosting via WCF or some other solution. I only want to focus on the waking logic.

Answer 1

One idea, in pseudo code

• Maintain a thread-safe list of connection id's, possibly use the session id
• Give every connection its own AutoResetEventWaitHandle
• In a thread safe manner, loop through those wait handles and set them when there is an update
• on session end, in a thread safe manner, remove that connection / session id from the list

I'd love to get some feedback on this

con's to this approach

• have to maintain a list of connections
• have to generate {n} wait handles for {n} clients

Answer 2

Check this out for a working implementation: http://msdn.microsoft.com/en-us/library/aa446486.aspx