Push Messages from Azure application to MonoTouch (iPhone) application without Apple Push Notifications

I'm currently designing an application for iOS (using MonoTouch) that will have a server component running on Windows Azure. The application will essentially be a chat type application where users will generate messages within their clients and send them to the server, which will then need to forward those messages out (as quickly as practicable) to other clients that the user might be sending the messages to.

My question is - is there a recommended practice for architecting an application like this, where clients need to receive 'push' messages from the server?

I've considered a few options but would appreciate feedback.

• The first option is to use Apple's Push Notifications service (APNs). I have two concerns about this - first, the clients only need to receive the messages when they're online (APNs sends messages through when the app is closed too, which I don't need or want); and second, there is a possibility that there will be a high volume of messages, which I know Apple would probably get unhappy about (perfectly fairly).

• A second option I considered is using a web service (WCF-based) and having the client call this service every (say) 2-3 seconds, which is the maximum delay we could tolerate. This would seem to involve a great deal of potentially unnecessary network traffic, though ("have you got anything for me?", "no", repeated ad nauseum).

• A third option is to maintain a persistent web service connection between the client and the server. When the client app starts it would call a web service method on a background thread. The server would hold the connection open (by not returning anything), and if any messages came through it would immediately return them. This connection might time out after, say, 2 minutes at which point it would be re-established. This seems to do what I want, but again, I'm concerned that there'd be a lot of connections open to the server at any moment, which could require server resources unnecessarily.

• A fourth option is to use a persistent connection over TCP (or UDP, although from what I've found, Windows Azure doesn't support this). This seems to be a good option, but again, might be overkill in terms of server usage - there could potentially be hundreds or even thousands of clients connected at any moment.

• A fifth option is to somehow have the server push messages directly to the client, perhaps by having the client run a mini web server or similar. However, as the app will be running on 3G and WiFi netwo开发者_StackOverflow中文版rks (beyond my control) I don't expect incoming ports will be open for this sort of thing.

If anyone has any other suggestions, or thinks one of the above options would be a good idea (or is a standard way of approaching this sort of problem) I'd be very interested to hear about it.

Thanks in advance,

John

You had a look at Pubnub http://www.pubnub.com/ ?