Services that run over HTTP don't allow for 'server push' as HTTP is a stateless, connectionless protocol (at least in principle). You'll need to poll for updates from the client.
You wouldn't use a web service to do this. What you want to be looking at is pushing out the data using something like a WebSocket or a FlashSocket. If I were you, I would take a look at SignalR[^] (if you were doing this with Node.JS, I would recommend using Socket.IO[^] instead).
