Silverdraw is realtime white board that can sync information between various participants in C#, using Silverlight + WCF Polling Duplex. Presently this is a just a POC implementation. Also, as we are using Polling Duplex, we need a callback interface too - so that the server can 'call back' or notify the clients. (In Polling Duplex, you may need to note that what is happening is not actually a direct callback from server to client. The client should poll the server periodically over HTTP to fetch any possible information that the server is willing to pass to the client, to simulate a callback.) Anyway, our 'call back' service interface (IDuplexDrawCallback) has a Notify method, for the server to notify the rest of the clients when some client calls the Draw method.