how to build wcf notification service

I have this situation: I have some wcf services which do stuffs, and have wcf RegistryService which enables me to get data about people, addresses etc. The client application is a silverlight 4 application. I want this:

that every time a client application updates a Registry (a p开发者_JAVA技巧erson or an address) the service sends a change notification to all connected clients that the registry has changed.

how to do that? thanks.