Silverlight 4 Data and Services Cookbook

Using duplex communication

Most types of communication between a client and a server are initiated by the client. The client sends a request to the server and the server sends its response back to the client. However, we may sometimes need to inform the client of changes that take place in the service. This will then be a server-initiated request to the client as the server needs to push information to the client. This leads us to the fact that sometimes we may need so-called duplex communication, in which both the server and the client side can initiate the communication. This isn't possible out of the box because of the HTTP stack. HTTP is based on a client-initiated request, on which the server can react.

The PollingDuplexBinding was introduced ...