Simply put, duplex operations provide a mechanism for allowing a service to callback to the client. When a contract is defined for a service, it is possible to specify a corresponding callback contract. The standard service contract defines the operations of the service that can be invoked by the client. The callback contract defines the operations of the client that can be invoked by the service. It is the client's responsibility to implement the callback contract and host the callback object. Each time the client invokes a service operation that is associated to the callback contract, the client supplies the necessary information for the service to locate and invoke the callback operation on the client.

One of the most popular uses for duplex operations seems to be for events. This is typically accomplished by using a publish-subscribe pattern. One or more clients will subscribe to the service. When something of interest occurs, the service will publish the information to the subscribed clients.

This is the approach used by the demo. It is based on the concept of assisting the host of a party by tracking guests and their consumption of beer. Although intentionally humorous, it effectively illustrates the key concepts involved with duplex operations. Each guest subscribes to the service by joining the party. When something interesting happens, such as other guests joining/leaving the party as well as the consumption of beer, the service publishes the information to the subscribed clients.