Sometimes, a CORBA server needs to invoke methods on a client. For example, when you publish stock quotes or aircraft positions, you don't want the clients polling the server over and over to get the new positions. You want the server to notify the clients when it receives data. One way to accomplish this is to use callback interfaces.

The idea is simple, a client program creates a CORBA object and registers the object with the local ORB. The client then invokes a method on the server, passing the CORBA object to the server. CORBA doesn't pass the whole object, of course; it just passes a remote reference to the object. The server then uses the remote reference to invoke methods on the client.

Figure 17.3 illustrates a typical use of ...

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