Client programs often need to react to changes or updates that occur on the server, such as changes in stock price. The client can either
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periodically call a method that checks for updated information
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ask to be notified by the server whenever a change occurs
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The second option is called a callback.
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In CORBA, you can create two interfaces in one module:
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the regular services interface that specifies which methods the client can call on the server
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a callback interface that specifies a method on the client side that the server can call to report updates.
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Note that this is really using the server/server architecture of the ORBs.
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