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Assume that the monitoring application registers a callback by passing an object reference along with information that indicates the circumstances under which a callback
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should occur The following example shows some hypothetical additions to the IDL for our CCS module to support callbacks
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module CCS { struct CallbackInfo { // contents omitted for this example }; interface Callback void notify(in any data); }; interface Thermometer { void register_callback( in Callback cb, in CallbackInfo why ); exception NotRegistered {}; void unregister_callback(in Callback cb) raises(NotRegistered); //
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To create a callback object, the monitoring application implements the Callback interface It then registers a reference for it by invoking register_callback on a Thermometer object, passing it a struct that indicates the conditions under which it wants to be called back When it wants to unregister the callback object, the application passes the registered object reference to unregister_callback Unfortunately, with this design the monitoring application will have trouble unregistering its callback objects because CORBA does not support comparison of object references as a way of determining whether they unequivocally refer to the same object CORBA provides the is_equivalent operation (in the CORBA::Object interface), which allows applications to ask whether two object references refer to the same object Because there are cases when determining equivalence is too expensive or is not possible such as when one object reference is indirected through a proxy in a firewall the CORBA specification does not require ORBs to perform this operation at all costs Rather, it allows is_equivalent to return false if the ORB for some reason cannot determine equivalency, even if the two object references in question actually refer to the same object In other words, a true return value means that the object references refer to the same object, but a false return means either that they do not or that the ORB was unable to make the determination Because of the weak semantics CORBA provides for is_equivalent, applications should not count on it as a tool for determining object identity and object reference equality Instead, applications must either avoid designs that require object reference
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comparison or support operations in their interfaces that allow object identity to be determined We can eliminate the need to compare object references in our callback example by returning an object from register_callback that allows the application to perform an unregister operation
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module CCS { struct CallbackInfo { // contents omitted for this example }; interface Callback { void notify(in any data); }; interface CBRegistration { void unregister(); }; interface Thermometer { CBRegistration register_callback( in Callback cb, in CallbackInfo why ); // };
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The CallbackInfo structure and the Callback interfaces are the same as before, but we have added a CBRegistration interface and changed the return type of the register_callback operation Now, a CBRegistration object is created as the result of register_callback When the application wants to unregister its callback, it invokes unregister on the CBRegistration returned from register_callback The CBRegistration object is created to represent only a single callback registration, so it leaves no ambiguity as to which callback object should be unregistered The unregister operation also implicitly destroys the callback registration, so invoking unregister after the callback has already been unregistered will result in an OBJECT_NOT_EXIST exception An additional benefit of this approach is that it does not open a hole for applications to cancel callbacks for other applications, something that the original solution allowed
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To maintain server activation transparency for the monitoring applications, we must modify the CCS server to save callback information in persistent storage Otherwise, if the CCS server application stopped because of inactivity or for maintenance or if it crashed because of an application defect, the monitoring applications would not know that their callback registrations had suddenly become invalid
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