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specific factory operation can be strongly typed, whereas a generic factory must by necessity return the reference as type Object The generic return type forces the receiving client to narrow the reference to its actual type, something that is inconvenient and not statically type-safe The copy and move operations present the same trade-offs as the generic factory Parameters are not statically type-safe, and the return value from copy is weakly typed (LifeCycleObject instead of a specific interface type, such as Thermometer) 1252 Date of Publication The Life Cycle Service was one of the first services to be defined and published by the OMG and, in some ways, is showing its age For example, the FactoryFinder interface provides a generic hook to implement a selection mechanism that can choose one or more factories that are suitable to create the required object Although this approach is valid, the problem is that it is too generic In addition, creating an even halfway sophisticated factory finder can be as much work as building an entire application As a result, we must make do with a simple factory finder unless we are prepared to expend a lot of effort More recently, in 1997, the OMG published an updated version of the CORBAservices specification [21] This document defines the OMG Trading Service, which provides a powerful and flexible object discovery mechanism A trader can (among many other things) act as a generic factory finder The significant advantage of the trader is that you do not have to implement it yourself It also provides interfaces that are far more powerful and flexible than a simple generic factory, yet they do not compromise type safety to the same degree We discuss the OMG Trading Service in detail in 19 1253 Problems with the move Operation The move operation presents two types of problems One type of problem is conceptual, and the other is technical
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Conceptual Problems with move
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The move operation is intended to enable object migration In other words, a client can use it to direct an object to disappear from one server and to reappear in another Even assuming that we have supplied sufficient information in the the_criteria parameter as to where and how the object should move, there are still serious conceptual issues associated with the idea of migration The notion of object migration does not rest easy with the CORBA object model One of the central features of CORBA, as we point out in 2, is the notion of location transparency In fact, CORBA does not embody the concept of object location in its
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object model at all Instead, CORBA goes to great lengths to hide the location of an object from clients and provides a notion of object identity that, together with an object's location, is encapsulated inside object references Attempts by application code to look inside an object reference to find out "where" it points to are illegal This raises the question of whether an operation such as move even makes sense within the object model If the object model has no sense of "here" and "there," why would clients, who are also part of the object model, want to move an object To impart meaning to the idea of object location, we must step out of the system and look at it from a different level of abstraction In other words, it is probably better to treat object location as an administrative aspect of CORBA rather than try to deal with it from inside the object model The server to which an object is moved may support the same protocol as the original server, but the client that instructs an object to move itself may not support the target server's protocol In other words, to guarantee that after a move the client will not lose connectivity to the object, the client would have to have knowledge of both the original server's and the target server's protocols However, making that knowledge available to the client destroys the protocol transparency of the CORBA object model The object to be moved may have persistent state in a database Assuming that you can redirect the reference to the object to now denote an object in a different server, the question remains of how the persistent state for the object can be moved Unless the source and the target server share a common data-base, it is difficult to see how this could be achieved without manual intervention You can treat the move operation as a logical copy of the physical object state, but you must be careful about its semantics The CORBA object model requires that a particular object reference must denote the same object throughout the object's lifetime After an object is destroyed, all its references must become permanently non-functional This means that an object's identity must not change during the move, and that the fact of its moving must be undetectable to all clients in the system as far as the object's semantics are concerned If you are not extremely careful, you might unwittingly violate this rule if some small detail of the object's state that is visible to clients is affected by the move The move operation raises the issue of object identity This topic is full of pitfalls and is very difficult to define precisely Object identity periodically becomes the subject of raging debate in the OMG, and it seems unlikely that agreement will ever arise from these discussions The issue is similar to a vexing identity question examined by philosophers (and science fiction writers!) If we were to record the complete physical makeup of all the matter in a person (in other words, completely capture the state of a person), we could destroy the person and keep only a recording of the person's state Assuming that later, by some miracle of technology, we could completely rebuild the state of the person so that the
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