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There is a lot happening here The code converts the passed object ID to an asset number and tests whether this device is known If the conversion fails or the asset number is not known, preinvoke throws OBJECT_NOT_EXIST, which is propagated back to the client Note that the code explicitly checks whether the request is for the controller object and, if it is, returns a pointer to the controller servant This step is necessary because we assume that the controller and all devices share a single POA We use a single POA because with separate POAs, invocations for the controller and a device may be processed in parallel
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even with the SINGLE_THREAD_MODEL policy on all POAs However, in this example, we are not dealing with issues of thread safety; we cover these in 21 If the device is real, we must locate its servant The code uses the find member function on our active object map to check whether we have a servant for this device in memory If the servant is not in memory, the evictor queue may already be at its maximum size (MAX_EQ_SIZE) If it is, the code retrieves the servant pointer in the element at the head of the evictor queue, removes the servant's entry from our active object map, removes the servant from the head of the queue, and deletes the servant This action evicts the least recently accessed servant from memory (Note that we have changed the servant's m_anum member variable to be public so that preinvoke can access it This is safe because m_anum is a const member) Now there is room for a new servant, so the code instantiates a servant for the current request, enqueues the servant's pointer at the tail of the evictor queue, and updates our active object map with the servant's asset number and queue position If the servant for the request is already in memory, the code simply moves the servant's element from its current position to the tail of the evictor queue and updates our active object map with the new queue position The preceding steps work for all operations except remove, for which we must take special steps If a remove causes a servant to be brought into memory, there is no point in placing that servant in our active object map or at the tail of the evictor queue because the servant is about to be destroyed If a remove finds that a servant is already in memory, that servant is immediately removed from our active object map, again because it is about to be destroyed This logic ensures that our active object map accurately keeps track of which servants are in memory The remainder of the source code is trivial, so we do not show it here (It creates a POA with the NON_RETAIN and USE_SERVANT_MANAGER policies, creates a DeviceLocator_impl instance, and calls set_servant_manager to inform the POA of the servant locator's existence) Before we go on, be warned that the preceding code takes advantage of a guarantee provided by the STL list container: insertion and removal of an element do not invalidate iterators to other elements This property is unique to the list container You cannot replace the list implementation of the evictor queue with a deque because a deque does not guarantee that iterators to other items in the container remain valid if any part of the container is modified
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1264 Evaluating the Evictor Pattern with Servant Locators Looking at the preceding code, you can see that it is remarkably easy to implement the Evictor pattern Ignoring the class header and a few type definitions, it takes barely 30 lines of code to implement sophisticated functionality Much of the credit for this goes to the Standard Template Library (STL),[2] which supplies us with the requisite data structures and algorithms But even ignoring STL, something else remarkable is happening here: to add the Evictor pattern to our code, we did not have to touch a single line of object implementation code The servants are completely unaware that we have added a new memory management strategy to the server, and they do not have to cooperate in any way
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[2] If you are not familiar with STL, we cannot overemphasize its importance and utility We strongly recommend that you acquaint yourself with this library as soon as you can See [14] for an excellent tutorial and reference
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Being able to make such modifications without disturbing existing code is a strong indicator of clean and modular design Moreover, it shows that the POA achieves correct separation of concerns Object activation is independent of the application semantics, and the servant locator design reflects this The most valuable feature of the Evictor pattern is that it provides us with precise control over the memory consumption and performance trade-off for the CCS server A longer evictor queue permits more servants to be active in memory and results in better performance; a shorter queue reduces performance but also reduces the memory requirements of the server You must be aware, however, of one potential pitfall: if the evictor queue is too small, performance will fall off dramatically This happens if there are more objects being used by clients on a regular basis than the server can hold in memory In that case, most operation invocations from clients cause one servant to be evicted and another servant to be instantiated, and that is expensive The problem is similar to that of thrashing in a demand-paged operating system if the working set of a process does not fit in memory [13]; if the "working set of objects" does not fit into the evictor queue, the server spends much of its time evicting and instantiating servants instead of servicing requests The Evictor pattern is an important tool that can help servers achieve high performance without consuming massive amounts of memory Object systems exhibit locality of reference (see [13]) just as ordinary processes do; it is rare for clients to be uniformly interested in all or almost all of the objects implemented by a server Instead, client activity is typically focused on a group of objects for quite some time and then shifts to a new group of objects The caching nature of the Evictor pattern makes it well suited to this behavior Another important way of achieving high performance is to use the USE_DEFAULT_SERVANT policy value, which allows you to handle invocations for many different CORBA objects with a single servant (see Section 1174) Default
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