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The second data structure we need to implement LRU eviction is a simple queue Each item on the queue represents a servant in memory For example, we could store a C++ pointer to a servant in the queue items, or we could store the servant's object ID instead The main point is that we can uniquely identify each instantiated servant with the information in each queued item Initially, when the server starts up, the evictor queue is empty Whenever a client request arrives, the servant locator's preinvoke operation is called, and it first looks in our STL map for the required servant If the servant is already in memory, preinvoke returns a pointer to the servant If the servant is not in memory, preinvoke instantiates it, adds an entry for the servant to our private active object map, and adds a new item for the servant to the tail of the queue Figure 122 shows the evictor queue after preinvoke has been called for the first five objects used by clients after server start-up The order of items in the queue indicates the order of instantiation The item corresponding to the servant that was instantiated first appears rightmost in the queue that is, as the oldest item
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Figure 122 An evictor queue after instantiating five servants
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Here is the sequence of events for instantiating a new servant as shown in Figure 122 A client invokes an operation The POA calls preinvoke on the servant locator The servant locator instantiates the servant The servant locator adds an item for the servant at the tail of the queue Note that the arrows from the queue items to the servants do not necessarily indicate pointers As pointed out earlier, we could store a C++ pointer in each queue item, but we also could store an asset number or the servant's object ID Let us assume that our queue is limited to holding only five items and that the client sends a request for object ID 6, which is not yet in memory Again, when the request arrives, the POA calls the preinvoke operation on the servant locator However, the implementation of preinvoke now realizes that the queue is full As a result, preinvoke removes the oldest servant's item from the head of the queue It then deletes
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this oldest servant before instantiating a new servant and adding the new servant's item to the tail of the queue The entire process is illustrated in Figure 123
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Figure 123 Eviction of servant 1 from the queue
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The sequence of events in Figure 123 is as follows A client invokes an operation on the object with ID 6 The POA calls preinvoke on the servant locator The servant locator's preinvoke realizes that the evictor queue is full and dequeues the item at the head (object 1) preinvoke either deletes the servant immediately or, in a multithreaded server, calls _remove_ref to decrement the servant's reference count preinvoke instantiates the servant for object 6 preinvoke adds an item for object 6 to the tail of the queue and returns control to the POA The POA dispatches the request to the new servant and then later invokes postinvoke on the servant locator (which does nothing in our implementation) The net effect of these events is that we start with five servants and we finish with five servants because we have evicted the oldest servant from memory to make room for the newest servant 1262 Maintaining LRU Order The remaining question is how to maintain the queue in LRU order Conceptually, we want to ensure that every operation that is dispatched to a servant causes that servant to be dequeued from its current queue position and to be moved to the tail of the queue Achieving this goal is simple in our implementation because the preinvoke operation is called on every request whether or not the servant is in memory If preinvoke finds a servant in memory, it moves the servant's item to the tail of the queue
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