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The client might, from maliciousness or ignorance, neglect to call destroy A bug in the client might cause it to crash before it can call destroy The network between client and server might be disrupted A power failure on the client side might prevent the client from ever calling destroy These are only some of the things that can cause a call to create without a corresponding call to destroy In a surprisingly short amount of time (possibly only minutes), such problems can cause a server to crash because it runs out of memory or can degrade performance to the point that the server might as well be dead What we are looking for is a way for the server to get rid of the unused servants, or to garbage-collect them We examine a number of techniques to do this in the following sections For the time being, we restrict ourselves to discussing the garbage collection of servants In Section 128, we turn to the issue of garbage collection of CORBA objects and explain how we might achieve it 1272 Garbage Collection by Shutting Down The suggestion may seem naive at first glance, but an entirely viable option can be to get rid of garbage by simply shutting down the server In fact, this is precisely the strategy used by many production systems to deal with memory leaks If the leaks are not too serious, it may be sufficient to shut down a server briefly, say at midnight each day, and to restart the server with a clean slate[3]
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[3] We are serious here We have seen more than one production system employing this strategy Especially for large systems that have been maintained over years, shutting down once per day can be much more cost-effective than trying to track down and fix all the memory leaks
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For a CORBA server, shutdown may well be a viable option In particular, as you will see in Section 141, most ORBs can automatically activate a server on demand and stop a server after a period of idle time These features are non-standard, but we might as well take advantage of them if they are available If some garbage objects have accumulated in the server but there are periods of idle time in between invocations that are longer than the server's idle time-out, the automatic server shutdown cleans up all the servants for transient objects (We are tacitly assuming that the server will shut down cleanly and properly destroy its servants Simply exiting is not an option in environments such as embedded systems or Windows 98, where the operating system does not guarantee to clean up after a process) Shutting down the server may not be an option because some servers simply cannot be switched off even for brief periods In addition, if clients present the server with a continuous work load, there may never be an idle period that is long enough for the server's idle time-out to trigger, so we need better solutions 1273 Using the Evictor Pattern for Garbage Collection
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The Evictor pattern we discuss in Section 126 can make an effective garbage collector The Evictor pattern takes care of automatically disposing of unused servants If servants are created and lost by clients, no more invocations arrive for these servants This means that, quite quickly, unused servants migrate to the head of the evictor queue, where they are reaped when an invocation arrives for a servant that is not yet in memory Using the Evictor pattern, the worst case is that all the servants on the evictor queue are garbage and therefore consume memory that may be better used elsewhere However, we can typically afford this, because we would be consuming the same amount of memory if all these servants were still in use Before we continue discussing other options for garbage collection, we strongly recommend that you give serious consideration to using the Evictor pattern for garbage collection of servants The Evictor pattern, with minor variations, is the only reliable option we are aware of that is easy to implement and non-intrusive The techniques that follow either are more difficult to design and implement correctly, or they pollute the IDL interfaces with garbage collection operations 1274 Using Time-Outs for Garbage Collection Another way to get rid of unused servants is to equip each servant with a timer When the client creates a new object, it can specify a time-out value via a parameter to the factory operation, or, alternatively, the server can assign a default time-out value The servant implementation in the server resets each servant's timer whenever a client invokes an operation (Doing this is especially easy if we use a servant locator's preinvoke operation to reset the timer) When a servant's timer expires, the servant commits suicide Time-outs are quite similar to the Evictor pattern In both cases, the server applies a heuristic to determine when a servant should be destroyed In the case of the Evictor pattern, the heuristic is the expected frequency with which new servants are activated, which determines how long it will take on average for an unused servant to get pushed off the end of the evictor queue With time-outs, the heuristic is simply the amount of time that must elapse before a servant is considered stale and is reaped The time-out approach shares some problems with the Evictor pattern In particular, choosing an appropriate time-out value can be difficult Allowing the client to select the time-out value is dangerous because clients are likely to play it safe and to select a long time-out Assigning a default time-out in the server can also be difficult because the server often has little idea of the behavior patterns of its clients If the time-out is too long, too many garbage servants may accumulate, whereas if the time-out is too short, the server can end up destroying a servant that is still in use by a client Apart from the problems shared with the Evictor pattern, time-outs add their own problems Time-outs can be delivered to a servant asynchronously in the form of a signal or other interrupt-like mechanism, or the server can ask for time-outs synchronously by calling an API call that delivers expired timers Neither approach is ideal
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