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Asynchronous timers have the habit of going off at the most inopportune moments Often, at the time the signal arrives, the server is not in a state in which it can react to the signal immediately and reap the servant whose timer has expired In that case, the signal handler must deposit the expired timer information in a global data structure that can be checked later for expired timers Implementing the required logic can be quite difficult Synchronous timers require the server to explicitly check to collect the expired timer information However, if a server is single-threaded, it may not be able to check For example, if the server uses the blocking ORB::run operation, an extended period of idle time will cause the server to go to sleep in the ORB's event loop During that time, there is no way for the server to invoke an API call because its only thread of control is blocked in the event loop The server could instead write its own event loop that polls the ORB for work items using ORB::work_pending and ORB::perform_work and polls its timers whenever the ORB is not busy, but writing and maintaining these loops can be tedious Depending on their implementation, timers can be quite heavyweight In addition, the number of available timers is often severely limited by the underlying OS, so there may not be enough timers for all servants that need them The timer approach is best suited for servers that are multithreaded In that case, we can use a separate reaper thread to clean up servants The reaper thread can block until a timer expires, synchronously clean up the expired servant, and go back to sleep until the next timer expires On the other hand, if a server is not multithreaded, the Evictor pattern is typically easier to use as a garbage collector than are time-outs 1275 Explicit Keep-Alive We can make clients responsible for keeping servants alive by adding a ping operation to each interface By default, the servant will be garbage-collected after some period of idle time If the client does not want to invoke ordinary operations on the servant for some time but still wants to ensure that the servant is not reaped by the server, the client must call the ping operation to reset the servant's timer Because this approach uses timers, it suffers all the drawbacks of the pure timer approach In addition, it makes the presence of garbage collection visible to the client and requires the client to actively participate This pollutes IDL interfaces with operations that have nothing to do with the interfaces' logical functions In addition, requiring the client to call a ping operation simply shifts the problem from server to client without solving it Having to call a ping operation periodically may be just as inconvenient to the client as polling a timer would be to the server 1276 Reverse Keep-Alive per Object Reverse keep-alive requires the client to pass a callback object to the server For example:
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interface ShortLived {
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short do_something(); void destroy();
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interface KeepAlive { void ping(in ShortLived obj); }; interface ShortLivedFactory { ShortLived create(in KeepAlive cb_obj); };
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When the client creates an object, it must also supply a reference to a KeepAlive object to the server The KeepAlive object is implemented by the client, and the server periodically invokes the ping operation to see whether the client still wants the object If the ping from server to client fails for example, with OBJECT_NOT_EXIST the server reaps the corresponding servant Although initially attractive, this technique has a number of serious drawbacks The client must somehow maintain the association between its KeepAlive objects and the references returned by the factory because there are as many KeepAlive objects in the client as there are ShortLived objects in the server The client must deliberately fail the server's ping operation if it no longer wants to use the corresponding ShortLived object for example, by destroying its own KeepAlive object However, garbage objects are often created when the client forgets to call destroy and not just because of network failure If the client forgets to call destroy, then it presumably will also forget to destroy its KeepAlive object, so the problem we have now may in fact be worse then the original one The keep-alive technique doubles the number of objects in the system because each ShortLived object created in the server is paired with its corresponding KeepAlive object in the client Doubling the number of objects in the system may be too expensive in terms of resources such as memory and network connections For the client to offer a callback object to the server, the client must act as a server for the duration of the callback This complicates the implementation of the client because, at the very least, the client must have a POA and run an event loop Depending on the features provided by the client's ORB, this may require the client to be multithreaded It is not reasonable to expect clients to add multithreading to their implementation just so that they can respond to callbacks The server is burdened by the need to invoke callbacks on the KeepAlive objects in the client This not only adds to the complexity of the server but also adds networking overhead If the number of objects in the system is large or if the time-out interval between keep-alive callbacks is too short, a substantial amount of network bandwidth can
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