SD5 Supporting Interfaces and Classes in Java

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SD5 Supporting Interfaces and Classes
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O'Reilly Network Safari Bookshelf Jini Specifications, The, Second Edition To understand this situation, it might help to first consider the conditions under which a service is normally discarded from the cache and then rediscovered An entity typically discards a service when the entity determines that the service has become unavailable Recall that a service usually becomes unavailable to an entity when the service crashes, the service is shut down, or the link between the entity and the service experiences a network partition Under normal circumstances, when a well defined service becomes unavailable because it has crashed or has been shut down, and the entity after determining that the service is unavailable discards the service, the cache will rediscover the service when the service comes back on line The service is rediscovered because a well behaved service will typically reregister with each lookup service with which it was registered prior to crashing or shutting down Note that such a service will reregister even when its original lease with a lookup service is still valid When the service reregisters with a lookup service, the lookup service notifies the cache's listener that a reregistration has occurred, and the service is then rediscovered A special case of the scenario just described involves services that choose to persist their leases Typically, when a service that persists its leases comes back on line after a crash or a shutdown, the service will not reregister with any lookup service for which the associated lease is still valid If none of the service's leases expire during the period in which the service is down, then when the service comes back on line, it will never reregister with any of the desired lookup services, and the cache will never be notified that the discarded service has become available once again Therefore it is important to note that there are conditions that may hinder rediscovering certain types of services that were discarded as a result of a crash or shutdown This situation should not occur with any frequency because services that persist their leases are expected to be less common than other types of services However, there is a common scenario in which any type of service may be discarded but never rediscovered This new scenario is characterized not by service crashes or shutdowns, but by communication failures In this situation, communication failures cause only the entity to view the service as unavailable; that is each lookup service in the managed set can still communicate with the service As with service crashes or shutdowns, communication failures between the entity and the service can also cause the entity to discard the service But prob lems can arise when the communication failures occur between the entity and the service, but not between the service and any of the lookup services in the managed set Although the service never goes down, it is still discarded by the entity because the inability to communicate with the service causes the entity to view the service as unavailable But because the service can still communicate with the lookup services, the service will continue renewing its residency in each lookup service Thus, since none of the service's leases expire, the service never reregisters with any of the lookup services, and the lookup services will never send events to the cache's listener that cause the service to be rediscovered To address the scenarios described above, all implementations must do the following when a service is discarded from the cache: Place the reference to the discarded service in separate storage, and remove the reference from the cache's storage (to guarantee that subsequent queries of the cache do not return that same unavailable reference) Wait an implementation dependent amount of time that is likely to exceed the typical service lease duration If a ServiceEvent with a transition equal to TRANSITION_MATCH_NOMATCH is received (indicating that the service's lease has expired), then the service reference that was set aside can be flushed, and the service is then truly discarded If such a ServiceEvent is not received (indicating that a transient communication failure probably occurred), the service reference that was set aside should be placed back in the cache's local storage, and if the entity is registered for events from the cache, the appropriate event should be sent to the entity's registered listener The terminate method performs cleanup duties related to the termination of the processing being performed by a particular instance of LookupCache For that instance, this method cancels all event leases granted by the lookup services that supplied the contents of the cache, and unexports all remote listener objects registered with those lookup services The terminate method is typically called when SD5 Supporting Interfaces and Classes 244
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O'Reilly Network Safari Bookshelf Jini Specifications, The, Second Edition the entity is no longer interested in the contents of the LookupCache Upon completion of the termination process, the semantics of all current and future method invocations on the current instance of LookupCache are undefined
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