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Distributed object systems are designed to support long-lived persistent objects. Given that these systems will be made up of many thousands (perhaps millions) of such objects, it would be unreasonable for object implementations to become active and remain active, taking up valuable system resources, for indefinite periods of time. In addition, clients need the ability to store persistent references to objects so that communication among objects can be re-established after a system crash, since typically a reference to a distributed object is valid only while the object is active.
Object activation is a mechanism for providing persistent references to objects and managing the execution of object implementations. In RMI, activation allows objects to begin execution on an as-needed basis. When an activatable remote object is accessed (via a method invocation) if that remote object is not currently executing, the system initiates the object's execution inside an appropriate JVM.
An active object is a remote object that is instantiated and exported in a JVM on some system. A passive object is one that is not yet instantiated (or exported) in a JVM, but which can be brought into an active state. Transforming a passive object into an active object is a process known as activation. Activation requires that an object be associated with a JVM, which may entail loading the class for that object into a JVM and the object restoring its persistent state (if any).
In the RMI system, we use lazy activation. Lazy activation defers activating an object until a client's first use (i.e., the first method invocation).
Lazy activation of remote objects is implemented using a faulting remote reference (sometimes referred to as a fault block). A faulting remote reference to a remote object "faults in" the active object's reference upon the first method invocation to the object. Each faulting reference maintains both a persistent handle (an activation identifier) and a transient remote reference to the target remote object. The remote object's activation identifier contains enough information to engage a third party in activating the object. The transient reference is the actual "live" reference to the active remote object that can be used to contact the executing object.
In a faulting reference, if the live reference to a remote object is
null, the target object is not known to be active. Upon method invocation, the faulting reference (for that object) engages in the activation protocol to obtain a "live" reference, which is a remote reference (such as a unicast remote reference) for the newly-activated object. Once the faulting reference obtains the live reference, the faulting reference forwards method invocations to the underlying remote reference which, in turn, forwards the method invocation to the remote object.
In more concrete terms, a remote object's stub contains a "faulting" remote reference type that contains both:
Note - The RMI system preserves "at most once" semantics for remote calls. In other words, a call to an activatable or unicast remote object is sent at most once. Thus, if a call to a remote object fails (indicated by a
RemoteExceptionbeing thrown), the client can be guaranteed that the remote method executed no more than once (and perhaps not at all).