The ServantLocator Interface

A servant locator has the following interface:

Slice
module Ice {
    local interface ServantLocator {
        ["UserException"]
        Object locate(Current curr, out LocalObject cookie);

        ["UserException"]
        void finished(Current curr, Object servant, LocalObject cookie);

        void deactivate(string category);
    };
};

Note that ServantLocator is a local interface. To create an actual implementation of a servant locator, you must define a class that is derived from Ice::ServantLocator and provide implementations of the locate, finished, and deactivate operations. The Ice run time invokes the operations on your derived class as follows:

  • locate

    Whenever a request arrives for which no entry exists in the active servant map (ASM), the Ice run time calls locate and supplies the Current object for the request. The implementation of locate (which you provide as part of the derived class) is supposed to return a servant that can process the incoming request. Your implementation of locate can behave in three possible ways:

    1. Locate an existing or instantiate a new servant, and return this servant for the current request. In this case, the Ice run time dispatches the request to this servant
    2. return null, which is equivalent to throwing ObjectNotExistException
    3. throw a system exception such as ObjectNotExistException, OperationNotExistException and FacetNotExistException

    You can also throw user exceptions from locate. If the user exception is in the corresponding operation's exception specification, that user exception is returned to the client. User exceptions thrown by locate that are not listed in the exception specification of the corresponding operation are also returned to the client, and then thrown in the client as UnknownUserException. Non-Ice exceptions are returned to the client as UnknownException.

    The cookie out-parameter to locate allows you to return a local object to the object adapter. The object adapter does not care about the contents of that object (and it is legal to return a null cookie). Instead, the Ice run time passes whatever cookie you return from locate back to you when it calls finished. This allows you to pass an arbitrary amount of state from locate to the corresponding call to finished.
  • finished

    If a call to locate has returned a servant to the Ice run time, the Ice run time dispatches the incoming request to the servant. Once the request is complete (that is, the operation being invoked has completed), the Ice run time calls finished, passing the servant whose operation has completed, the Current object for the request, and the cookie that was initially created by locate. This means that every call to locate is balanced by a corresponding call to finished (provided that locate actually returned a servant).

    If you throw an exception from finished, the Ice run time propagates the thrown exception back to the client. As for locate, you can throw user exceptions from finished. If a user exception is in the corresponding operation's exception specification, that user exception is returned to the client. User exceptions that are not in the corresponding operation's exception specification are also returned to the client, and then thrown by the client as UnknownUserException.

    finished can also throw run-time exceptions. However, only ObjectNotExistException, OperationNotExistException, and FacetNotExistException are propagated without change to the client; other run-time exceptions are returned to the client as UnknownLocalException.

    Non-Ice exceptions thrown from finished are returned to the client as UnknownException.

    If both the operation implementation and finished throw a user exception, the exception thrown by finished overrides the exception thrown by the operation.
  • deactivate

    The deactivate operation allows a servant locator to clean up once it is no longer needed. (For example, the locator might close a database connection.) The Ice run time passes the category of the servant locator being deactivated to the deactivate operation.

    The run time calls deactivate when destroying the object adapter to which the servant locator is attached. More precisely, deactivate is called when you call destroy on the object adapter, or when you call destroy on the communicator (which implicitly calls destroy on the object adapter).

    Once the run time has called deactivate, it is guaranteed that no further calls to locate or finished can happen, that is, deactivate is called exactly once, after all operations dispatched via this servant locator have completed.

    This also explains why deactivate is not called as part of ObjectAdapter::deactivate: ObjectAdapter::deactivate initiates deactivation and returns immediately, so it cannot call ServantLocator::deactivate directly, because there might still be outstanding requests dispatched via this servant locator that have to complete first — in turn, this would mean that either ObjectAdapter::deactivate could block (which it must not do) or that a call to ServantLocator::deactivate could be followed by one or more calls to finished (which must not happen either).

It is important to realize that the Ice run time does not "remember" the servant that is returned by a particular call to locate. Instead, the Ice run time simply dispatches an incoming request to the servant returned by locate and, once the request is complete, calls finished. In particular, if two requests for the same servant arrive more or less simultaneously, the Ice run time calls locate and finished once for each request. In other words, locate establishes the association between an object identity and a servant; that association is valid only for a single request and is never used by the Ice run time to dispatch a different request.

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