TAO/docs/forwarding.html

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   4     <title>Implementation of location forwarding</title>
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  14         <h1>Location forwarding</h1>
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  16           <h2>Context</h2>
  17
  18           The motivation to support location forwarding for objects is
  19           to allow objects to move or forward certain requests to
  20           other objects.  Moving of objects is very important for the
  21           Common Object Services LifeCycle Service. An object
  22           complying to the LifeCycleObject interface, defined by the
  23           LifeCycle Service should support the move() operation. The
  24           move() operation allows the client to keep its object
  25           reference to the object, but the object is going to be
  26           relocated on the same or a different server.  Making
  27           location forwarding transparent to the client is the most
  28           important issue.
  29
  30           <h2>Communication between server and client</h2>
  31
  32           GIOP defines a message named "LOCATION_FORWARD", which
  33           should be used to inform the client stub, that the object
  34           has been moved. The message body has to contain an object
  35           reference to the new location of the forwarded object.
  36
  37           <h2>Server side implementation</h2>
  38
  39           Two approaches are possible, one is to use a POA servant
  40           locator and the other is to use a servant activator.
  41
  42           <h2>Client side implementation</h2>
  43
  44           The client has to expect the location forwarding GIOP
  45           message and should respond to it in setting the Profile of
  46           its Object right.
  47
  48           <h2>Conclusion</h2>
  49
  50           Changing the Profile is transparent to the client. So the
  51           client gets no idea that the object now resides somewhere
  52           else.
  53
  54           The result of the above mentioned solution is that if an
  55           object moves the client will notice it with the next call to
  56           the object. If the object moves again, the original location
  57           is not bothered at all again. Say if the original location
  58           was A, then it moved to B and finally to C. First location A
  59           responded with a GIOP Location Forward to the client, then B
  60           gets used and finally after moving the object to C, B will
  61           send a GIOP Location Forward and location C gets used.
  62
  63           There is "no" concept of a home location. If the object
  64           moves very often and old servers die it might be a problem,
  65           because clients, which did not call on the object lately
  66           will not know where to search. Though in the situation of a
  67           home location, there is also the risk that this server might
  68           die and the object is not reachable any more.
  69
  70           <h2>Optimization</h2>
  71
  72           In the case, when the object moves several times, a chain of
  73           forwarding servers is created. But this chain might be
  74           easily disturbed by just one server failing. A good idea is
  75           to give the servant locator more intelligence to tell all
  76           the other/previous servers where the object is now.  This
  77           will of course increase the communication overhead in the
  78           case of a move, but we get a high reliability against dying
  79           hosts.
  80
  81           <HR>
  82             For more details and questions,
  83             <p>
  84
  85               <address><a href="mailto:mk1@cs.wustl.edu">Michael Kircher</a></address>
  86               <p>
  87                 <address><a href="mailto:irfan1@cs.wustl.edu">Irfan Pyarali</a></address>
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