TAO/docs/leader_follower.html

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   4     <title>Implementation of leader follower model for global resources</title>
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  14         <h1>Leader follower model for global resources</h1>
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  16           <h2>Context</h2>
  17           Global resources in TAO mean one ORB and using the reactive strategy
  18           one Reactor. TAO uses the ACE Select Reactor for this purpose.
  19           One main problem with multithreading and only one Reactor is that
  20           only one thread can wait in handle_events at a time.
  21           Handle_events is called, when a thread expects input, meaning
  22           it is blocking to wait for it.
  23
  24           <h2>Idea</h2>
  25           One solution to this problem is to use a leader-follower model,
  26           which partitions the set of threads wanting to wait for input
  27           into one leader and followers. Every thread is eligible to become
  28           a leader. A thread wanting to wait while nobody else is waiting
  29           becomes the leader. If the leader gets its input, which is in
  30           this case its response, it will select a new leader out of the
  31           set of followers. The followers wait on a condition variable they
  32           own and register it with the ORB core. The ORB core is thereby
  33           responsible for making this access thread-save by providing
  34           a lock for the list of followers with some flags, like
  35           leader_available.
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  37           <h2>Implementation</h2>
  38           The above mentioned condition variables are owned by the
  39           connection handlers, because responses are expected per connection,
  40           not necessarily, thinking about other resource models, per thread.
  41           <p>
  42             The involved classes are TAO_ORB_Core, TAO_Client_Connection_Handler
  43             and TAO_ORB. In the TAO_ORB_Core class the list of followers, a lock,
  44             a leader reference counter and a leader thread ID were added.
  45             The handle_input and send_request methods on the TAO_Client_Connection_Handler
  46             contain 95% of the code used for implementing the leader-follower
  47             model. Care has to be taken, that the connection handlers are
  48             suspended and resumed, properly. A special concern is that
  49             no deadlocking occurs.
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  52             For more details and questions,
  53             <p>
  54
  55               <address><a href="mailto:mk1@cs.wustl.edu">Michael Kircher</a></address>
  56               <p>
  57                 <address><a href="mailto:irfan1@cs.wustl.edu">Irfan Pyarali</a></address>
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