Figure 6.6:Maekawa’s algorithm

• Fault tolerance
  • Main points in evaluating the above algorithms with respect to fault tolerance are:
    • What happens when message are lost?
    • What happens when a process crashes?
  • None of the algorithms above would tolerate the loss of messages, if the channels were unreliable.
  • The ring-based cannot tolerate a crash failure of any single process. 
  • Maekawa’s algorithm can tolerate some process crash failures
    • If a crashed process is not in a voting set that is required, then its failures will not affect the other processes.
  • The central server algorithm can tolerate the crash failure of a client process that neither holds nor has requested the token. 
  • The Ricart and Agrawala algorithm can be adapted to tolerate the crash failure of such a process, by taking it to grant all requests implicitly. 
  • Even with a reliable failure detector, care is required to allow for failures at any point (including during a recovery procedure), and to reconstruct the state of the processes after a failure has been detected.