Election Algorithms

Many of the algorithms for distributed systems require some centralization,
    some site with leader/coordinator activities.

All other sites need to recognize this leader;
    often there is accomplished by an initial agreement/election.

When the site of the leader fails or goes down for some reason,
    it is necessary to elect a new leader.

This is the purpose of election or agreement algorithms.

There are two basic criteria for an election/agreement algorithm.

• One way to decide the leader is to use some global priority.
      The Bully algorithm by Garcia-Molina (1982)
          falls into this category.
• The second is a more general, preference-based algorithm,
      that permits some nodes to have heavier votes.
  Chang & Roberts's Token Ring Election algorithm belongs here.

Assumptions for most election algorithms:

• A complete topology,
      i.e. one message hop between any two processes.
• All process ids are unique and known to all other processes.
• All communication networks are reliable,
      i.e. only communicating processes may fail.
  This assures that no messages are
  • lost,
  • duplicated,
  • corrupted.
• A recovering process is aware that it failed.
  • Failure is reliably detected by setting the time-out interval to be a little larger than the sum of the round-trip message delay and the message processing time.
  • A failed process can rely on the coordinator to poll periodically for recovered process so that they may rejoin the pool of processes.

The following pages discuss a few such algorithms:

• The Bully algorithm by Garcia-Molina (1982)
• The Token Ring Election algorithm by Chang & Roberts
• A Tree-based Election algorithm

Yet another algorithm is related to the Byzantine General paper by Lamport et al.

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