Election Algorithms

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Election Algorithms
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Topics

• Issues
• Detecting Failures
• Bully algorithm
• Ring algorithm

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Readings

• Van Steen and Tanenbaum 5.4
• Coulouris 11.3

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Election Algorithms

• Remember using Lamport clocks for total order
• Can you think of another way to do this?
• It turns out that you can use a sequencer.
• All operations go to a sequencer
• The sequencer assigns numbers to each message
  before the message goes to each replica
• What if the sequencer goes down?

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Election Algorithms

• Many distributed algorithms require a process to
  act as a coordinator.
• The coordinator can be any process that organizes
  actions of other processes.
• A coordinator may fail
• How is a new coordinator chosen or elected?

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Election Algorithms

• Assumptions
• Each process has a unique number to distinguish
  them.
• One process per machine (which suggests that an
  IP address can be the unique identifier)
• Processes know each others process number
• Processes do not know which ones are currently up
  and which ones are down.
• General Approach
• Locate the process with the process with the
  highest process number and designate it as the
  coordinator.
• Election algorithms differ in how they do this.

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Issues in Dealing with Coordinator Failure

• Detecting Failure
• Any node might detect failure first
• Multiple processes might detect failure at once.
• Election
• Must run without coordination
• Must deal with arbitrary process failures
• All nodes must agree on when election is over and
  who the new coordinator is.

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Detecting Failures

• Timeouts are used to detect failures
• T 2Ttrans Tprocess
• Where Ttran is maximum transmission delay and
  Tprocess represents the maximum delay for
  processing a message.
• If a process fails to respond to a message
  request within T seconds then an election is
  initiated.

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Bully Algorithm

• When a process, P, notices that the coordinator
  is no longer responding to requests, it initiates
  an election.
• P sends an ELECTION message to all processes with
  higher numbers.
• If no one responds, P wins the election and
  becomes a coordinator.
• If one of the higher-ups answers, it takes over.
  Ps job is done.

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Bully Algorithm

• When a process gets an ELECTION message from one
  of its lower-numbered colleagues
• Receiver sends an OK message back to the sender
  to indicate that he is alive and will take over.
• Receiver holds an election, unless it is already
  holding one.
• Eventually, all processes give up but one, and
  that one is the new coordinator.
• The new coordinator announces its victory by
  sending all processes a message telling them that
  starting immediately it is the new coordinator.

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Bully Algorithm

• If a process that was previously down comes back
• It holds an election.
• If it happens to be the highest process currently
  running, it will win the election and take over
  the coordinators job.
• Biggest guy always wins and hence the name
  bully algorithm.

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The Bully Algorithm (Example)

• The bully election algorithm
• Process 4 holds an election
• Process 5 and 6 respond, telling 4 to stop
• Now 5 and 6 each hold an election

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The Bully Algorithm (Example)

1. Process 6 tells 5 to stop
2. Process 6 wins and tells everyone

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Bully AlgorithmAnalysis

• Best case
• The node with second highest identifier detects
  failure
• Total messages N-2
• One message for each of the other processes
  indicating the process with the second highest
  identifier is the new coordinator.
• Worst case
• The node with lowest identifier detects failure.
  This causes N-1 processes to initiate the
  election algorithm each sending messages to
  processes with higher identifiers.
• Total messages O(N2)

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Bully Algorithm Discussion

• How many processes are used to detect a
  coordinator failure?
• As many as you want. You could have all other
  processes check out the coordinator.
• It is impossible for two processes to be elected
  at the same time.

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Ring Algorithm

• Use a ring (processes are physically or logically
  ordered, so that each process knows who its
  successor is).
• Algorithm
• When a process notices that coordinator is not
  functioning
• Builds an ELECTION message (containing its own
  process number)
• Sends the message to its successor (if successor
  is down, sender skips over it and goes to the
  next member along the ring, or the one after
  that, until a running process is located).
• At each step, sender adds its own process number
  to the list in the message.

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Ring Algorithm

• Algorithm (continued)
• When the message gets back to the process that
  started it all
• Process recognizes the message that contains its
  own process number
• Changes message type to COORDINATOR
• Circulates message once again to inform everyone
  else Who the new coordinator is (list member
  with highest number) Who the members of the new
  ring are.
• When message has circulated once, it is removed.
• Even if two ELECTIONS started at once, everyone
  will pick same leader since node with highest
  identifier is picked.

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Ring Algorithm
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Ring Algorithm
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Ring Algorithm contd
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Ring Algorithm Analysis

• At best 2(N-1 ) messages are passed
• One round for the ELECTION message
• One round for the COORDINATOR
• Assumes that only a single process starts an
  election.
• Multiple elections cause an increase in messages
  but no real harm done.

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Summary

• Synchronization between processes often requires
  that one process acts as a coordinator.
• The coordinator is not fixed.
• Election algorithms determine the coordinator.