Elections in a Distributed Computing System

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Elections in a Distributed Computing System

• Hector Garcia-Molina

Presenter Srinath Rao
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Introduction to Elections

• Strategies to deal with a node failure
• Have s/w which can operate continuously even as
  failures occur
• Halt temporarily, reorganize the system
• Need for the coordinator and hence election
• Election protocols can be used to start up a
  system, add/remove nodes
• Issues ?

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Issues

• Constituent nodes may fail after election
• What does it mean to be a coordinator?
• How to cope up with failures during the election
  itself?
• Always possible to select a unique coordinator?
• Might wish to have more than one coordinator

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Outline

• Assumptions
• Elections with no commn. failures
• The Bully Election Algorithm
• Elections with commn. failures
• The Invitation Election Algorithm
• Related Work
• Conclusions

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Assumptions

• All nodes cooperate
• Election algorithm makes use of bug-free
  software facilities
• Communication subsystem will not spontaneously
  generate messages
• Nodes have safe storage cells
• Node halts processing when it fails
• No transmission errors

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Assumptions (contd..)

• Messages are processed in the order they are
  received
• Communication subsystem does not fail
• Node never pauses

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State Vector of node

• A collection of safe storage cells
• Principal components of Vector S(i)
• Status of node i S(i).s
• Down, Election, Reorganization, Normal
• Coordinator according to node i S(i).c
• Definition of the Task being performed S(i).d

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Outline

• Assumptions
• Elections with no commn. failures
• The Bully Election Algorithm
• Elections with commn. failures
• The Invitation Election Algorithm
• Related Work
• Conclusions

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Desired Characteristics

• Assertion 1 For two nodes i and j
• S(i).c S(j).c if nodes i and j are in one of
  the states Normal or Reorganization
• S(i).d S(j).d if both i and j are in Normal
  state
• States what it means to be a coordinator

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Desired Characteristics (contd..)

• Assertion 2 If no failures, election will
  eventually transform a system in any state to a
  state
• There exists node i with S(i).s Normal and
  S(i).c i
• Other active nodes j have S(j).s Normal and
  S(j).c i

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The Bully Election Algorithm

• Each node has an unique id no.
• Algorithm uses id no. as priorities
• Two step algorithm
• Node i tries to contact all nodes with higher
  priorities. If no reply received, then assume the
  role of coordinator
• Inform all the lower priority nodes
• Send halt message, force state of j to
  Election
• Send I am elected message, node j sets S(j).cI
• and S(j).s Reorganization
• Distribute new algorithms to nodes, all status
  changed to Normal

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Bully (contd..)

• Let the recovering node k attempt to become the
  coordinator using the same algorithm
• Halts all lower priority nodes which may be in
  the process of becoming coordinators
• Step 1 ensures no conflict with higher priority
  nodes

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Outline

• Assumptions
• Elections with no commn. failures
• The Bully Election Algorithm
• Elections with commn. failures
• The Invitation Election Algorithm
• Related Work
• Conclusions

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Discussion

• Failures
• Partitioning of nodes
• A node can only send/receive message
• Node i and node j can talk to node k but not with
  each other
• Node may pause and then resume
• Observation Impossibility of consensus in the
  event of failure of commn. subsystem or node
  pausing.
• Inference Redefine the meaning of an election

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Discussion (contd..)

• Notion of a group of nodes and group id
• Node i stores group id in its state vector
  S(i).g
• Nodes are free to change groups
• Identify messages with group id
• Coordinator is unique within a group

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New Desired Characteristics

• Assertion 3 For two nodes i and j
• S(i).c S(j).c if nodes i and j are in the same
  group and are in one of the states Normal or
  Reorganization
• S(i).d S(j).d if both i and j are in Normal
  state and are in the same group

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Desired Characteristics (contd..)

• No requirements for nodes with only one-way
  communication
• Assertion 4 If no failures, election will
  eventually transform a set of nodes R that have
  two way communication in any state to a state
• There exists node i with S(i).s Normal and
  S(i).c i
• Other active nodes j in R have S(j).s Normal
  and S(j).c i and S(j).g S(i).g

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The Invitation Election Algo.

• A node invites other nodes to join it in
  forming a new group
• A node may accept or decline an invitation
• Make a receiving node form a new group with
  itself the coordinator and the only member
• Objective to merge groups
• Coordinators periodically send invite message
• Can be used instead of the Bully algorithm

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Related Work

• Scott D. Stoller (2000)
• Modifies Bully algorithm to work with crash
  failures
• Points out a flaw and proposes a new
  specification
• Gurdip Singh (1996)
• Proposes an algorithm for leader election in the
  presence of link failures

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Conclusions

• Meaning of an election depends on the possible
  types of failures
• Paper studied elections in two representative
  failure environments
• Postulated assertions that define concept of an
  election
• Presented an election algorithm for each
  environment