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Leader Election Breaking the symmetry in a system

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Ring based leader election The network is known to be a ring Ring size is unknown

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Chang-Robert’s algorithm Every process sends an election message with its id to the left process if it has not seen a message from a higher process Forward any message with an id greater than own id to the left If a process receives its own election message it is the leader It then declares itself to be the leader by sending a leader message

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Chang Roberts Leader Election Worst case message complexity Best caseWorst case

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Hirschberg-Sinclair algorithm Assume ring is bidirectional Carry out elections on increasingly larger sets Algorithm works in asynchronous rounds Only processes that win the election in round r can proceed to round r+1 Algorithm: P i is the leader in round r iff it has the largest id of all nodes that are at a distance 2 r or less from P i

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Hirschberg-Sinclair algorithm Initially: All processes are leaders Round 0: 6, 7 and 8 are leaders Round 1: 7, 8 are leaders Round 2: 8 is the only leader At most log(N) rounds

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Election on general graphs Totally connected graph – all nodes are mutually connected Homework assignment: Give a randomized algorithm to elect a leader Analyze its message and time complexity

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