Centralized mutual exclusion Problem : What if the coordinator fails? Solution : Elect a new one

Leader Election : Problem Statement Given is a set of n processes, 1.. n. Each process j has variables : –l.j : true iff j is a leader –up.j : true iff j has not failed This is called an auxiliary variable. Requirements –In any state, there is at most one non-failed leader always (up.j  up.k a  l.j  l.k  j = k) –eventually some process is elected a leader eventually (exists j :: up.j  l.j)

Solving Leader Election via Tree Construction You are a leader iff you are a root –Problems? In connected (but not fully connected) networks, tree construction and leader election are almost same. –Everyone needs to know how to get to the leader –The path to leader (often forms a tree)

Algorithms for Leader Election There are several algorithms for leader election We will discuss one of them: Bully Algoirthm

Bully Algorithm The goal is to choose the process with highest ID as the leader. When a process is repaired or it suspects that the current leader has failed, it starts `election' Election process : –[Step 1 :] Make sure that processes with higher ID have failed –[Step 2 :] If successful, inform all processes with lower ID that a new leader is elected

Bully Algorithm (continued) Step 1 –When process j enters the election mode, it sends an `election' message to j+1,.. n. –If process k receives the election message from j, it enters the election mode, sends an OK message to j, and sends election message to k+1, n. –If j receives an OK message, j has lost the election. –If j does not receive any OK message, j can proceed to step 2.

Bully Algorithm (continued) Step 2 : When process j enters the second step, it has checked that processes j+1.. n have failed. It needs to make sure that no process in 1..(j-1) is a leader. Process j forces processes (j-1).. 1 to accept j as the leader. Garcia-Molina suggests that this be done using RPC; contact (j-1), (j-2),..., 1. –This ensures that if two processes `know’ who the leader is then their information is the same.

Other Approaches Probabilistic Algorithm –Each node chooses a random number between [0..N] –Send the number to all –Leader = (sum of values received) mod N –Repeat if this process has failed.

Other Approaches Utilize tree construction algorithm –Provides nonmasking fault-tolerance for leader election Eventually at most one leader Will focus on the idea of diffusing computation that can help ensuring at most one leader at all times

Results So Far Lack of global clocks Logical clocks Vector Clocks Mutual Exclusion Quorum Based Token Based Leader election Causal Delivery