-a group of processes on different machines need to choose a coordinator
-peer to peer communication: every process can send messages to every other process.
-Assume that processes have unique IDs, such that one is highest
-Assume that the priority of process Pi is i
(a) Bully Algorithm
Background: any process Pi sends a message to the current coordinator; if no response in T time units, Pi tries to elect itself as leader. Details follow:
Algorithm for process Pi that detected the lack of coordinator
Algorithm for other processes (also called Pi)
If Pi is not the coordinator then Pi may receive either of these messages from Pj
if Pi sends “Elected Pj”; [this message is only received if i < j]
Pi updates its records to say that Pj is the coordinator.
Else if Pj sends “election” message (i > j)
Pi sends a response to Pj saying it is alive
Pi starts an election.
(b) Election In A Ring => Ring Algorithm.
-assume that processes form a ring: each process only sends messages to the next process in the ring
- Active list: its info on all other active processes
- assumption: message continues around the ring even if a process along the way has crashed.
Background: any process Pi sends a message to the current coordinator; if no response in T time units, Pi initiates an election
If a process receives an “Elect(j)” message
(a) this is the first message sent or seen
initialize its active list to [i,j]; send “Elect(i)” + send “Elect(j)”
(b) if i != j, add i to active list + forward “Elect(j)” message to active list
(c) otherwise (i = j), so process i has complete set of active processes in its active list.
=> choose highest process ID + send “Elected (x)” message to neighbor
If a process receives “Elected(x)” message,
set coordinator to x
Suppose that we have four processes arranged in a ring: P1 à P2 à P3 à P4 à P1 …
P4 is coordinator
Suppose P1 + P4 crash
Suppose P2 detects that coordinator P4 is not responding
P2 sets active list to [ ]
P2 sends “Elect(2)” message to P3; P2 sets active list to 
P3 receives “Elect(2)”
This message is the first message seen, so P3 sets its active list to [2,3]
P3 sends “Elect(3)” towards P4 and then sends “Elect(2)” towards P4
The messages pass P4 + P1 and then reach P2
P2 adds 3 to active list [2,3]
P2 forwards “Elect(3)” to P3
P2 receives the “Elect(2) message
chooses P3 as the highest process in its list [2, 3] and sends an
P3 receives the “Elect(3)” message
P3 chooses P3 as the highest process in its list [2, 3] + sends an “Elected(P3)” message
Byzantine Generals Problem
Intuition: Only want to proceed with the plan of attack if they are sure everyone else agrees
Can't trust other generals.
-If generals can't trust one another they can never be sure if they should attack.