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An Election Algorithm In Distributed Systems Information Technology Essay
The Bully Algorithm is a method to elect a new coordinator in a distributed system when the current coordinator disappears for some reason. I am just wondering what happens when two processes realise the demise of the coordinator simultaneously and both decide to hold an election using the Bully Algorithm? Both processes will anounce themselves. However, the one with the highest indentifier will become the leader. For example, 3 and 4 announce themselves. Then 3 knows 4 is the new leader.
No process replies. The Stack Overflow podcast is back! After a certain time or period, process Q does not receives any Coordinator message, it will repeat the algorithm again. Now 3 comparing the priority number and select the highest process process 6 and send a message to its GRANT. Token ring algorithm is totally different with Bully algorithm. It achieves mutual exclusion by creating a bus network of processes in distributed system. It does not have a real ring in the network but a logical ring is constructed with all processes and all processes are assigned a position in the ring.
All the processes know who is before them and next after them in the line. So it starts an election, sending a message containing its id to the next node in the ring. Process 3 receives the coordinator message, and stops it. This algorithm is modified from Token Ring algorithm.
It is modified to reduce the number of message passing and additional message being sent to the elected leader. When the leader has crashed and been noticed by a node, it will send its ID number to the node next to it in the ring.
Sachi Choudhary Shukla - Department of Computer Science | UPES
It is not necessary for all nodes to send their IDs into the ring. The receiving node compares the received ID with its own, and forwards whichever is the greatest at this moment. But this comparison is done by all the nodes such that only the greatest ID remains in the ring. After that, the greatest ID will return to the initial node. It declares itself as the leader by sending a coordinate message into the ring if the received ID equals that of the initial sender.
This method will dramatically reduce the overhead involved in message passing. Besides that, if there is many nodes notice the leader crashed or absence at the same time, only the message of greatest ID node circulates in the ring and it will prevent the smaller IDs from being sent message in the ring. Both of these algorithms have almost the same advantages.
They can always check the liveness of the leader by the assumption of message delivery. The process is chosen as the non-crashed process at the end of the run with the largest process number if no process is replaced. It is impossible for two processes to decide which coordinator among them is, since the process with the lower number will discover that the other exits and defer to it.
However, Modified Bully algorithm has another advantage that Bully algorithm does not have. That is the lower traffic passing. It reduce the traffic in the message communicate passing compare to Bully algorithm. It does not let the lower numbered process involve in the election but just higher numbered process.
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Then the process which wins will announce the message that declares itself as leader to all the nodes. It is more less traffic flow in the process and it is much better than Bully algorithm. The Bully algorithm also suffers from many shortcomings. This algorithm is not guaranteed to meet the safety condition if processes that had crashed and replaced by processes with the same number. A process that replaces the crashed process Q may decide which has the highest number just as another process has decided that it has the highest number.
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Both of them will announce themselves as the coordinator in a same time. But unfortunately, there are no guarantees on message passing order, and the recipients of these messages may give out different conclusion on which is the coordinator process. The condition may be broken if the assumed timeout values turn out to be accurate. Token Ring and Modified Token Ring algorithm are having very special routing. It is totally different with the Bully algorithm.
They know the process next to them and just passing the message to the process which next to them. It is much more simple compare to other algorithm. They just need to take the information from beside, add the value and pass to another side. The message passing traffic is very much lower than other algorithms. This is because they just pass the information to the next process after that no more communicate with other processes.
This reason make the algorithm become much easier to carry out. Compare with the bully algorithm which need keep communicate with all the process, Token Ring just need the message passing around the ring and know all the information about which one process having the higher numbered process.
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Algorithm — Suppose process P sends a message to the coordinator. If coordinator does not respond to it within a time interval T, then it is assumed that coordinator has failed. Now process P sends election message to every process with high priority number. It waits for responses, if no one responds for time interval T then process P elects itself as a coordinator.
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Then it sends a message to all lower priority number processes that it is elected as their new coordinator. Algorithm — If process P1 detects a coordinator failure, it creates new active list which is empty initially. It sends election message to its neighbour on right and adds number 1 to its active list. If process P2 receives message elect from processes on left, it responds in 3 ways: I If message received does not contain 1 in active list then P1 adds 2 to its active list and forwards the message.
II If this is the first election message it has received or sent, P1 creates new active list with numbers 1 and 2. It then sends election message 1 followed by 2. III If Process P1 receives its own election message 1 then active list for P1 now contains numbers of all the active processes in the system.
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