Leader Election Bismita Srichandan

Leader Election What is Leader Election Why Leader election is needed Different design topologies Algorithms used – Bully Algorithm – Ring Algorithm – Invitation Algorithm References

What is Leader Election? In distributed computing, leader election is the process of designating a single process as the organizer, coordinator, initiator or sequencer of some task distributed among several computers (nodes).

Why Leader Election is required The existence of a centralized controller greatly simplifies process synchronization However, if the central controller breaks down, the service availability can be limited The problem can be alleviated if a new controller (leader) can be chosen How leader is chosen? Ans: Different Algorithms are used for leader election.

Leader Election criteria(Chow et al, 1997) 1: Extrema finding - Leader election is based on global priority. 2: Preference-based - Processes in the group can vote for leader based on personal preference (locality, reliability, estimation).

Leader Election Algorithms for Wireless Ad Hoc Networks[10] Two algorithms, Secure Extrema Finding Algorithm (SEFA) and Secure Preference-based Leader Election Algorithm (SPLEA). SEFA assumes that all elector-nodes share a single common evaluation function that returns the same value. In SPLEA, the scenario becomes more complicated because elector-nodes can have different preferences for a candidate- node.

Leader Election Algorithms for Wireless Ad Hoc Networks One relaxed assumption in synchronous distributed systems, Asynchronous Extrema Finding Algorithm (AEFA) is used. In this we allow the topology to change during the election process. In AEFA, nodes can start the process of election at different times, but after topological changes stop long enough for the algorithm to terminate, all nodes agree on a unique leader.

Election vs. Synchronization Election In election, all participants must know who owns the token(i.e. who is the leader). Election algorithms are performed only if failure occurs. Synchronization In synchronization, the nontoken holders only need to know that they do not hold the token. Synchronization algorithms are designed to work well in absence of failures.

Leader Election vs. Mutual Exclusion Leader Election Process may yield to others and execute normally as long as leader is selected. Concerned with fast and successful termination of election process. Result of leader election must be known to other processes. Mutual Exclusion Process competes until it succeeds. Must ensure that no process is starved. Does not care which process is running in critical section.

Design Topologies Complete Topology Logical Ring Topology Tree Topology Complete Topology: Each process in the group can reach any other process in the same group in one message hop. Assumptions taken – All process ids are unique and known to every process – Second assumption is communication network is reliable and only the communicating processes may fail. – Process takes a known finite amount of time to handle a message, in other words, the process is considered as crashed if it doesn’t respond within specified time-out period. *Note – Bully algorithm is based on the assumptions of complete topology

Bully Algorithm(Author : Tanenbaum) This is an extrema-finding algorithm. In this algorithm, the highest-numbered process becomes coordinator. Thus the biggest guy in town always wins, hence the name “Bully Algorithm.” When a process notices that the coordinator is not responding to requests, it initiates an election. Bully algorithm makes note of time of by which the process should respond. Election is held as follows: – P sends an ELECTION message to all processes with higher numbers. – If no one responds, P wins the election and becomes coordinator. – If one of the higher-ups answers, it takes over. P’s job over!

Bully Algorithm 1.Process 4 holds an election. 2.Process 5 and 6 respond, sending OK message that, we are UP. 3.Now 5 and 6 each hold an election.

Bully Algorithm Process 6 tells 5 to stop Process 6 wins and tells everyone Process 6 tells 5 to stop Process 6 wins and tells everyone. 13

Logical Ring Topology Easy to construct Unique property is that a message initiated by any node will return to the node.

Ring Algorithm In Ring algorithm if any process notices that the current coordinator has failed, it starts an election by sending message to the first neighbor on the ring. The election message contains the node’s process identifier and is forwarded on around the ring. Each process adds its own identifier to the message. When the election message reaches the originator, the election is complete. Coordinator is chosen based on the highest numbered process.

Ring Algorithm Initiation: 1. Process 4 sends an ELECTION message to its successor (or next alive process) with its ID 2. Each process adds its own ID and forwards the ELECTION message

Ring Algorithm contd… Leader Election: 1. Message comes back to initiator, here the initiator is Initiator announces the winner by sending another message around the ring

Tree Topology To construct a logical ring structure is easy if the underlying network supports broadcast facilities. A tree is used as representative topological structure. Each node is considered an autonomous entity that exchanges messages with adjacent nodes A minimum-weight spanning tree(MST) is used for leader election in tree topology Gallager, Humbelt, and Spira’s algorithm is based on searching and combining. It works by merging fragments, starting from each node and attaching level by level till it ends up with the MST A leader can be designated as the last node that merges and yields to the final MST.

Invitation Algorithm Invitation algorithm is a strong algorithm and it is basically designed for asynchronous systems. It eliminates possible drawbacks of Bully algorithm. Few drawbacks of Bully Algorithm – Bully algorithm strongly uses the facts that timeouts can accurately detect failed processors. – An arbitrary timing glitch (lost messages, overfull buffers, temporary overloads etc), the bully algorithm can elect two leaders. – Another possibility can be the node that fails to respond might not have failed.

Invitation Algorithm contd… Each node in a group periodically checks whether the leader is alive or not, by sending a message to the leader and waits for reply. If the node does not reply within a timeout period, the node invokes a Recovery procedure. The recovery procedure puts node i into a singleton group with node i as the leader. periodically, each leader i calls a check procedure, which sends messages to every other node asking whether that node is a leader. If many nodes responds that it is a leader, node i pauses for some time and then calls for a merge procedure.

Invitation Algorithm contd… Merge procedure sends message to all the leaders, inviting them to join a new group with the inviting node as leader. Example :Group communication system in Amoeba uses Invitation Algorithm to reconfigure a group after a node crashes[8].

References [1] Distributed Operating Systems & Algorithms, Randy Chow and Theodore Johnson, Addison Wesley, [2] [3] [4] [5] H. Garcia-Molina. Elections in a distributed computing system. IEEE Transactions on Computers, C- 31(1):47-59, January [6] [7] Distributed Systems principles and paradigms by Andrew S. Tanenbaum, Maarten van Steen, [8] M.F. Kaashoek and A.S. Tanenbaum, “Group Communication in the Amoeba Distributed Operating System,” Proc. IEEE 11th Int'l Conf. Distributed Computing Systems (ICDCS), pp , [8] M.F. Kaashoek and A.S. Tanenbaum, “Group Communication in the Amoeba Distributed Operating System,” Proc. IEEE 11th Int'l Conf. Distributed Computing Systems (ICDCS), pp , 1991.

References Ring Algorithm [9] [10]Vasudevan, S.; DeCleene, B.; Immerman, N.; Kurose, J.; Towsley, D.; “Leader election algorithms for wireless ad hoc networks”, DARPA Information Survivability Conference and Exposition, ProceedingsDARPA Information Survivability Conference and Exposition, Proceedings [11] Optimal Leader Election Scheme for Peer-to-Peer Applications Networking, ICN '07. Sixth International Conference on Volume, Issue, April

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