CSE 486/586 Distributed Systems

Transcription

1 CSE 486/586 Distributed Systems Leader Election Slides by Steve Ko Comuter Sciences and Engineering University at Buffalo CSE 486/586

2 Reca: Mutual Exclusion Centralized Ring-based Ricart and Agrawala s Maekawa s

3 Why Election? Examle 1: sequencer for TO multicast Examle 2: leader for mutual exclusion Examle 3: grou of NTP servers: who is the root server?

4 What is Election? In a grou of rocesses, elect a leader to undertake secial tasks. What haens when a leader fails (crashes) Some rocess detects this (how?) Then what? Focus of this lecture: election algorithms 1) Elect one leader from among only the non-faulty rocesses 2) All non-faulty rocesses agree on which is the leader We ll look at 3 algorithms

5 Assumtions Any rocess can call for an election. A rocess can call for at most one election at a time. Multile rocesses can call an election simultaneously. All such elections combined must yield a single leader Election results should not deend on which rocess called for it Messages are eventually delivered.

6 Problem Secification An election rotocol selects the non-faulty rocess with the best selected election attribute at termination. Attribute examles: CPU seed, load, disk sace, ID Must be unique Each rocess maintains a variable elected. An execution of the election algorithm should ideally guarantee at the end: Safety: For each non-faulty : 's elected = q: a articular nonfaulty rocess with the best attribute value, or Liveness: election terminates and for each non-faulty rocess, s elected is eventually not

7 Algorithm 1: Ring Election [Chang & Roberts 79] N Processes are organized in a logical ring i has a communication channel to i +1 mod N. All messages are sent clockwise around the ring. To start election Send election message with my ID When receiving message (election, id) If id > my ID: forward message Set state to articiating If id < my ID and state is not articiating: send (election, my ID) Set state to articiating If id = my ID: I am elected (why?) send elected message elected message forwarded until it reaches leader again

8 Ring-Based Election: Examle The election was started by rocess The highest rocess identifier encountered so far is (final leader will be 33)

9 Ring-Based Election: Examle The worst-case scenario occurs when? the counter-clockwise neighbor (@ the initiator) has the highest attribute. In the examle: 9 24 The election was started by rocess The highest rocess identifier encountered so far is (final leader will be 33)

10 Ring-Based Election: Analysis In a ring of N rocesses, in the worst case: N-1 election messages to reach the new coordinator Another N election messages before coordinator decides it s elected 9 Another N elected messages to announce winner Total Message Comlexity = 3N Turnaround time = 3N-1

11 Correctness? Safety: highest rocess elected Liveness: comlete after 3N-1 messages What if there are failures during the election run?

12 Examle: Ring Election Election: 4 Election: 4 Election: 2 Election: 3 1. initiates election after old leader failed 2. receives "election", dies Election: 4 3. Election: 4 is forwarded forever? May not terminate when rocess failure occurs during the election! Consider above examle where attr==highest id

13 Algorithm 2: Modified Ring Election election message tracks all IDs of nodes that forwarded it, not just the highest Each node aends its ID to the list Once the message goes all the way around a circle, a new coordinator message is sent out Coordinator is chosen by the highest ID in the election message Each node aends its own ID to coordinator message When coordinator message returns to initiator Election is a success if the new coordinator is in the ID list Otherwise, start the election anew

14 Examle: Ring Election Election: 2,3,4,0,1 Election: 2,3,4 Election: 2 Election: 2,3 1. initiates election Coord(4) 2,3,0,1 4. receives "Coord", but is not included Election: 2,3,0 2. receives "election", dies Election: 2,3,0,1 Election: 2 Election: 2,3 5. re-initiates election Coord(4): 2,3 Coord(3): 2,3,0 Coord(4): 2 3. selects 4 and announces the result Coord(3): 2,3,0,1 Coord(3): 2 Coord(3): 2,3 6. is finally elected

15 Modified Ring Election How many messages? 2N Is this better than original ring rotocol? Messages are larger Reconfiguration of ring uon failures Can be done if all rocesses "know" about all other rocesses in the system What if the initiator fails? Successor notices a message that went all the way around (how?) Starts new election What if two eole initiate at once Discard initiators with lower IDs

16 What about that Imossibility? Can we have a totally correct election algorithm in a fully asynchronous system (no bounds) No! Election can solve consensus Where might you run into roblems with the modified ring algorithm? Detecting leader failures Ring reorganization

17 Algorithm 3: Bully Algorithm Assumtions: Synchronous system Election attribute is rocess ID Each rocess knows all the other rocesses in the system (and thus their IDs)

18 Algorithm 3: Bully Algorithm 3 message tyes election starts an election answer acknowledges a message coordinator declares a winner Start an election Send election messages to rocesses with higher IDs than self If no one relies after timeout: declare self winner If someone relies, wait for coordinator message Restart election after timeout When receiving election message Send answer Start an election yourself If not already running

19 Examle: Bully Election answer=ok Election Election Election OK OK Election Election Election 1. initiates election 2. receives relies 3. & initiate election coordinator OK 4. receives rely 5. receives no rely 5. announces itself

20 The Bully Algorithm election election 4 fails, and 1 detects this Stage 1 1 election answer 2 answer 3 election C 4 Stage election answer 3 election C 4 3 fails Stage 3 timeout Eventually... coordinator C Stage

21 Analysis of The Bully Algorithm Best case scenario? The rocess with the second highest id notices the failure of the coordinator and elects itself immediately. N-2 coordinator messages are sent. Turnaround time is one message transmission time.

22 Analysis of The Bully Algorithm Worst case scenario? When the rocess with the lowest id in the system detects the failure. N-1 rocesses ultimately begin elections, each sending messages to all rocesses with higher IDs. The message overhead is O(N 2 ).

23 Summary Coordination in distributed systems sometimes requires a leader rocess Leader rocess might fail Need to (re-)elect leader rocess Three Algorithms Ring algorithm Modified Ring algorithm Bully Algorithm

24 References Textbook Section Required Reading.

25 Acknowledgements These slides by Steve Ko, used and lightly modified with ermission by Ethan Blanton These slides contain material develoed and coyrighted by Indranil Guta (UIUC).