L11.sp11 - Distributed Systems CS 425 / CSE 424 / ECE 428...

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Distributed Hash Tables Reading: Chord paper on website (Sections 1, 2, 3, 4, 6, 7) 2010, I. Gupta (slides modified by N. Vaidya) Distributed Systems CS 425 / CSE 424 / ECE 428
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2 DHT=Distributed Hash Table A hash table allows you to insert, lookup and delete objects with keys A distributed hash table allows you to do the same in a distributed setting (objects=files) DHT also sometimes called a key-value store when used within a cloud Performance Concerns: Load balancing Fault-tolerance Efficiency of lookups and inserts
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3 Chord Intelligent choice of neighbors to reduce latency and message cost of routing (lookups/inserts) Uses Consistent Hashing on node’s (peer’s) address (ip_address,port) hashed id ( m bits) Called peer id (number between 0 and ) Not unique but id conflicts very unlikely Can then map peers to one of logical points on a circle m 2 1 2 - m
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4 Ring of peers N80 N112 N96 N16 0 Say m=7 N32 N45 6 nodes
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5 Peer pointers (1): successors N80 0 Say m=7 N32 N45 N112 N96 N16 (similarly predecessors)
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6 Peer pointers (2): finger tables N80 80 + 2 0 80 + 2 1 80 + 2 2 80 + 2 3 80 + 2 4 80 + 2 5 80 + 2 6 0 Say m=7 N32 N45 i th entry at peer with id n is first peer with id >= n + 2 i (mod2 m ) N112 N96 N16 i ft[i] 0 96 1 96 2 96 3 96 4 96 5 112 6 16 Finger Table at N80
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7 What about the files? Filenames also mapped using same consistent
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L11.sp11 - Distributed Systems CS 425 / CSE 424 / ECE 428...

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