L5.sp11 - CS 525 Advanced Distributed Systems Spring 2011...

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1 CS 525 Advanced Distributed Systems Spring 2011 Indranil Gupta (Indy) Lecture 5 Peer to Peer Systems II February 1, 2011 All Slides © IG
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2 Systems that work well in practice but with no big/famous names Non-academic P2P systems e.g., Gnutella, Napster (previous lecture) Systems with big/famous names but that may or may not work well Academic P2P systems e.g., Chord (this lecture) Two types of P2P Systems
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3 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) Performance Concerns: Load balancing Fault-tolerance Efficiency of lookups and inserts Locality Napster, Gnutella, FastTrack are all DHTs (sort of) So is Chord, a structured peer to peer system that we study next
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4 Comparative Performance Memory Lookup Latency #Messages for a lookup Napster O(1) ( O(N) @server) O(1) O(1) Gnutella O(N) O(N) O(N)
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5 Comparative Performance Memory Lookup Latency #Messages for a lookup Napster O(1) ( O(N) @server) O(1) O(1) Gnutella O(N) O(N) O(N) Chord O(log(N)) O(log(N)) O(log(N))
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6 Chord Developers: I. Stoica, D. Karger, F. Kaashoek, H. Balakrishnan, R. Morris, Berkeley and MIT Intelligent choice of neighbors to reduce latency and message cost of routing (lookups/inserts) Uses Consistent Hashing on node’s (peer’s) address SHA-1 (ip_address,port) 160 bit string Truncated to 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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7 Ring of peers N80 N112 N96 N16 0 Say m=7 N32 N45 6 nodes
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8 Peer pointers (1): successors N80 0 Say m=7 N32 N45 N112 N96 N16 (similarly predecessors)
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9 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 (mo d2 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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10 What about the files? Filenames also mapped using same consistent
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L5.sp11 - CS 525 Advanced Distributed Systems Spring 2011...

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