CS 525 Advanced Topics in Distributed Systems Spring 08Two types of P2P SystemsDHT=Distributed Hash TableComparative PerformanceSlide 5ChordRing of peersPeer pointers (1): successorsPeer pointers (2): finger tablesWhat about the files?Mapping FilesSearchSlide 13Slide 14AnalysisAnalysis (contd.)Search under peer failuresSlide 18Slide 19Search under peer failures (2)Slide 21Need to deal with dynamic changesNew peers joiningNew peers joining (2)New peers joining (3)Experimental ResultsLoad BalancingLookupsStabilization ProtocolFault-toleranceWrap-up NotesWrap-up Notes (2)Wrap-up Notes (3)SummaryAdministrative AnnouncementsAnnouncements (contd.)Slide 37Next LectureA question before you goCS 525 Advanced Topics in Distributed SystemsSpring 08Indranil GuptaLecture 3Introduction to Peer to Peer Systems - IIJanuary 22, 2008Systems that work well in practice but with no big/famous names• Non-academic P2P systemse.g., Gnutella, Napster (previous lecture)Systems with big/famous namesbut that may or may not work well• Academic P2P systemse.g., Chord (this lecture)Two types of P2P SystemsDHT=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•So is Chord, a structured peer to peer system that we study nextComparative PerformanceMemory LookupLatency#Messagesfor a lookupNapster O(1)(O(N)@server)O(1) O(1)Gnutella O(N) O(N) O(N)Comparative PerformanceMemory LookupLatency#Messagesfor a lookupNapster 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))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 circlem212 mRing of peersN80N112N96N160Say m=7N32N456 nodesPeer pointers (1): successorsN800Say m=7N32N45N112N96N16(similarly predecessors)Peer pointers (2): finger tablesN8080 + 2080 + 2180 + 2280 + 2380 + 2480 + 2580 + 260Say m=7N32N45ith entry at peer with id n is first peer with id >= )2(mod2min N112N96N16i ft[i]0 961 962 963 964 965 1126 16Finger Table at N80What about the files?•Filenames also mapped using same consistent hash function–SHA-1(filename) 160 bit string (key)–File is stored at first peer with id greater than its key•File cnn.com/index.html that maps to key K42 is stored at first peer with id greater than 42–Note that we are considering a different file-sharing application here : cooperative web caching–The same discussion applies to any other file sharing application, including that of mp3 files.Mapping FilesN800Say m=7N32N45File with key K42 stored hereN112N96N16SearchN800Say m=7N32N45File cnn.com/index.html with key K42 stored hereWho has cnn.com/index.html?(hashes to K42)N112N96N16SearchN800Say m=7N32N45File cnn.com/index.html with key K42 stored hereAt node n, send query for key k to largest successor/finger entry < kif none exist, send query to successor(n) N112N96N16Who has cnn.com/index.html?(hashes to K42)SearchN800Say m=7N32N45File cnn.com/index.html with key K42 stored hereAt node n, send query for key k to largest successor/finger entry < kif none exist, send query to successor(n) All “arrows” are RPCsN112N96N16Who has cnn.com/index.html?(hashes to K42)AnalysisSearch takes O(log(N)) timeProof –(intuition): at each step, distance between query and peer-with-file reduces by a factor of at least 2 (why?)Takes at most m steps: is at most a constant multiplicative factor above N, lookup is O(log(N)) –(intuition): after log(N) forwardings, distance to key is at most (why?)Number of node identifiers in a range of is O(log(N)) with high probability (why? SHA-1!)So using successors in that range will be okNm/2Nm/2m2HereNext hopKeyAnalysis (contd.)•O(log(N)) search time holds for file insertions too (in general for routing to any key)–“Routing” can thus be used as a building block for other applications than file sharing [can you name one?]•O(log(N)) time true only if finger and successor entries correct•When might these entries be wrong?–When you have failuresSearch under peer failuresN800Say m=7N32N45File cnn.com/index.html with key K42 stored hereXXXLookup fails (N16 does not know N45)N112N96N16Who has cnn.com/index.html?(hashes to K42)Search under peer failuresN800Say m=7N32N45File cnn.com/index.html with key K42 stored hereXOne solution: maintain r multiple successor entriesIn case of failure, use successor entriesN112N96N16Who has cnn.com/index.html?(hashes to K42)Search under peer failures•Choosing r=2log(N) suffices to maintain lookup correctness w.h.p.–Say 50% of nodes fail–Pr(at given node, at least one successor alive)=–Pr(above is true at all alive nodes)=2log211)21(1NN1)11(212/2NNeNSearch under peer failures (2)N800Say m=7N32N45File cnn.com/index.html with key K42 stored hereXXLookup fails (N45 is dead)N112N96N16Who has cnn.com/index.html?(hashes to K42)Search under peer failures (2)N800Say m=7N32N45File cnn.com/index.html with key K42 stored hereXOne solution: replicate file/key at r successors and predecessorsN112N96N16K42 replicatedK42 replicatedWho has cnn.com/index.html?(hashes to K42)Need to deal with dynamic changesPeers fail•New peers join•Peers leave–P2P systems have a high rate of churn (node join, leave and failure)•25% per hour in Overnet (eDonkey)•100% per hour in Gnutella•Lower in managed clusters, e.g., CSILSo, all the time, need to: Need to update successors and fingers, and copy keysNew peers joiningN800Say m=7N32N45N112N96N16N40Introducer directs N40 to N32N32 updates successor to N40N40 initializes successor to N45, and inits fingers from itN40 periodically talks to neighbors to update finger tableStabilization Protocol(followed byall nodes)New peers joining (2)N800Say m=7N32N45N112N96N16N40N40 may need to copy some files/keys from N45(files with fileid between 32 and
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