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PEER TO PEER AND DISTRIBUTED HASH TABLES CS 2711

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Distributed Hash Tables Challenge: To design and implement a robust and scalable distributed system composed of inexpensive, individually unreliable computers in unrelated administrative domains CS 2712 Partial thanks Idit Keidar)

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Searching for distributed data Goal: Make billions of objects available to millions of concurrent users – e.g., music files Need a distributed data structure to keep track of objects on different sires. – map object to locations Basic Operations: – Insert(key) – Lookup(key) CS 2713

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Searching Internet N1N1 N2N2 N3N3 N6N6 N5N5 N4N4 Publisher Key=“title” Value=MP3 data… Client Lookup(“title”) ? CS 2714

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Simple Solution First There was Napster – Centralized server/database for lookup – Only file-sharing is peer-to-peer, lookup is not Launched in 1999, peaked at 1.5 million simultaneous users, and shut down in July CS 2715

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Napster: Publish I have X, Y, and Z! Publish insert(X, ) CS 2716

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Napster: Search Where is file A? Query Reply search(A) --> Fetch CS 2717

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Overlay Networks A virtual structure imposed over the physical network (e.g., the Internet) – A graph, with hosts as nodes, and some edges Overlay Network Node ids Hash fn Keys CS 2718

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Unstructured Approach: Gnutella Build a decentralized unstructured overlay – Each node has several neighbors – Holds several keys in its local database When asked to find a key X – Check local database if X is known – If yes, return, if not, ask your neighbors Use a limiting threshold for propagation. CS 2719

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I have file A. Gnutella: Search Where is file A? Query Reply CS 27110

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Structured vs. Unstructured The examples we described are unstructured – There is no systematic rule for how edges are chosen, each node “knows some” other nodes – Any node can store any data so a searched data might reside at any node Structured overlay: – The edges are chosen according to some rule – Data is stored at a pre-defined place – Tables define next-hop for lookup CS 27111

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Hashing Data structure supporting the operations: – void insert( key, item ) – item search( key ) Implementation uses hash function for mapping keys to array cells Expected search time O(1) – provided that there are few collisions CS 27112

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Distributed Hash Tables (DHTs) Nodes store table entries lookup( key ) returns the location of the node currently responsible for this key We will mainly discuss Chord, Stoica, Morris, Karger, Kaashoek, and Balakrishnan SIGCOMM 2001 Other examples: CAN (Berkeley), Tapestry (Berkeley), Pastry (Microsoft Cambridge), etc. CS 27113

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CAN [Ratnasamy, et al] Map nodes and keys to coordinates in a multi- dimensional cartesian space source key Routing through shortest Euclidean path For d dimensions, routing takes O(dn 1/d ) hops Zone

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Chord Logical Structure (MIT) m-bit ID space (2 m IDs), usually m=160. Nodes organized in a logical ring according to their IDs. N1 N8 N10 N14 N21 N30 N38 N42 N48 N51 N56 15

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DHT: Consistent Hashing N32 N90 N105 K80 K20 K5 Circular ID space Key 5 Node 105 A key is stored at its successor: node with next higher ID CS Thanks CMU for animation

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Consistent Hashing Guarantees For any set of N nodes and K keys: – A node is responsible for at most (1 + )K/N keys – When an (N + 1)st node joins or leaves, responsibility for O(K/N) keys changes hands CS 27117

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DHT: Chord Basic Lookup N32 N90 N105 N60 N10 N120 K80 “Where is key 80?” “N90 has K80” Each node knows only its successor Routing around the circle, one node at a time. CS 27118

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DHT: Chord “Finger Table” N80 1/2 1/4 1/8 1/16 1/32 1/64 1/128 Entry i in the finger table of node n is the first node that succeeds or equals n + 2 i In other words, the i th finger points 1/2 n-i way around the ring CS 27119

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DHT: Chord Join Assume an identifier space [0..8] Node n 1 joins i id+2 i succ Succ. Table CS 27120

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DHT: Chord Join Node n 2 joins i id+2 i succ Succ. Table i id+2 i succ Succ. Table CS 27121

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DHT: Chord Join Nodes n 0, n 6 join i id+2 i succ Succ. Table i id+2 i succ Succ. Table i id+2 i succ Succ. Table i id+2 i succ Succ. Table CS 27122

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DHT: Chord Join Nodes: n 1, n 2, n 0, n 6 Items: f 7, f i id+2 i succ Succ. Table i id+2 i succ Succ. Table i id+2 i succ Succ. Table 7 Items 1 i id+2 i succ Succ. Table CS 27123

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DHT: Chord Routing Upon receiving a query for item id, a node: Checks whether stores the item locally? If not, forwards the query to the largest node in its successor table that does not exceed id i id+2 i succ Succ. Table i id+2 i succ Succ. Table i id+2 i succ Succ. Table 7 Items 1 i id+2 i succ Succ. Table query(7) CS 27124

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Chord Data Structures Finger table First finger is successor Predecessor What if each node knows all other nodes – O(1) routing – Expensive updates CS 27125

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Routing Time n f p finger[i] Node n looks up a key stored at node p p is in n’s ith interval: p ((n+2 i-1 )mod 2 m, (n+2 i )mod 2 m ] n contacts f=finger[i] – The interval is not empty so: f ((n+2 i-1 )mod 2 m, (n+2 i )mod 2 m ] f is at least 2 i-1 away from n p is at most 2 i-1 away from f The distance is halved at each hop. n+2 i-1 n+2 i 26

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Routing Time Assuming uniform node distribution around the circle, the number of nodes in the search space is halved at each step: – Expected number of steps: log N Note that: – m = 160 – For 1,000,000 nodes, log N = 20 CS 27127

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P2P Lessons Decentralized architecture. Avoid centralization Flooding can work. Logical overlay structures provide strong performance guarantees. Churn a problem. Useful in many distributed contexts. CS 27128

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