PEER TO PEER AND DISTRIBUTED HASH TABLES

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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 271 2Partial thanks Idit Keidar)

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)

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Searching

Internet

N1

N2 N3

N6N5

N4

Publisher

Key=“title”Value=MP3 data… Client

Lookup(“title”)

?

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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 2001.

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Napster: Publish

I have X, Y, and Z!

Publish

insert(X, 123.2.21.23)...

123.2.21.23

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Napster: Search

Where is file A?

Query Reply

search(A)-->123.2.0.18Fetch

123.2.0.18

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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 idsHash fn Hash fn

Keys

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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.

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I have file A.

I have file A.

Gnutella: Search

Where is file A?

Query

Reply

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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

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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

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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.

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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(dn1/d) hops

Zone

Chord Logical Structure (MIT)

• m-bit ID space (2m IDs), usually m=160.• Nodes organized in a logical ring according

to their IDs.N1

N8

N10

N14

N21

N30N38

N42

N48

N51N56

15

DHT: Consistent Hashing

N32

N90

N105

K80

K20

K5

Circular ID space

Key 5Node 105

A key is stored at its successor: node with next higher ID

CS 271 16Thanks CMU for animation

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

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DHT: Chord Basic Lookup

N32

N90

N105

N60

N10N120

K80

“Where is key 80?”

“N90 has K80”

Each node knows only its successor • Routing around the circle, one node at a time.

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DHT: Chord “Finger Table”

N80

1/21/4

1/8

1/161/321/641/128

• Entry i in the finger table of node n is the first node that succeeds or equals n + 2i

• In other words, the ith finger points 1/2n-i way around the ring

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DHT: Chord Join

• Assume an identifier space [0..8]

• Node n1 joins0

1

2

34

5

6

7

i id+2i succ0 2 11 3 12 5 1

Succ. Table

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DHT: Chord Join

• Node n2 joins0

1

2

34

5

6

7

i id+2i succ0 2 21 3 12 5 1

Succ. Table

i id+2i succ0 3 11 4 12 6 1

Succ. Table

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DHT: Chord Join

• Nodes n0, n6 join 0

1

2

34

5

6

7

i id+2i succ0 2 21 3 62 5 6

Succ. Table

i id+2i succ0 3 61 4 62 6 6

Succ. Table

i id+2i succ0 1 11 2 22 4 0

Succ. Table

i id+2i succ0 7 01 0 02 2 2

Succ. Table

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DHT: Chord Join

• Nodes: n1, n2, n0, n6

• Items: f7, f1 01

2

34

5

6

7 i id+2i succ0 2 21 3 62 5 6

Succ. Table

i id+2i succ0 3 61 4 62 6 6

Succ. Table

i id+2i succ0 1 11 2 22 4 0

Succ. Table

7

Items 1

Items

i id+2i succ0 7 01 0 02 2 2

Succ. Table

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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

01

2

34

5

6

7 i id+2i succ0 2 21 3 62 5 6

Succ. Table

i id+2i succ0 3 61 4 62 6 6

Succ. Table

i id+2i succ0 1 11 2 22 4 0

Succ. Table

7

Items 1

Items

i id+2i succ0 7 01 0 02 2 2

Succ. Table

query(7)

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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

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Routing Timen

f

p

finger[i]

• Node n looks up a key stored at node p

• p is in n’s ith interval: p ((n+2i-1)mod 2m, (n+2i)mod 2m]

• n contacts f=finger[i]– The interval is not empty so:

f ((n+2i-1)mod 2m, (n+2i)mod 2m] • f is at least 2i-1 away from n• p is at most 2i-1 away from f• The distance is halved at each

hop.

n+2i-1

n+2i

26

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

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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.

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