Chord: A Scalable Peer-to-Peer Lookup Service for Internet

Applications

Ion Stoica Robert MorrisDavid Liben-Nowell David R. Hari Balakrishnan KargerM. Frans Kaashoek Frank Dabek

1097304151 莊宜豐

IntroductionChord ProtocolHashing Finger Table

Lookup Simple Scalable

Conclusion

Outline

Introduction

Slide 3

Client / server architecture Peer-to-Peer

Chord Protocol : System Model

• Load balance:– Chord acts as a distributed hash function, spreading keys evenly over the nodes.

• Decentralization:– Chord is fully distributed: no node is more important than any

other.• Scalability:

– The cost of a Chord lookup grows as the log of the number of nodes, so even very large systems are feasible.

• Availability:– Chord automatically adjusts its internal tables to reflect newly

joined nodes as well as node failures, ensuring that, the node responsible for a key can always be found.

• Flexible naming:– Chord places no constraints on the structure of the keys it looks

up.

Chord: Distributed Hash Table• Chord provides support for just

one operation: given a key, it maps the key onto a node.

• Hashing function assigns each node and key an m-bit identifier using SHA-1 base hash function.

• Node’s IP address is hashed.

• Each node has a m-bits node ID ( 0~2m-1)

• Keys, too, are m-bit identifiers

m= 6

k10

CircularID Space

N1

N8

N14

N21

N32N38

N42

N48

N51

N56

k24

k30k38

k54

An ID Ring of length 2m-1

Chord: Distributed Hash Table

• Each node, n, maintains a routing table with (at most) m entries, called the finger table.

Finger Table

Finger Table

Successor()

Bit=6

i=1i=2i=3i=4i=5i=6

start.node()

start.node ()

Successor()

When Node=8,i=3start.node(n+2i-1=N12)

start.node()

i=3

When Node=8,i=4start.node(n+2i-1=N16)

i=4

Node=8,i=3

start.node(N12)<= next.node (N14)

Successor(N12)=next.node (N14) =N14

When Node=8,i=3start.node(n+2i-1=N12)

next.node(N14)

i=3

Successor ()

Key/node space partitions

N8

N16

N14

i ＝ 3i ＝ 4

N14N14 N21

N12

start.node()

start.node()

Successor() Successor()

i ＝ 2i ＝ 1 N21

Simple Lookup

CircularID Space

N1

N8

N14

N21

N32N38

N42

N48

N51

N56k54

lookup(k54)

Scalable Lookup

k10

N1

N8

N14

N21

N32N38

N42

N48

N51

N56

k24

k30k38

k54

Finger tableN8+1 N14N8+2 N14N8+4 N14N8+8 N21N8+16 N32N8+32 N42+1

+2+4+8+16

+32

Chord : Scalable Lookup

Find (K54)

N1

N8

N14

N21

N32N38

N42

N48

N51

N56k54

Finger table at N8N8+1 N14N8+2 N14N8+4 N14N8+8 N21N8+16 N32N8+32 N42

+1

+2+4+8+16

+32

lookup(k54)

Lookup(K54) :N8// ask node n to find the successor of idn.find_successor(id)

if (id belongs to (n, successor]) // k54 belongs to (N8.N14)return successor;

elsen0 = closest preceding node(id);return n0.find_successor(id);

// search the local table for the highest predecessor of idn.closest_preceding_node(id)

for i = m downto 1 // i =6if (finger[i] belongs to (n, id)) // finger[6] =42 belongs to (N8.K54)

return finger[i];return n;

Chord : Scalable Lookup

Find (K54)

N1

N8

N14

N21

N32N38

N42

N48

N51

N56k54

Finger table at N42N42+1 N48N42+2 N48N42+4 N48N42+8 N51N42+16 N1N42+32 N14

lookup(k54)

Lookup(K54) :N42// ask node n to find the successor of idn.find_successor(id)

if (id belongs to (n, successor]) // k54 belongs to (N42.N48)return successor;

elsen0 = closest preceding node(id);return n0.find_successor(id);

// search the local table for the highest predecessor of idn.closest_preceding_node(id)

for i = m downto 1 // i =4if (finger[i] belongs to (n, id)) // finger[4] =51 belongs to (N42.K54)

return finger[i];return n;

Chord : Scalable Lookup

Find (K54)

N1

N8

N14

N21

N32N38

N42

N48

N51

N56k54

Finger table at N51N51+1 N56N51+2 N56N51+4 N56N51+8 N1N51+16 N8N51+32 N21

lookup(k54)

Lookup(K54) :N51// ask node n to find the successor of idn.find_successor(id)

if (id belongs to (n, successor]) // k54 belongs to (N51.N56)return successor; // return N56

elsen0 = closest preceding node(id);return n0.find_successor(id);

// search the local table for the highest predecessor of idn.closest_preceding_node(id)

for i = m downto 1 if (finger[i] belongs to (n, id))

return finger[i];return n;

• IP Hash NoID• Data Hash DataID • Search Data Hash Key• DataID Hash Key• Search Data （ Key ） NoID DataID(Key) DataIP

• A’s NoID=212• 如果 A有檔案”寶貝 .mp3”經過 hashing =88555• B’s NoID=234• 如果 B有檔案”屋頂 .mp3”經過 hashing = 45651• C’s NoID=555• 如果 A有檔案”麵包 .mp3”經過 hashing =14212• D’s NoID=651• 如果 A有檔案”籃子 .mp3”經過 hashing = 66234

• 當搜尋者輸入欲搜尋的檔案，經過 hash除以 1000產生餘數，• 而餘數就是 KEY值• 假設 A要搜尋籃子這首歌，經過 hash得到 KEY值 234，所以尋找 K234• 而 KEY234是歸 B管，所以當 A路由到 B的時候， B會跟 A說，檔案在 D那

• 所以 A就跑過去 D那詢問，給不給下

Conclusion

• Chord protocol solves it in a efficient decentralized manner– Routing information: O(log N) nodes– Lookup: O(log N) nodes– Update: O(log2 N) messages

THX