Chord

1
Chord

• Ion Stoica, Robert Morris, David Liben-Nowell,
  David Karger, M. Frans Kaashoek, Frank Dabek, and
  Hari Balakrishnan
• IEEE/ACM Transactions on Networking
• February 2003

2
Chord Ring
N1

• Map nodes into m-bit keyspace
• hash(IP Addr) ? Node ID
• Map item IDs into same keyspace
• hash(File Name) ? key
• Key assigned to first node with Node ID key
• Lookup(key) Operations
• Lookup(9) ? N21

N55
(1,8
N51
N8
(8,21
N21
N42
N32
3
Routing in Chord

• Each peer maintains
• succ(p)
• pred(p)
• finger table
• for i1 to m
• fingeri succ(n 2k-1)
• At each hop, look for next hop that brings
  message closest to destination
• At N8 lookup(54)
• N51 returns successor N55
• Routing requires O(log N) hops

N1
N55
N51
N8
N21
N42
N32
4
Node Joins and Stabilization

• Node Join
• New node knows about some node in the system
• Uses that node to perform lookup(NID) to find its
  successor
• Join process does not make rest of network aware
  of new node
• Stabilization
• To ensure correct lookups, only need consistent
  successor pointers
• Periodically each peer performs stabilization
  protocols to update successor pointers and finger
  table

5
Stabilize and Notify
6
Fix Fingers
7
Handling Failures

• Each node maintains list of r successor pointers
• If succi fails, use succi1
• All r successors need to fail in order to disrupt
  Chord ring
• Can replicate a key at every successor in list to
  provide fault tolerance
• Details of replication left to the application
  layer

8
Virtual Nodes

• To increase number of nodes in system, each
  physical node can run v virtual nodes
• Improves load balancing
• Each virtual node has its own ID and routing
  structure