P2P and DHT

Peer-to-peer

• Practical distributed systems in early 2000s
  • eMule 2000, BitTorrent 2001
• BitTorrent
  • tracker servers exchange info “who has the file”
  • client contact tracker to contact the file owners
  • after finish, the client report to tracker as an owner
  • centralized bottleneck: some servers are more important than others.

Consistent hashing

• deterministic hashing function
  • uniform distribution
  • choose m-bit in results so that the collision rate is low
  • each key is hashed to m bits
  • each server (ip, port) is also hashed to m bits
• a ring connects 0 and 2^m-1
  • a key (its hash) is stored on the succeeding server
  • if every node has the membership info, then it can route the request to the right node with one hop
• on membership change
  • if a node joins, it takes over some keys from its immediate successor
  • if a node leaves, it gives all its keys to its immediate successor.
• what about failures?
  • use replicas: for a key who hash is k, use k + F, k + 2F as extra replicas (F is a constant)
• if everyone has the membership, this is it
  • Dynamo is implemented like this
  • reasonable for a moderate sized system with closed membership
    • what about a million nodes?
    • what about more open network?

Chord, a scalable lookup service

• no node stores the full membership
• the minimum: a node only stores its predecessor and successor
  • with N nodes, search O(N) times.
• Idea: store multiple “fingers” on the ring, look up similar a binary search
  • how can we distribute the fingers?
• store m fingers, divided by increasing index of 2
  • e.g., m=3, every node (node 0, 1, 3) store three fingers
    • node 0: 1, 2, 4
    • node 1: 2, 3, 5
    • node 3: 4, 5, 7
  • every finger has the successor and interval (range to next finger)
• search rule for k
  • if find a match (the immediate successor is the one), return
  • else, find the interval that includes k, forward the search to that successor, and repeat
• each forward will halve the search range, hence seach time is O(logN)
  • search node: n, i-th finger node: f, succ node: p
  • distance between n and f >= 2^(i-1)
  • distance between f and p <= 2^(i-1)
  • f to p is at most half of n to p
• membership change (join), base version
  • init finger table
    • delegate to exisiting nodes
    • quicker to ask a neighbor
  • update fingers of other nodes
  • transfer keys
• dynamic stablization
  • a new node joins: notify immediate successor
  • exisiting node calibrates: ask the immediate successor about its predecessor