Pastry

Pastry

• big p2p lookup system
• routing is one of the hardest challenges
• each node has an ID, and when presented with a message, pastry efficiently routes the messages to the closest to that node
• takes into account network locality
  • minimize routing hops
• SCRIBE - pub/sub built on pastry
• PAST - storage built on pastry

Design

• self-organizing overlay network
• each node routes requests and interacts with local application instances
• nodes with adjacent node IDs may be divers in geography, ownership, network, etc.
• route to given node in \(log_{2^b}(N)\) hops
• routing guaranteed unless L/@ nodes with adjacent nodeIds fail simultaneously
• pastry node state
  • each node has a routing table, neighborhood set, and leaf set
  • routing table has \(log_{2^b}(N)\) rows
  • \(2^b - 1\) entries per row
  • the entries at row \(n\) of the table refer to a node whose nodeId shares the present node’s nodeId in the first \(n\) digits.
  • the neighborhood set contains nodeIds and IP addresses of M nodes closest to the local node
    • neighborhood set not normally used in routing - useful for maintaining locality properties
    • leaf set used for routing
• routing
  • when a message arrives
  • check if key falls within range of nodeIds covered by the leaf set
    • if within range, message is forwarded directly to the destination node
  • if the key is not covered by the leaf set, the routing table is used and the message is forwarded to a node that shared a common prefix by at least one additional digit
  • in the case where the routing table is empty or the associated node is not reachable, then forward the message to a node that shares a prefix with the key at minimum as long as the local node and is numerically closer to the key than the present node’s ID.
  • in the case of many simultaneous node failures, number of routing steps required is at worst linear in N.
  • API
    • nodeId = pastryInit(credentials, app)
    • route(msg, key)
    • deliver(msg, key)
    • forward(msg, key, nextId)
    • newLeafs(leafSet)
• self-organization -