A brief introduction to Pastry

1. A brief introduction to Pastry Based on: A. Rowstron and P. Druschel, Pastry: Scalable, decentralized object location and routing for large-scale peer-to-peer systems, Middleware, Nov. 2001. Presenter: Fabián E. Bustamante Fabián E. Bustamante, Fall 2005

2. Distributed Systems Architectures • System architecture – Component placement and their interactions • Client/Server • Server implementing a specific service • Client requests the service • Peer-to-Peer • All peers are equal • Symmetric interaction between processes • Focusing on data-sharing systems CS 343 Operating SystemsEECS,Northwestern University

3. Data Sharing P2P • Data sharing P2P apps rely on overlay network protocols for object storage/retrieval & message routing • Classes of overlay protocols • Unstructured • Few constraints on overlay construction & data placement • Could support arbitrary complex queries, highly resilient to churn • Restricted to inefficient, near-blind search strategies • Structured (DHT ~ Distributed Hash Tables) • Constraining overlay structure & data placement • Efficient object discovery • Potential problems handling churn, exploiting node heterogeneity & supporting complex queries CS 343 Operating SystemsEECS,Northwestern University

4. Pastry DHT • Pastry • “Scalable, decentralized object location and routing for large-scale P2P systems” • Nodes assigned a unique 128-bit numeric node identifier (nodeId) (circular id space) • NodeID can be expressed in based 2b, b = 4 • Given a msg & its 128-bit key, Pastry routes the msg to the node with numerically closest nodeId (in less than ) • At each step, forward to node sharing with a key a 1 larger prefix or is numerically closer CS 343 Operating SystemsEECS,Northwestern University

5. Pastry Node State • Each Pastry node maintains • Routing table • Log N rows with 2b - 1 entries • Entries at row n share n digit with current node • Choice of b involves a trade-off between size of populated portions of the routing table & maximum number of hos to route between two nodes (e.g. b = 4 & 106, avg. 75 entries and 5 hops) • Neighborhood set • nodeIds and IP addresses of |M| nodes closest (proximity metric) to the local node (|M| ~ 2 * 2b) • Leaf set • Set of |L|/2 numerically closest larger/smaller nodeIds relative to the present node’s nodeId (|L| ~ 2b) CS 343 Operating SystemsEECS,Northwestern University

6. Routing in Pastry • Routing a message with key D arriving at a node with nodeId A If (D is within range of leafset) Forward to Li such that |D - Li| is minimal Else // use routing table Let l be the length of prefix shared bet/ A and D If != null Forward to Else Forward to T in L U R U M such that the length of prefix between T and D is >= l and |T - D| < |A - D| CS 343 Operating SystemsEECS,Northwestern University

7. Pastry API • Pastry exports (downcalls) • nodeId = pastryInit(Credentials, Applications) • To join the local node (application is a handle to a set of procedures to invoke when certain events happen) • route(msg, key) • Applications must export (upcalls) • deliver(msg, key) • Call by Pastry when msg arrived for node • forward(msg, key, nextId) • Call by Pastry just before forwarding the msg to node with nodeId = nextId • newLeafs(leafSet) • Call by Pastry whenever there’s a change in local node leafset CS 343 Operating SystemsEECS,Northwestern University

8. Dynamics – Node Arrival • Node X knows of nearby node A • X asks A to route a join msg with key X, A routes it to node Z • In response to join msg, A, Z and all in between send Z their state tables • X initializes its own state tables • A’s neighborhood set is X’s • Z’s leafset is basis for X’s leafset (closest nodeId) • X’s R0 (X0) = A0, X1 = B1, … where B is the first node in the route from A to Z • X informs everyone who should know of its arrival CS 343 Operating SystemsEECS,Northwestern University

9. Example Applications • PAST – A persistent storage service • fileId a function of file’s name & owner • Replicas of a file are stored on the k Pastry nodes with nodeId numerically closest to fileId • Scribe – A publish/subscribe system • Subscribers are kept in node with nodeId numerically closest to topicId) • Others • SplitStream • Squirrel • … CS 343 Operating SystemsEECS,Northwestern University