Peer-to-Peer Infrastructure and Applications Andrew Herbert Microsoft Research, Cambridge

Microsoft and the Grid Shared vision of the virtual organization But focused on e-Business rather than e-Science Grid investments Windows clusters for large scale computations TerraServer projects for large data sets Globus port to Windows Globus implementation of OGSA Wrap Grid services as Web services Leverage web services as Grid infrastructure E.g., Hailstorm user authentication

Microsoft Research Web services enable wide area integration How to extend this to enable efficient wide scale information sharing and collaboration? Move to a model of peer-to-peer service implementation in contrast to todays server-based model Necessarily scalable and self-organizing Necessarily simple developer framework No conflict with WSDL, SOAP etc

Peer-to-Peer today Music / video download Napster, Morpheus, Gnutella Distributed computing Research community looking for general purpose frameworks discovering useful applications

Peer-to-Peer applications Publish/Subscribe Event Notification (SCRIBE) Share load of supporting topics and disseminating messages from publishers to subscribers Distributed document archive (PAST) Share load of storing documents reliably Web caching (SQUIRREL) Share load of caching web pages Dynamic directory (OVERLOOK) Share load of storing directory entries for dynamic data

A P2P framework requires: Content-based addressing Hash content to key Route message to computer hosting that key Dynamic caching and proxying Local computers stand in for remote ones Faster access, reduced load on key holder Replication and automatic failover Store at K computers adjacent to key holder Multicast cascade for group communication Each computer needs a spanning tree of routes for reaching every other computer

Overlay Networks Peer-to-peer requires richer routing semantics than IP IP routes to destination computer, not content URLs route to destination computer, not content IP multicast isnt widely deployed Solution: Overlay networks allow applications to participate in hop-by-hop routing decisions Ideal overlay is efficient, self-organizing, scalable, and fault-tolerant

Pastry outline Computers (Nodes) have unique Id Typically 128 bits long Primitive: Route (msg, key) Deliver msg to currently alive node with Id closest numerically to key Scalable, efficient Per node routing table O(log(N)) entries Route in O(log(N)) steps Fault tolerant Self-fixes routing tables when nodes added, deleted or fail

0XXX1XXX2XXX3XXX START 0112 routes a message to key First hop fixes first digit (2) Second hop fixes second digit (20) END 2001 closest live node to Pastry routing

Routing table: For each level, nearest peer for other domains Namespace leaf set: nearest Ids to left and right in name space Each entry gives IP address for host associated with Id Pastry routing table

Pastry Routing Demo

Pastry node addition Want to add new node to the system Invent a new random nodeId X Go to a nearby or well-known node A 1.Route to key X via A (finds node Z with Id closest to X) 2.Obtain leaf set from Z and rebuild 3.Obtain routing table entries from each node along the route from A to Z and rebuild 4.Register with each member of As namespace leaf set so they adjust their leaf sets and rebuild 5.Find nearest leaf set node and use its routing table to improve locality

Scribe: A Pastry Application Publisher Subscriber Topic of interest Publish / subscribe is a popular model for event driven systems with volatile membership Decouple event publishers from event subscribers Publishers dont know in advance who subscribers are Subscribers dont know in advance who publishers are Challenge is how to multicast notifications from topics efficiently

Scribe: architecture Topic hashed to a key Construct a multicast tree based on the Pastry network Have the (Pastry) node with the closest Id to the topic key be the root This node replicates knowledge of the topic to its k nearest neighbours for resilience Pass event notification down through the tree Each parent forwards event to its children Avoids over stressing network links close to the topic node

Scribe: Topic creation Each topic is assigned a topicId Root of the multicast tree= node with nodeId numerically closest Create(topic): route through Pastry to the topicId T Create(T) Root

Scribe: subscribing

Scribe: event dissemination Publish(topic, event) Route through the Pastry network using the topicId as the destination Dissemination along the multicast tree starting from the root E

Scribe demo

Summary Peer-to-peer techniques are good for wide area information sharing and collaborative computation Overlay networks enable peer-to-peer distributed computing Pastry is an efficient, scalable, self-organizing peer-to- peer framework Pastry makes it easy to build powerful peer-to-peer applications For more see: