Peter R Pietzuch and Jean Bacon Peer-to-Peer Overlay Networks in an Event-Based Middleware DEBS’03, San Diego, CA, USA, June 2003

1 Overlay Broker Networks Today  Static neighbour lists –Difficult at deployment time –Require global view –Depend on physical network topology  Hierarchical topologies –Hard to maintain global properties –No redundancy Tomorrow  Self-managing overlays –Add more brokers on demand –Not only useful for large-scale  Adaptive overlay networks –Reflect current network situation –Lead to more efficient event dissemination  Evaluation Distributed pub/sub systems –Mapping of brokers to physical nodes –Specification of overlay topology Efficiency, reliability, manageability, …

2 Overview Overlay Broker Networks Peer-to-Peer Techniques Hermes –Type- and Attribute-Based Routing Simulational Evaluation –Routing Efficiency –Space Efficiency and Distribution –Message Complexity Conclusions

3 Peer-to-Peer Techniques Distributed hash tables (Pastry, CAN, Chord, …) –Overlay network of nodes with unique ids –Hash operation from key to nodeid –Scalable and efficient –Locality properties Advantages of P2P for publish/subscribe –Higher abstraction for building pub/sub systems –Content-based routing algorithm deals with hash keys –P2P overlay handles neighbouring set for event brokers route (msg, key) Easy to manage Dynamic mapping Efficient routing Fault-tolerance

4 Hermes Hermes, an event-based middleware –P2P overlay network –Evaluation of its efficiency vs. Siena-like approach –Type- and attribute-based publish/subscribe Event Clients Event Brokers –Local Broker, Rendezvous Node Rendezvous Nodes –Set up on a per type basis –Hash of event type name gives key for DHT –Ensure that advertisements and subscriptions join in the network B B B B B B S S S S S S P P P P P R P P P

5 Type- and Attribute-Based Routing Advertisement Messages –Routed towards RN by publishers –Create entries in advertisement routing tables along the way Subscription Messages –Routed towards RN by subscribers –Follow the reverse path of advertisements –Create entries in subscription routing tables along the way Publication Messages –Follow the reverse path of subscriptions –Get filtered along the way

6 Simulational Evaluation Evaluation of content-based pub/sub in simulator –Large-scale deployment for experiments difficult –Realistic network topologies and model for simulation E.g. notification latency, hop count, routing cost, … –Scale reflects corporate deployment (10 2 event brokers) Keep number of subscribers small if routing unaffected CovAdv Siena-like pub/sub Static set of neighbours Acyclic topology DSSim Discrete event simulator Transit stub network model Visualisation plug-ins Pan Pastry-like routing Hermes Pub/sub

7 Routing Efficiency Overlay networks used in experiments –Hermes: Sequential addition to closest existing broker w.r.t latency –CovAdv: Pre-computed minimum spanning tree Latency per notification (500 brokers; single subscriber per broker) Quality of the overlay Decreases  tree is more populated CovAdv (closest broker) has worst latency CovAdv (min span) is optimal Hermes is in between

8 Space Efficiency and Distribution Routing table sizes Hermes does not broadcast advertisements Hermes has slightly less subscription state due to better routing Converge to same value as tables become full Routing table distribution CovAdv: Majority of broker has 20 routing table entries Hermes: No broker has more than 15; some have none

9 Message Complexity Message numbers (Advs, Subs, Pubs) (100 event types) Hermes sends fewer publications than CovAdv due to its better routing Hermes sends more subscriptions than CovAdv due to RNs Number of advertisements stays constant

10 Conclusions Self-managing & adaptive overlay networks are needed –Distributed Hash Tables are helpful Evaluation through simulation –Contrast different kinds of pub/sub approaches Peer-to-peer routing with RNs is efficient Future Work –Fault-tolerance –Dynamic network environments

11 Thank You Any Questions?