Peer-to-Peer Overlay Networks in an Event-Based Middleware

1
Peer-to-Peer Overlay Networks in an Event-Based
Middleware

• DEBS03, San Diego, CA, USA, June 2003

2
Overlay Broker Networks

• Distributed pub/sub systems
• Mapping of brokers to physical nodes
• Specification of overlay topology
• Efficiency, reliability, manageability,

• 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

3
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

4
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
5
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

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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

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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 (102 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

8
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

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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

10
Message Complexity

• Message numbers (Advs, Subs, Pubs)
• (100 event types)
• Hermes sends fewer publications than CovAdv due
  to its better routing
• Hermes sends moresubscriptions than CovAdv due
  to RNs
• Number of advertisements staysconstant

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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

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Thank You

• Any Questions?