Hermes: A Distributed Event- Based Middleware Architecture Peter Pietzuch and Jean Bacon 1st DEBS Workshop, Vienna, Austria, July 2002

1 Motivation How do we build Event-Based Distributed Applications? –Software Engineering Issues –Middleware Abstraction Event-Based Middleware vs. Simple Event System –Language Integration –Complete Environment for Application Programmer –Additional Middleware Services Scalability for Large-Scale, Internet-Wide Systems –Keep Little State –No Global Broadcasts –Fault-Tolerance

2 Overview Motivation Event-Based Middleware Hermes –Design –Architecture –Event Routing Algorithms –Fault-Tolerance –Implementation Future Work Conclusions

3 Event-Based Middleware M/W following Pub/Sub Communication Paradigm –loose coupling, asynchronous messaging, many-to-many communication, source/sink anonymity +Additional Middleware Services –Programming Model, Programming Language Integration –Reliability, Fault-tolerance, QoS –Security –Transactions –Support for Mobile Event Clients +Additional Pub/Sub Services –Reflection (Meta Events) –Composite Event Patterns –Event Persistence

4 Logical Network of Self-organising Event Brokers (P2P) Scalable Design and Routing Algorithms Expressive Content-Based Filtering Clean Layered Design Traditional M/W functionality –strong typing, fault-tolerance, reliability… Extensible to provide additional M/W services Hermes – A Distributed Event-Based M/W

5 Design I Event Brokers –provide middleware functionality –logical overlay network with content-based routing and filtering –easily extensible Event Clients (Event Publishers Event Subscribers ) –connect to Event Broker –light-weight language independence mobility

6 Event Model –Typed Events with Attributes –Inheritance Subscription Model –Type- and Attribute-Based Pub/Sub –Super-Type Subscriptions Routing Model –Peer to Peer Overlay Routing –Application-Level Routing more flexible –Several Layers Design II PrinterEvent (printerID > 42) class PrinterFinished extends PrinterEvent { attribute int jobID; attribute string userID; }

7 Architecture

8 Routing Overlay Network (Pastry, Tapestry, CAN, …) –Event Brokers get unique Node IDs – route (message, nodeID) Properties –always routes to numerically closest existing Node ID –log N hops –logical topology reflects physical topology –adapts to node and link failure Event Brokers become Rendezvous Nodes –make sure that subscriptions find advertisements –created on a per-type basis –hash function of event type name –scalable event dissemination algorithm Algorithms I - Peer to Peer Routing

9 Algorithms II – Topic-Based Pub/Sub Type Msg, Advertisements, Subscriptions, Notifications Rendezvous Nodes Reverse Path Forwarding –Notifications follow Advs and then the reverse path of Subs R

10 Filtering State Notifications follow reverse paths of subscriptions Covering and Merging supported Algorithms III – Content-Based Pub/Sub R

11 Fault-Tolerance I Heartbeat Messages + Soft State (Leases) Link Failure –adapt at P2P routing layer Event Broker Failure –recreate filtering state by resending subscriptions and advertisements Event Client Failure –Soft State in Brokers will expire Rendezvous Node Failure –replicate Rendezvous Nodes (weak consistency)

12 Fault-Tolerance II Event Broker Failure –resend Advertisements and Subscriptions –recreate Filtering State B 4 B 2 B 5 B 1 B 3 P 1 S 1 R

13 Implementation Actual Implementation –Java Implementation of Event Broker and Event Clients –Event Types defined in XML Schema –Java Language Binding for Events using Reflection Implementation within a Simulator –Large-Scale, Internet-Like Topologies –up to 10 4 Nodes so far

14 Experiments within the Simulator –Verify Scalability Claims –Test Fault-Tolerance Scenarios Other Pub/Sub Middleware Services –Distributed Composite Event Detection Changes to the P2P Routing Layer –Taking Advantage of an Existing Multicast Service –Structuring into Domains Future Work

15 Future Internet-Wide Applications require new M/W Paradigms  Event-Based Middleware –Traditional Middleware functionality –Scalable Event Dissemination Algorithms Application-Level P2P Routing gives a useful abstraction Type- and Attribute-Based Pub/Sub results in good language integration Fault-Tolerance is essential for wide-area networks Conclusions