1. A Framework for Object-Based Event Composition in Distributed Systems Peter Pietzuch and Brian Shand {Peter.Pietzuch,Brian.Shand}@cl.cam.ac.uk June 2002 – 12 th PhDOOS Workshop, Malaga, Spain

2. 1 Large-scale systems require new middleware paradigms –need: scalability, loose coupling of components, reliability O-O middleware (request/reply) limitations: –synchronous, tight coupling, one-to-one communication New m/w paradigm: Event-Based Communication –require additional, expressive middleware services Framework for Composite Event Detection –handle complexity of many events –distributed composite event detection (scalability) Motivation

3. 2 Introduction to Event-Based Systems Event-based Middleware Systems Composite Event Detection Our Composite Event (CE) Framework –Composite Event Language –Composite Event Objects –Mobile Detection Objects Distributed Composite Event Detection Future Work Overview

4. 3 Notion of an Event: Examples: PrinterFinishedEvent, SalesOrderEvent Event Publishers publish events; Event Subscriber subscribe to events with a subscription (filter expression) Event-based Systems Event Publisher Event Subscriber publish notify subscribe Pub/Sub System asynchronous occurrence in time contains data that describes the occurrence can be implemented as an object

5. 4 Middleware using the Pub/Sub paradigm –built on top of an existing O-O/M-O middleware –Scalable and fault-tolerant event dissemination algorithms Properties: Must be able to: –integrate with O-O app programming language –handle large number of events Event-Based Middleware Publisher/Subscriber are decoupled asynchronous notification many-to-many communication  Composite Event Detection

6. 5 Composite Event Detection Management Plant A Plant B Sales Department Plant C Supplier A CED Supplier B Client A Composite Events denote patterns of events Composite Event Detectors Example: “Client A submits new order” ; “Sales Dept. authorises order” ; “Plant A rejects order” Scalability  Distributed detection

7. 6 Design Guidelines –Integration with an O-O middleware –Facilitate distributed detection –Little assumptions about pub&sub mechanisms Components: Our CE Framework – Composite Event Language – Composite Event Objects – Mobile Detection Objects

8. 7 Regular-expression based –can be detected by (extended) finite state automata –bounded resource usage –minimal set of operators Examples –A followed by B, or C (A;B)|C –Any number of A in parallel with B Distribution: –Decomposable along syntax tree –explicit input alphabet e.g. [A|B in A,B,C] Composite Event Language Operators AtomA ConcatenationA;B DuplicationA* AlternationA|B Parallelization Timing(A,B)!1s

9. 8 Represent composite events Published by detectors Example: (A|B);(C;D) Interface to access primitive event instances Composite Event Objects

10. 9 Detect composite events Are Event Subscriber + Publisher Encapsulate extended FSM with explicit input alphabet Example: Distribution: Agent-like behaviour –migrate to different location to optimise detection –can cooperate by decomposing expressions Mobile Detection Objects

11. 10 Distribution Example I MDO A|B MDO C;D notify Composite Event Object (CEO) MDO Mobile Detection Object (MDO) AB CD A|B C;D notify Example: new expression (A|B);(C;D)

12. 11 Distribution Example II MDO (A|B);(C;D) MDO A|B MDO C;D notify Composite Event Object (CEO) MDO Mobile Detection Object (MDO) AB CD A|B C;D (A|B);(C;D) notify

13. 12 Distribution Example III

14. 13 Distribution Policy handles –MDO placement and migration (optimise for bandwidth, latency, …) –Degree of CE expression decomposition –Degree of replication (reliability and load-balancing) Distribution Issues (Network delay) –Out of order arrival of events due to network delays –Best-Effort Detection vs. Guaranteed Detection –Event Publisher failure? Distributed Detection

15. 14 Implementation in Hermes Pub/Sub Simulator –investigate different Distribution Policies –performance/cost of the framework? –temporal issues… Higher-level, domain-specific CE specification languages –user-friendly language –language-binding –GUI Future Work

16. 15 Event-Based Middleware –good for building large-scale systems –must provide additional services Composite Event Detection –handle large number of events –should be part of the middleware Framework for Distributed CE Detection –object-based (CEOs & MDOs) –distributed (CE detectors as agents) –composite event language (extended FSMs) –distribution policies Conclusions