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 – 12th PhDOOS Workshop, Malaga, Spain

Motivation 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) large-scale systems require new communication paradigms scalability, loose coupling, reliability o-o request/reply limitations: synchronous, tight-coupling, only one-to-one communication need for new powerful middleware abstractions event-based communication composite event detection distributed composite event detection

Overview 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 Introduction to Event-based Systems Event-based Middleware Composite Event Detection Our Framework Distributed Composite Event Detection

Event-based Systems Event Publisher Event Subscriber notify Event Publisher Pub/Sub System Event Subscriber subscribe Notion of an Event: Examples: PrinterFinishedEvent, SalesOrderEvent Event Publishers publish events; Event Subscriber subscribe to events with a subscription (filter expression) asynchronous occurrence in time contains data that describes the occurrence can be implemented as an object notion of an event event sinks; event sources publish/subscribe good properties: asynchronous, loosely-coupled, many-to-many communication

Event-Based Middleware 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 Publisher/Subscriber are decoupled asynchronous notification many-to-many communication Middleware using the pub/sub paradigm Integration with o-o app programming language Scalable & fault tolerant event dissemination algorithms must come with traditional m/w functionality + must come with new pub/sub functionality like composite event detection  Composite Event Detection

Composite Event Detection 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 Management Plant A Plant B Sales Department Plant C Supplier A CED Supplier B Client A composite events composite event detection (finding patterns) example helps to deal with scale and complexity more efficient system (argument from paper) why is it important to do it distributed?

Our CE Framework Design Guidelines Components: Integration with an O-O middleware Facilitate distributed detection Little assumptions about pub&sub mechanisms Components: Composite Event Language Composite Event Objects Mobile Detection Objects overview of our framework nicely integrates with o-o m/w services composite event objects mobile detection objects design guidelines

Composite Event Language 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 <A*,B> Distribution: Decomposable along syntax tree explicit input alphabet e.g. [A|B in A,B,C] Operators Atom A Concatenation A;B Duplication A* Alternation A|B Parallelization <A,B> Timing (A,B)!1s regular expression based (why?) show operators examples useful properties: decomposable, understood, bound resource usage,

Composite Event Objects Represent composite events Published by detectors Example: (A|B);(C;D) Interface to access primitive event instances represent composite events can be manipulated by the o-o programming language example

Mobile Detection Objects 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 contains detector agent-like behaviour show interface

Distribution Example I Example: new expression (A|B);(C;D) notify A|B C;D MDO MDO A|B C;D Composite Event A B C D Object (CEO) - distribution example using mdo from paper notify notify Mobile Detection MDO Object (MDO)

Distribution Example II notify (A|B);(C;D) MDO (A|B);(C;D) notify A|B C;D MDO MDO A|B C;D Composite Event A B C D Object (CEO) - distribution example using mdo from paper notify notify Mobile Detection MDO Object (MDO)

Distribution Example III - distribution example using mdo from paper

Distributed Detection 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? distribution issues distribution policies heuristics take static and dynamic properties of system into account (mention properties) speculate

Future Work 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 simulation environment implementation on top of Hermes distribution results

Conclusions Event-Based Middleware Composite Event Detection 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 - summary