Peter R. Pietzuch, Brian Shand, and Jean Bacon A Framework for Distributed Event Composition Middleware’03, Rio de Janeiro, Brazil, June 2003

1 Motivation I Event-based systems provide loosely-coupled many-to-many communication –This makes it easier to build large scale systems But subscribers may be overwhelmed by the vast number of primitive, low-level events –They need a higher- level view Composite events (CEs) represent patterns of events –Analogous to database triggers Client Network Supplier Network Management Plant A Sales Dept Plant C Corporate Backbone Event Message  Plant B              

2 Motivation II To date, event subscribers must do this themselves –This makes them complex and error prone to implement –They need to subscribe to all relevant events –Common patterns can be reused –CE Detectors can be close to publishers Reduces bandwidth consumption, minimizes latency Improves reliability –Distributed detection increases scalability, efficiency –Independence from publish/subscribe event system Supports any pub/sub system  A generic middleware service for distributed composite event detection

3 Overview Motivation Background to publish/subscribe systems –Active Office Scenario Introduction to composite events Composite Event Detection Framework –Architecture –Composite Event Language –CE Detectors –Distributed Detection Evaluation –Implementation over JMS –Results

4 Publish/Subscribe Systems Event publishers publish events Event subscribers subscribe to events using subscriptions Pub/sub system notifies subscribers of matching events Expressiveness of pub/sub system can be topic-based, content-based, … Event Publisher Event Subscriber publish notify subscribe Pub/Sub System  

5 Active Office Scenario The Active Office is a ubiquitous computerized environment. Sensors monitor activity in the building. Users want context- aware notifications. Scenario: Jean wants a PDA record of the whiteboard from meetings she attends, sent to her wireless PDA (but not if she returns to her workstation afterwards). Office FE01 Meeting Room 1 A W A A T T D D D Location Sensor (Active Bat) Whiteboard Sensor T Temperature Sensor W Door Sensor D A

6 Composite Events Composite events are patterns of simpler events –e.g. Jean is seen in the kitchen, followed by Ken –Constituent events could be primitive or composite Time model –In distributed systems, a linear time model is insufficient –Composite events are partially ordered by interval timestamps –Intervals represent uncertainty or event duration JK J;K time

7 CE Detection Framework –Subscribes to primitive events (and simpler CE’s) –Automata detect event patterns, publish composite events –Automata created dynamically, and managed by CE framework Architecture Pub/Sub System –Minimal assumptions –Composite events disguised as primitive Distributed Application –Submits composite event subscriptions to CE framework –Can also use underlying pub/sub system directly CE Detection Publish/Subscribe System Network Transport sub(CE) pub Distributed Application subnotifypubsubnotify

8 Composite Event Language I Human Specification Factorisation and Distribution Execution and Matching Higher- level Languages Core Composite Event Language CE Detection Automata Expressiveness CE detectors need bounded resource usage –Automata should be statically analyzable Requirements for a CE language –At least as expressive as other languages –Easily factorizable, canonical representation We use automata which extend FSM’s / RE’s –Regular expressions are a well understood construct –Extensions: parallelisation, timing, parameterisation –Cannot consider ALL events Higher level, domain- specific languages

9 Atoms –Describable events are what can be expressed by the pub/sub system –E.g. Consider all events in input alphabet A,B,C, but accept only A’s Composite Event Language II [PersonEvent(room=‘FE01’)] Sequence –Event C 1 occurred wholly before event C 2 Alternation, Iteration Timing –Allows event combinations within or not within an interval –E.g. J followed by K within 5 minutes –A special time event t is generated internally Parallelisation –Both expressions must occur, possibly concurrently ([J], [K  t,K]) t=5 min C 1 ||C 2 (C 1,C 2 ) t=X min C 1 |C 2 C1*C1* C 1 ;C 2 [E][E] [A  A,B,C] Examples

10 Composite Event Detectors CE language expressions compile into detection automata The automaton for C 1 =[B];[P] is: S0S0 {B}P} B;P B C 1 P { If someone subscribes to C 2 =[B];[P]|[M] then C 1 can be reused S0S0 { C 1 |M C 2 [B ] ; [P ], M [B ] ; [P ] [B ] ; [P ], M } [B ] ; [P ]

11 Efficiency is improved by distributing detectors throughout the network –An overlay network of event brokers supports CE detectors Distribution policies dictate detector placement –Migrating subexpressions near to sources –Excessive factorisation increases overheads –Detector replication Distributed Detection high-bandwidth link low-bandwidth link Src Sink Wide-Area Network CED Sink

12 Scenario: Jean wants a PDA record of the whiteboard from meetings she attends, sent to her wireless PDA (but not if she returns to her workstation afterwards). Implementation over JMS Each CE type is associated with a JMS topic –CE expressions can use JMS content-based filtering Broker nodes host CE detectors –An administration topic is used to control detector instantiations [Pers][Board on ] [Login] [Board off ] CED 1 JMS 2 JMS 1 Sink PDA wireless CED 2 C1C1 C

13 Results As the number of subscribers increases, so do the advantages of the CE framework –Bandwidth is used efficiently, with CE detectors close to event sources – especially wireless bandwidth CE notification delay remained low (<220ms) Subscribers CEPECEPECEPECEPECEPECEPE Wireless Wired Traffic (KB)

14 Conclusions Generic CE detection is a novel, useful middleware service –Can be added to existing pub/sub systems Extended finite state automata provide a tractable computational model Distributed detection increases robustness for large scale ubiquitous systems Future work –Higher level languages for expressing CE’s –CE detection for federating pub/sub frameworks

15 Thank You Any Questions?