Looking into the Past: Enhancing Mobile Publish/Subscribe Middleware Exponent: Pablo E. Guerrero Thesis Supervisor: Mariano A. Cilia Universidad Nacional del Centro de la Provincia de Buenos Aires Facultad de Ciencias Exactas Tandil, Argentina, October, 2004

2 Agenda  Motivation  Problem Statement  Proposed Approach  Design & Implementation  Experimental Results  Conclusions

3 Motivation  Convergence of Technologies  Usage of Large-Scale Distributed Systems  Mobile Devices & Wireless Technologies  Client/Server (Request/Reply) Predominance  New Paradigm for Information-Driven Applications  WWW, Sensor Networks, Enterprise Strategies  Pull-based (Request/Reply) Does Not Fit  Polling implies resource waste  Leads to Network Saturation or Server Breakdown!  Not Suitable for Mobile Devices  Push-based  Event-based Dissemination Initiator of Interaction ConsumerProducer Knowledge about Counterpart Full Request/ Reply One-to-One Messaging No Anonymous Request/Reply Event-Based Dissemination

4 Motivation (II)  Publish/Subscribe Middleware provides  Asynchronous communication  Loose coupling  Producers don’t know who is subscribed  Clients can “dynamically” join and leave  Doesn’t require publishers and subscribers at the same time  Reflects intrinsic behavior of Information- driven Application  Support for roaming clients  Notion of Location  Location Transparency  Pub/Sub needs an Intermediator

5 Publish/Subscribe Systems Structure  Independent Process Data Flow Style  Message Manager or Notification Service  Clients (producers/consumers/both)  Communication links  Service Semantics  Implementation  Decentralized brokers  Connected graph  Client’s proxies

6 Problem Statement  Problem Applications Startup  Initial Bootstrapping Sequence phase  Getting a Consistent State  Request/Reply (combination)  Observation phase (pure Pub/Sub)  Cannot assume how long will it take Meaningful Event :anEventBasedApp:aNotificationService

7 Problem Statement (II)  Problem Aggravated in Mobile Environments  Disconnections  Context Changes (new location)  Pervasive environment’s dynamic behavior  A new bootstrapping phase must be initiated!  System responsiveness degraded

8 Proposed Approach  Goal:  Reduce bootstrapping latency  Use already published (past) notifications  Extend an existing pub/sub system, R EBECA  Store notifications in its Infrastructure  Caches Distributed in the Network  Minimize underlying system changes Meaningful Event :aNotificationService:anEventBasedApp

9 The R EBECA Notification Service

10 R EBECA Brokers Network

11 R EBECA Routing Algorithms  Flooding  Filter Forwarding  Simple Routing  Identity-based Routing  Covering-based Routing  Merging-based Routing  Routing with Advertisements

12 Proposed Approach  Approach Analysis  Caching Requirements  Caching in Event-Based Systems  Notifications Storage  Cache Querying Strategies  Guidelines

13 Proposed Approach Analysis  Store Notifications in Distributed Caches  Extending External Behavior  Past Bound  Interface Extension

14 Proposed Approach Analysis (II)  Extend Internal Behavior  Extend System Behavior  Support for Different Strategies

15 Caching Strategies Requirements  Response Time  Time Overhead  Space Usage  Integrability  Correctness  Transparence  Dynamism  Separation of Concerns  Specification of the Requirements from the Infrastructure

16 Caching in Event-Based Systems  Caching applied in a different fashion than traditional systems  Main differences:  Cache Coherency and Consistency  Cache Sizes  Information Lifetime  Caching vs. Buffering

17 Notifications Storage  When? Locality Principle not Applicable!  Distributed decision based on:  Subscriptions and Unsubscriptions  Advertisements and Unadvertisements  Notifications  How? Behaving as FIFO queues  Circular Ring Buffer

18 Notifications Storage (II)  Parallel Circular Ring Buffers  Take advantage of Content-based NS  Filter Identity, Covering and Merging

19 Notifications Storage (III)  A Combination, which  avoids Replicating Notifications  allows Ordering Notifications

20 Cache Querying  NS is an Undirected Acyclic Graph  How deep to search? (i.e. How many levels k?)  Link traversals involve costs similar to RPC’s:  De / Serialization, Un / Marshalling  RPC Runtime Costs, Physical Network Transfer  RPC requires ~ machine instructions  LPC 100 times faster than RPC  Strategies Classification:  k = 1  Restricted to Local Event Brokers  1 < k  m  Restricted to maximum depth ‘m’  k =   Allowed to perform exhaustive search

21 Cache Querying: LocalBrokerCaching

22 Cache Querying: BorderBrokerCaching

23 Cache Querying: MergingCaching

24 Cache Querying  Caching w/More Complex Routing Algorithms  Routing Algorithms selects which neighbors to forward a subscription  Solution: Force Forwarding an explicit Fetch message  Caching w/Advertisements  Avoid asking every peer by using advertisements information  Considering the Time Dimension  Border Brokers’ clocks synchronization  Attach a Timestamp to notifications

25 Cache Querying Guidelines

26 Design & Implementation  OOP Aproaches  Class Inheritance  Decorator Design Pattern

27 AOP Architectural Approach  Usage of AOP tools

28 AOP Architectural Approach (II)  Method Interceptions  Advices  Inter-Type Declarations  Aspects pointcut initRoutingEngine(): initialization(RoutingEngine.new()); pointcut subscription(): withincode(void LocalEventBroker.subscribe(..)) && call(void RoutingEngine.addSubscription(..)); before (RoutingEngine routingEngine, Subscription sub, Subscription sub, EventProcessor processor): EventProcessor processor): subscription(routingEngine, sub, processor) subscription(routingEngine, sub, processor){ if (sub.isBounded()) if (sub.isBounded()) routingEngine.getCachingStrategy().beforeSubscribe(sub, processor); routingEngine.getCachingStrategy().beforeSubscribe(sub, processor);} public void EventBroker.subscribe(Subscription sub, EventProcessor source, EventProcessor source, PastBound bound); PastBound bound); private PastBound Subscription.bound;

29 AOP Architectural Approach (III)  Caching Class Hierarchy

30 Experimental Results  Provide details on Caching Behavior  Evaluations Setup  Hierarchical Network  Consumers issue Random Subscriptions  Producers publish Notifications of Random Types  Caching Efficacy Metric  Measures Best-Effort

31 LocalBrokerCaching Evaluation

32 BorderBrokerCaching Evaluation

33 MergingCaching Evaluation

34 Caching Strategies Comparison

35 Conclusions  Notification Service was Extended  Reduced Bootstrap Latency  Used a pure pub/sub Approach  Minimal Changes on the pub/sub Subscription Interface  Incorporated Distributed Caching  Efficient, Content-based Notification Storage  Several Strategies Analyzed & Implemented  Aspect-Orientation  Achieved good modularity for a crosscutting concern  Allowed a plug-and-play functionality  Evaluation  Measured & Compared Caching Efficacy  Gave Insights of Caching Behavior

36 Notification Service Future Work  Caching Functionality Integration  On-line Integration through Dynamic AOP  Experiment with other Notification Services  Extend Evaluations  Develop Analytical Response Time Models  Perform Large Scale Simulations  Consider Other Enhancements  Priorization and Interactions with other Aspects of the Notification Service