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Distributed Publish/Subscribe Network

Distributed Publish/Subscribe Network. Presented by: Yu-Ling Chang. Overview. Introduction Basic Overview of Pub/Sub System Classification of Pub/Sub Architecture Event filtering and Event Routing SIENA Pros and Cons Conclusion and Future work References.

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Distributed Publish/Subscribe Network

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  1. Distributed Publish/Subscribe Network Presented by: Yu-Ling Chang

  2. Overview • Introduction • Basic Overview of Pub/Sub System • Classification of Pub/Sub Architecture • Event filtering and Event Routing • SIENA • Pros and Cons • Conclusion and Future work • References

  3. Introduction: Motivations for Pub/Sub model • Traditional Client/Server communication model (Employs RPC, message queue, shared memory etc..) • Synchronous, tightly-coupled request invocations. • Very restrictive for distributed applications, especially for WAN and mobile environments. • When nodes/links fail, system is affected. Fault Tolerance must be built in to support this. • Require a more flexible and de-coupled communication style that offers anonymous and asynchronous mechanisms.

  4. What is a Publish/Subscribe System? • Distributed Pub/Sub System is a communication paradigm that allows freedom in the distributed system by the decoupling of communication entities in terms of time, space and synchronization. • An event service system that is asynchronous, anonymous and loosely-coupled. • Ability to quickly adapt in a dynamic environment.

  5. Key components of Pub/Sub System • Publishers : Publishers generate event data and publishes them. • Subscribers : Subscribers submit their subscriptions and process the events received • P/S service: It’s the mediator/broker that filters and routes events from publishers to interested subscribers.

  6. Publish/Subscribe System

  7. Decoupling in time, space and synchronization • Provides decoupling in time, space and synchronization.

  8. Classification of Pub/Sub Architectures • Centralized Broker model • Consists of multiple publishers and multiple subscribers and centralized broker/brokers (an overlay network of brokers interacting with each other). • Subscribers/Publishers will contact 1 broker, and does not need to have knowledge about others. • E.g. CORBA event services, JMS, JEDI etc…

  9. Classification of Pub/Sub Architectures • Peer-to-Peer model • Each node can be publisher, subscriber or broker. • Subscribers subscribe to publishers directly and publishers notify subscribers directly. Therefore they must maintain knowledge of each other. • Complex in nature, mechanisms such as DHT and CHORD are employed to locate nodes in the network. • E.g. Java distributed event service

  10. Key functions implemented by P/Smiddleware service • Event filtering (event selection) • The process which selects the set of subscribers that have shown interest in a given event. Subscriptions are stored in memory and searched when a publisher publishes a new event. • Event routing (event delivery) • The process of routing the published events from the publisher to all interested subscribers

  11. Event Filtering (Subscription Model)Topic based VS Content based • Topic based • Generally also known as topic based, group based or channel based event filtering. • Each event is published to one of these channels by its publisher. • Subscribers subscribes to a particular channel and will receive ALL events published to the subscribed channel.

  12. Topic-based subscription • Simple process for matching an event to subscriptions. However, limited expressiveness. • Event filtering is easy, event routing is difficult (Heavy load on the network). The challenge is to multicast event effectively to subscribers.

  13. Event Filtering- Subscription ModelTopic based VS Content based • Content based • More flexibility and power to subscribers, by allowing more expression in arbitrary/customized query over the contents of the event. • Event publication by a key/value attribute pair, and subscriptions specify filters using a explicit subscription language. • E.g. Notify me of all stock quotes of IBM from New York stock exchange if the price is greater than 150

  14. Content-based Subscription • Added complexity in matching an event to subscriptions. (Implementation: Subscription arranged in a matching tree, where each node is a partial condition. • However, more precision is provided and event routing is easier

  15. Event Routing • After filtering the events, the broker/brokers must route the events to the corresponding subscribers. • Can be done in the following ways: • Unicast • Multicast • Server push/ client pull

  16. Event Routing • The broker makes the decision: how to route the message to the subscriber. • Several optimization schemes are available. • Profile forwarding scheme – brokers only forward the event to their neighbor broker which fulfill their subscription • Filtering the total covering of the subscription of the system – accept publisher events only if a subscriber has subscribed this event.

  17. Example: SIENA • SIENA is a wide area notification service that uses covering-based routing. • Consists of Nodes and servers (access points), Event notifications & filters, Publish/subscribe protocol + advertisements, Identities and handlers, Filtering • Siena system can be configured in three types of inter-connection topologies: • Hierarchical client/server architecture • Acyclic P2P architecture • General P2P architecture

  18. SIENA: Hierarchical Architecture • Servers interact with each other in an asymmetric client-server fashion. • Server is not distinguished from objects of interest or interested parties • Potential overloading of server stationed at higher level of hierarchy • Failure of one node in hierarchy causes all the nodes below that node to fail

  19. Acyclic P2P architectureand General P2P architecture • The acyclic P2P architecture and General P2P architecture are very similar. • Both represented by an undirected graph and allows bidirectional communication. • Scaling an issue for both. • Acyclic P2P • Restriction on the configuration of connections between servers to forming acyclic graph representation • Therefore no redundant connections/ multiple paths are not allowed. (Enforcement by a cycle avoiding algorithm) • Can be difficult to maintain and not as robust as general P2P architecture. • General P2P architecture • Requires less coordination among servers. • Redundancy enforces robustness of Siena system with respect to failure of single servers. • Drawback: Special algorithms must be run to choose the best path.

  20. Siena: Routing • Simplest strategy is to maintain the subscriptions at their access point and broadcast the notification throughout the network • Least efficient • Consumes lots of bandwidth • Send the notification towards the event servers that have clients that are interested in that notification (possibly using shortest path)

  21. SIENA : Routing • Downstream Replication • Events are kept as one copy as long as possible and only replicated when it is as close as possible to the subscribing servers/clients.

  22. SIENA : Routing • Upstream Evaluation • Applying filters upstream, that is as close to the event publisher as possible

  23. Advantages of Pub/Sub • Highly suited for mobile applications, ubiquitous computing and distributed embedded systems • Robust – Failure of publishers or subscribers does not bring down the entire system • Scalability- Suited to build distributed applications consisting a large number of entities • Adaptability- can be varied to suit different environments (mobile, internet game, embedded systems etc…)

  24. Disadvantages of Pub/Sub • Reliability – no strong guarantee on broker to deliver content to subscriber. After a publisher publishes the event, it assumes that all corresponding subscribers would receive it. • Potential bottleneck in brokers when subscribers and publishers overload them. (Solve by load balancing techniques) • Security an issue: • Encryption hard to implement when the brokers has to filter out the events according to context. • Brokers might be fooled into sending notifications to the wrong client, amplifying denial of service requests against the client.

  25. Conclusion • Distributed Pub/Sub System provides a loosely-coupled, asynchronous model which is useful in many fields of network utilization. • Several areas are still open for research: • Effective routing and filtering algorithms for better performance • Scalability versus expressiveness issue • Fault tolerance • Security

  26. References • Papers: • P.Eugster, P.Felber,RGuerraoui and A.Kermarrec. The Many Faces of Publish/Subscribe. In ACM Computing Surveys, Vol. 35, No.2, June 2003. • A.Carzaniga, D.Rosenblum, A.Wolf . Design and Evaluation of a Wide-Area Event Notification Service. ACM Transactions on Computer Systems, Vol. 19, No. 3, August 2001. • Wikipedia: • http://en.wikipedia.org/wiki/Publish/subscribe

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