Msc course in advanced distributed systems session 1 2b publish subscribe communication service
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MSc Course in Advanced Distributed Systems Session 1.2b : Publish/Subscribe Communication Service. Session 1.2b :Publish/Subscribe Communication Service. Overview of Lecture Motivation for publish/subscribe communication service What is publish/subscribe service?

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Msc course in advanced distributed systems session 1 2b publish subscribe communication service l.jpg

MSc Course in Advanced Distributed SystemsSession 1.2b : Publish/Subscribe Communication Service


Session 1 2b publish subscribe communication service l.jpg
Session 1.2b :Publish/Subscribe Communication Service

  • Overview of Lecture

    • Motivation for publish/subscribe communication service

    • What is publish/subscribe service?

    • key functions and issues associated with publish/subscribe service

    • Classifications of publish/subscribe service and their differences

    • Suitability of publish/subscribe service for different environments

    • Research at Lancaster

  • Additional reading

    • http://www.cs.tcd.ie/publications/tech-reports/reports.02/TCD-CS-2002-14.pdf

    • Advanced CORBA programming with C++, M.Henning, S. Vinoski . Chapter 20


Motivation for publish subscribe communication model l.jpg
Motivation for publish/subscribe communication model

  • Limitations of client/server communication model, specially for WAN and mobile environments

  • Client/server communication model are based on synchronous request invocations

  • Many distributed applications find synchronous request invocation too restrictive

  • Wide area networks (WAN) and mobile computing environment requires more flexible and de-coupled communication style

  • Large scale distributed systems involve thousands of entities potentially distributed in WAN and /or wireless settings

  • Increasing number of distributed applications require anonymous and asynchronous communication style


Synchronous request invocations l.jpg
Synchronous request invocations

request

Server

Client

Operation()

response

-Remote method invocations result in synchronous execution of an operation provided by an object

-Both client( requestor) and server ( provider) must be present and running

-Client blocks until operation returns

-Client are aware of destinations of request, since they hold object references to target object

-Only supports uni-cast communication( one to one)

E.g. Standard CORBA method invocations, Microsoft DCOM, Java Remote method invocations (RMI)


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What is publish/subscribe communication model ?

  • P/S model represents an emerging paradigm for de-coupled and asynchronous connections between application components

  • The publish/subscribe system contain information providers, which publish events to the information consumers. The Consumers subscribe to particular categories of event

  • The P/S service ensures the timely delivery of published events to all interested subscribers

  • P/S service is also known as event service


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Publish/Subscribe Middleware Service

Want champions league football news

REAL MADRID 4-2 MARSEILLE

Subscribe

Publish

Notify

Notification

Publication

Want weather news for Lancaster

Publish/Subscribe

Service

Weather Lancaster : sunny intervalsmin 11°C max 20°C

Subscription

Want Traffic update for junction A6

Publisher

Subscriber


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Role of each participant

  • Publishers role : Publishers generate event data and publishes them

  • Subscribers role : Subscribers submit their subscriptions and process the events received

  • P/S service: It’s the mediator/broker that routes events from publishers to interested subscribers


Key attributes of p s communication model l.jpg
Key attributes of P/S communication model

  • The publishing entities and subscribing entities are anonymous

  • The publishing entities and subscribing entities are highly de-coupled

  • Asynchronous communication model

  • The number of publishing and subscribing entities can dynamically change without affecting the entire system


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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

  • Event routing (event delivery) – The process of routing the published events from the publisher to all interested subscribers


Event filtering subject based vs content based 1 l.jpg
Event filtering Subject based vs. Content based(1)

  • Subject based:

    • Generally also known as topic based, group based or channel based event filtering.

    • Here each event is published to one of these channels by its publisher

    • A subscriber subscribes to a particular channel and will receive all events published to the subscribed channel.

    • Simple process for matching an event to subscriptions


Slide11 l.jpg

Event filtering Subject based vs. Content based(2)

  • Content based:

    • More flexibility and power to subscribers, by allowing to express as an arbitrary query over the contents of the event.

    • E.g. Notify me of all stock quotes of IBM from New York stock exchange if the price is greater than 150

    • Added complexity in matching an event to subscriptions


Event routing l.jpg
Event routing

  • The basic P/S system consists of many event publishers, an event broker (or mediator) and many subscribers.

  • An event publisher generates an event in response to some change it monitors

  • The events are published to an event broker which matches events against all subscriptions forwarded by subscribers in the system.

  • Event broker system could have either a single event broker or multiple distributed event brokers coordinating among themselves


Event routing13 l.jpg
Event routing

C1,C2,C3 are subscriptions of S1,S2,S3 respectively

P

C1

S1

Event broker

system

P

S2

C2

P

C3

S3

P

OR

Single broker

P- Event publisher

S – Event subscriber

Cluster of cooperating brokers


Basic elements of p s model l.jpg
Basic elements of P/S model

  • Event data model

    • Structure

    • Types

  • Subscription model

    • Filter language

    • Scope (subject, content ,context)

  • General challenge

    - Expressiveness vs. scalability


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Classification of publish/subscribe systems(1)

  • Peer to Peer /Listener model

    • Subscribers register at specific named publishing entities

    • Publishing entities deliver events to specific named subscribing entities directly

    • Requires subscribing entities to have some references of publishing entities

    • Requires publishing entities to have references of all subscribers it intends to notify about events

    • E.g. Java distributed event service, JMS, Used in JINI


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Classification of publish/subscribe systems(2)

  • Broker ( Mediator) model

    • Subscribers register their subscriptions with a common event mediator

    • Publishers forwards events to the common event mediator

    • Mediator takes care of receiving events from publishers and delivering them to all interested subscribers

    • Mediators can be single mediator or multiple mediators

    • Multiple mediators co-operate among themselves to achieve the common brokering facility provided to publishers and subscribers.

    • E.g. CORBA Event service, JMS, CORBA Notification service, SIENA, JEDI etc


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Classification of publish/subscribe systems(3)

  • Implicit model

    • Subscribing entities subscribe( or register) to particular event types rather than with a mediator or publishing entity

    • Publishing entities generate event of some type which are delivered to the subscribed entities

    • Publishers or subscribers need not hold explicit references to mediators or pub/sub entities.

    • E.g. STEAM, Cambridge event model (CEM)


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Sampling of applications

  • Highly suited for mobile applications, ubiquitous computing and distributed embedded systems

  • Examples include

    • Gaming

    • Workflow

    • Auctioning

    • Intrusion detection

    • Intelligent office

    • News distribution

    • File sharing

    • Resource discovery

    • Sensor grids


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Suitability of publish/subscribe middleware

  • Highly suited for mobile computing

  • Highly suited for ubiquitous computing and distributed embedded systems

  • Suited to build distributed applications consisting a large number of entities

  • Robust – Failure of publishers or subscribers does not bring down the entire system

  • Many variations to suit different environments

    • LAN – CORBA Event service, Java distributed event service ,Real time CORBA event service

    • WAN – SIENA, JEDI etc Still require significant research advances

    • Mobile environments – JEDI, STEAM Still require significant research advances

      http://www.cs.colorado.edu/serl/dot/siena.html

      http://www.research.ibm.com/gryphon/


Early experience at lancaster cortex project l.jpg
Early experience at Lancaster-CORTEX project

  • Publish/subscribe middleware suited for mobile environment

  • Uses implicit model

  • No reliance on fixed event brokers or system wide services

  • Supports subject, content, context based event filtering

  • XML based event data model

  • Utilised in two scenarios:

    • Smart room ( Interactive innovations lab)

    • Cooperating cars with traffic light

      http://www.comp.lancs.ac.uk/computing/research/mpg/projects/cortex/


Cooperating cars application l.jpg
Cooperating Cars Application

Satellites

Car publishes on Carcontrol channel

TL publishes on Carcontrol channel

Car subscribes to: CarControlChannel &

Receives events from other cars, Traffic lights

IEEE 802.11b(ad-hoc)

---Event Channel---CarControlChannel

Car A

Car B



Analysis l.jpg
Analysis

  • The existing mature publish/subscribe solution for LAN ( E.g. CORBA Event service, Java event service ) have some limitations

  • New standards are advancing such as CORBA Notification service, JMS to address some of the limitations for LAN

  • Research on P/S required for WAN and specially for mobile environments still not mature


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Expected Learning Outcomes

  • To understand the benefits provided by publish/subscribe communication model

  • To be able to compare publish/subscribe with other communication models

  • To understand the core functions and key features related to publish/subscribe

  • To understand the various classifications of publish/subscribe models and their advantages/disadvantages


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