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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

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

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?
    • 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
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
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)

what is publish subscribe communication model
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
publish subscribe middleware service
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

role of each participant
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
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
key functions implemented by p s middleware service
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
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

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
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
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
Basic elements of P/S model
  • Event data model
    • Structure
    • Types
  • Subscription model
    • Filter language
    • Scope (subject, content ,context)
  • General challenge

- Expressiveness vs. scalability

classification of publish subscribe systems 1
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
classification of publish subscribe systems 2
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
classification of publish subscribe systems 3
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)
sampling of applications
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
suitability of publish subscribe middleware
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
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
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
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
expected learning outcomes
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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