MSc Course in Advanced Distributed Systems Session 1'2b : PublishSubscribe Communication Service

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MSc Course in Advanced Distributed
SystemsSession 1.2b Publish/Subscribe
Communication Service
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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/rep
  orts.02/TCD-CS-2002-14.pdf
• Advanced CORBA programming with C, M.Henning,
  S. Vinoski . Chapter 20

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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

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Synchronous request invocations
Server
request
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 11C max
20C
Subscription
Publisher
Want Traffic update for junction A6
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 Its the mediator/broker that routes
  events from publishers to interested subscribers

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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

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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

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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

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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

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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
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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/

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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/

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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---CarControl
Channel
Car A
Car B
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Video clip
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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