Publish Subscribe Systems in Mobile Environments

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Publish / Subscribe Systems in Mobile Environments

• Christiana Christophi

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Outline

• Publish / Subscribe systems.
• P/S systems in Mobile Environments.
• Types of P/S systems.
• Architectures
• Mobile Adaptation
• Improvements
• Summary Conclusions

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What is a P/S system?

• A system gets events for information publisher
  and event requirements from subscriber, and sends
  events to subscribers whose requirements match
  the events.

4
Main Characteristics

• Anonymous Communication (Space decoupling).
• Asynchronous (Time Flow decoupling).
• Multicasting.
• ! Ideal for mobile / wireless networks
• High Compositionality.
• Dynamic Reconfigurability.

5
Mobile Computing

• Diffusion of mobile computing.
• Context awareness
• Middleware manage connectivity changes and
  location awareness.
• Two scenarios of mobile networks
• Base stations
• Ad-Hoc network
• New dimensions for middleware design
• No. of mobile nodes, Range of movement, Frequency
  of movement.

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Framework

• Group-based (channel-based)System designates a
  set of channels.
• Subject-based (topic-based)Each event is
  characterized by a random or domain specific
  subject.

Subscriber.subscribe(UCY.CSGroup)
Subscriber.subscribe(LondonStockMarket/Stock/Stoc
kQuotes)
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Framework

• Content-based (condition monitoring)Express
  interest in arbitrary way. Example Predicate

Subscriber.subscribe(issueIBM pricelt100
volumegt1000)
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Basic Model
(2) Create Event
ES
(3) Publish event
(6) Notify user
Event Brokering System

• Change in a real
• world variable

ED
(5) There is a match
ES
ED
ES
ED
(4) Match set against subscriptions
ES Event Sources ED Event Displayers
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Architectures

• Centralised Architecture
• Mobile Adaptation
• Quenching
• Distributed Architecture
• Distributed Broadcasting
• Distributed Multicasting
• Mobile Adaptation

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Centralised Architecture

• A single Event Broker
• Stores all subscriptions
• Responsible of matching them against events.
• Forwards the event to Event Displayer.
• How do we adapt such an architecture to operate
  in mobile environment?

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Mobile adaptation

• Where does ES,ED,EB reside?
• NOT EB and ES.
• EB requires large computing resourcesES sensor
  device
• ES autonomous
• EB needs to be online continuously
• NOT EB and ED.
• ED/PDA disconnectedEB constantly listens to
  events.
• Fixed network (single point of failure)

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Challenges posed by Mobile P/S

• Frequent disconnections of ES and ED.
• ES not all events published
• ED unreachable
• Ad-hoc network
• EB also mobile
• ES search for existing EB
• ED periodically poll EB

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Quenching
ES
ED
Event Brokering System
Call
C1
Call
ED
ES
C2
Call
C3
ED
ES
ES Event Sources ED Event Displayers
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Architectures

• Centralised Architecture
• Mobile Adaptation
• Quenching
• Distributed Architecture
• Distributed Broadcasting
• Distributed Multicasting
• Mobile Adaptation

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Distributed Broadcast
ES
ED
EB
EB
ED
ES
EB
EB
ED
ES
Event Brokering System
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Distributed Broadcast

• Each EB processes all events.
• Matching against a fraction of subscriptions.
• ?delivering and matching workload reduced

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Distributed Multicast
ES
ED
c1?c2 ?cN
EB
c1
EB
ED
ES
c2
EB
EB
cN
ED
ES
Event Brokering System
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Distributed Multicast

• An EB forwards an event to outgoing branches only
  if there could be a match with a subscription of
  at some EB leading from this branch.

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Mobile adaptation

• ED moves ? it connects to a different EB.
• New EB has to be informed of the EDs
  subscription adjust routing tree.
• New EB needs to obtain all queued events for ED.

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Mobile adaptation

• Alternatively
• ED carries its own subscription.
• ED receives new events even when the old EB is
  unreachable.
• New EB must inform the old new to cancel
  subscription
• An ED may end up with more than one EB monitoring
  its subscription.
• Mechanisms to avoid duplicates.

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Mobile adaptation

• Subscription handoff protocol
• No event should be lost
• The same event should not be delivered to both
  old and new EB, or duplicate elimination
  techniques.
• Important to know about the semantic of a
  subscription.
• Integrated approach.

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Replication
ES
ED
c1
EB1
ES
c2
c1
EB2
c2
ED
ES
Event Brokering System
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Replication

• Problems
• Consistency
• Unordered event sequence
• Goal Rule out deliveries of events to a user
  that could not have occurred with non-replicated
  system.

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Replication-Properties of P/S

• Orderedness
• Events from the same ES are delivered in the
  order they were generated.
• ConsistencyThe set of events it displays to the
  user must be a set that can be generated by a
  non-replicated system.
• CompletenessDisplay all events that would be
  displayed by a non-replicated system that
  received the same events.

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Replication - Filtering at ED
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Summary -Conclusions

• P/S systems are ideal for mobile environments
• Architectures
• Centralised
• Distributed (improved scalability)
• Replication, (increased availability, consistency
  problems)

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References

• Yongqiang Huang, Hector Garcia-Molina
  Publish/Subscribe in a Mobile Environment
• Gianpaolo Cugola, Elisabetta Di Nitto, Gian
  Pietro Picco Content-Based Dispatching in a
  Mobile Environment
• P.Th Eugster, P.Felber, R.Guerraoui, A.M.
  Kermarrec The Many Faces of Publish/Subscribe