Publisher Mobility in Distributed Publish/Subscribe SystemsVinod Muthusamy, Milenko Petrovic, Dapeng Gao, Hans-Arno Jacobsen

University of Toronto

June 10, 2005

4th International Workshop on Distributed Event-Based Systems (DEBS'05)

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Motivation Explosion of information producers

Blogs, wikis, podcasting, photo sharing

Mobility of users Cell phones, PDAs, sensors

Mobile information producers Traditionally wired publishers can increasingly be mobile New types of publishers

SMS, camera phones, location based services

Pub/sub data dissemination Well suited to mobile clients

Decoupling, filtering

Mobility of information producer has not been studied in pub/sub Breaks common pub/sub assumption

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Publisher Mobility Scenarios Journalists with blogs

Update blogs on location Upload pictures from camera

phone

Police patrol car Send status updates

Traffic, accidents, parts failures

Mail delivery Track delivery status, location

updates

Publisher

1 2

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Agenda

Background Context Subscriber mobility

Publisher mobility Problem Solutions

Evaluation Setup Results

Conclusions

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Context Part of Toronto Publish/Subscribe System (ToPSS)

Improve expressiveness Approximate matching, location queries, XML, RDF, composite

subscriptions, historic subscriptions, etc. Distributed issues

Fault tolerance, load balance, reliability New environments

MANETs, P2P overlays, sensor networks

Mobile-ToPSS project Subscriber mobility [MDM’04]

Based on JEDI, SIENA work Publisher mobility [DEBS’05] Effects of routing computations [Mobicom’05] Content based routing in MANET [Mobiquitous’05]

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Distributed Publish/Subscribe Advertisements

flooded Create adv tree

Subscriptions along reverse adv path Create

multicast tree

Publications along reverse sub path

PublisherSubscriber Subscriber

. . .. . .

Advertisements Subscriptions Publications

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Subscriber Mobility Problem Matching publications during

disconnection Stored by broker Replayed upon reconnection

“State” transfer is expensive Double message load with

only 10% of mobile subscribers [MDM’04]

No state lost when publishers are disconnected No problem with mobile

publishers?

1 2

SubscriberSubscriber SubscriberSubscriber

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Publisher Mobility Problem Adv and sub trees Moveout: both trees torn down Movein: both trees rebuilt Expensive

Network load: May be # ads > # subs

No delivery until tree constructed Distinguish temporary disconnections

t1

At Old Broker

t3

Disconnected At New Broker

t5t4

Can publishnew events

Connect(movein)

Disconnect(moveout)

t2

moveout

Publisher

1 2

. . . . . .

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Publisher Mobility Problem Adv and sub trees Moveout: both trees torn down Movein: both trees rebuilt Expensive

Network load: May be # ads > # subs

No delivery until tree constructed Distinguish temporary disconnections

t1

At Old Broker

t3

Disconnected At New Broker

t5t4

Can publishnew events

Connect(movein)

Disconnect(moveout)

t2

movein

Publisher

1 2

. . . . . .

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Prefetching Optimization Exploits knowledge of future mobility

patterns Concurrent

Construction at new broker Teardown at old broker

Tree construction time hidden from user

t1

At Old Broker

t3

Disconnected At New Broker

t5t4

Can publishnew events

Connect(movein)

Disconnect(moveout)

t2

moveout

Publisher

1 2

. . . . . .

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Prefetching Optimization Exploits knowledge of future mobility

patterns Concurrent

Construction at new broker Teardown at old broker

Tree construction time hidden from user

t1

At Old Broker

t3

Disconnected At New Broker

t5t4

Can publishnew events

Connect(movein)

Disconnect(moveout)

t2

movein

Publisher

1 2

. . . . . .

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Proxy Optimization

Maintain trees from several brokers

Advantageous if restricted mobility region

t1

At Old Broker

t3

Disconnected At New Broker

t5t4

Can publishnew events

Connect(movein)

Disconnect(moveout)

t2

moveout

Publisher

1 2

. . . . . .

movein

Publisher

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Delayed Optimization

Maintain trees at old broker for some time

Allow new tree to graft onto old tree

Remove extraneous portions of old tree

t1

At Old Broker

t3

Disconnected At New Broker

t5t4

Can publishnew events

Connect(movein)

Disconnect(moveout)

t2

moveout

Publisher

1 2

. . . . . .

movein

Publisher

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Evaluation: Setup Simulation Environment

ns-2 network simulator Implemented mobility optimizations

Parameters Topology

Metropolitan Area Network 4 levels of degree 4 64 leaf brokers

Subscribers: 500 Publishers: 50 Locality: random, 30%, 60%, 90% Mobility

Static subscribers, mobile publishers Random speeds (5km/h, 50km/h, 100km/h)

Metrics Tree rebuild load Tree rebuild time, delivery ratio

• • •

64• • •

1

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Publisher Scalability

Standard and Prefetching >>Proxy and Delayed

Prefetching worse due to extra control messages

Delayed better due to smaller tree deltas

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Publisher Scalability Probe tree

completion

Prefetching is fastest Starts early

Standard is slowest Almost 4s

Delayed close to Prefetching

Note: time is not known to publisher

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Publisher Scalability

Tree rebuilding cost Best: Delayed, Proxy Worst: Standard, Prefetching

Tree rebuilding time Best: Prefetching, Delayed Worst: Standard

Prefetching Good for the user Bad for the network

Delayed Good for user and network Practical

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Publication Locality

250 publishers

Vary publication similarity

Standard and Prefetching approach Proxy and Delayed

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Publication Locality

Time from publish to notification

Again, Standard and Prefetching approach Proxy and Delayed

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Publication Locality

With sufficient publication similarity, optimizations have diminishing benefit Tree rebuilding cost Delivery latency

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Conclusions

The publish/subscribe model is well suited to mobile applications But publisher mobility has not been evaluated

Publisher mobility is expensive Breaks conventional assumptions Tree rebuilding imposes large cost

Must distinguish temporary vs. permanent disconnection Delayed has best performance and is most practical

Future Work Other scenarios: realistic traces, mobile subscribers Develop more optimizations

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Publisher Mobility in Distributed

Publish/Subscribe Systems

Thank you