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The Cougar Approach to In-Network Query
Processing in Sensor Networks
By Yong Yao and Johannes GehrkeCornell University
The Cougar Approach to In-Network Query
Processing in Sensor Networks
By Yong Yao and Johannes GehrkeCornell University
Presented by Penelope BrooksPresented by Penelope Brooks
OverviewOverview
• Motivation• Sensor Networks Overview• Applications• Sensor Data• Problems in Sensor Networks• Cougar
– Architecture– Approach
• Related Projects
• Motivation• Sensor Networks Overview• Applications• Sensor Data• Problems in Sensor Networks• Cougar
– Architecture– Approach
• Related Projects
MotivationMotivation
• Distributed database approach to sensor networks
• Why?– Declarative queries are well-suited to
sensor networks– Energy conservation in sensor
networks is crucial
• Distributed database approach to sensor networks
• Why?– Declarative queries are well-suited to
sensor networks– Energy conservation in sensor
networks is crucial
The Big IdeaThe Big Idea
Local computation is much cheaper than communication, so push
computation to the network and improve energy consumption
Local computation is much cheaper than communication, so push
computation to the network and improve energy consumption
Sensor Networks OverviewSensor Networks Overview
• Thousands of sensors connected through wireless communication– Multi-hop routing protocol used– Limited computation and storage– Limited energy supply
• Sensor nodes connected to one or more physical sensors
• Distributed to measure and monitor physical environment
• Communication and computation biggest energy drains
• Thousands of sensors connected through wireless communication– Multi-hop routing protocol used– Limited computation and storage– Limited energy supply
• Sensor nodes connected to one or more physical sensors
• Distributed to measure and monitor physical environment
• Communication and computation biggest energy drains
ChallengesChallenges
• Communication• Power consumption• Computation• Uncertainty in sensor readings
• Communication• Power consumption• Computation• Uncertainty in sensor readings
Some ApplicationsSome Applications
• Besides temperature…• Intelligent building management• Hostile environments
– Battlefield– Disaster regions/Early warning systems
• Tracking items in transit• Automatic target recognition and
tracking
• Besides temperature…• Intelligent building management• Hostile environments
– Battlefield– Disaster regions/Early warning systems
• Tracking items in transit• Automatic target recognition and
tracking
Sensor DataSensor Data
• Uncertainty of data values– Measurements accurate within range– Addressed by aggregation
• Historically - sensor networks collect data and transfer to central node for querying and analysis
• Uncertainty of data values– Measurements accurate within range– Addressed by aggregation
• Historically - sensor networks collect data and transfer to central node for querying and analysis
Problems in Sensor NetworksProblems in Sensor Networks• Aggregation
– Must complete at leader node– Data has to be delivered from source nodes– Computation approaches
• Send all data to leader and compute there• Some computation at nodes along path
• Query Languages– Diverse applications, data, query classes – Look at properties of sensor data, abstract
computational patterns that fit
• Aggregation– Must complete at leader node– Data has to be delivered from source nodes– Computation approaches
• Send all data to leader and compute there• Some computation at nodes along path
• Query Languages– Diverse applications, data, query classes – Look at properties of sensor data, abstract
computational patterns that fit
Problems in Sensor Networks (cont’d)Problems in Sensor Networks (cont’d)• Query Optimization
– Large space of possible query plans– Cost of plan is energy consumed– Make decisions with uncertainty
• Catalog Management – Metadata for optimizer– Sensor position, density, connectivity,
system workload, network stability
• Multi-Query Optimization– Share results from similar queries
• Query Optimization– Large space of possible query plans– Cost of plan is energy consumed– Make decisions with uncertainty
• Catalog Management – Metadata for optimizer– Sensor position, density, connectivity,
system workload, network stability
• Multi-Query Optimization– Share results from similar queries
Cougar ArchitectureCougar Architecture
• Loosely-coupled, distributed• Supports in-network computation• Query optimizer on sensor gateway
– Describes data flow in network – Computation flow in each sensor
• Query proxies on sensor nodes– Register query– Create local operator tree– Activate relevant sensors– Return applicable results
• Loosely-coupled, distributed• Supports in-network computation• Query optimizer on sensor gateway
– Describes data flow in network – Computation flow in each sensor
• Query proxies on sensor nodes– Register query– Create local operator tree– Activate relevant sensors– Return applicable results
contribution
contribution
Cougar ArchitectureCougar Architecture
Query Proxy Layerhere
Query Optimizerhere
ApproachApproach
• Query presented to optimizer• Query optimizer
– Merge with existing query OR
– Generate new query plan
• Query presented to optimizer• Query optimizer
– Merge with existing query OR
– Generate new query plan
Approach (cont’d)Approach (cont’d)
• Designate leader for computation– Methods
• Fixed • Randomly selected node
– Leader selection policy• Dynamically maintained in case of failure• Minimize communication distance
• Two plans: leader, other• Query plans disseminated to all nodes
• Designate leader for computation– Methods
• Fixed • Randomly selected node
– Leader selection policy• Dynamically maintained in case of failure• Minimize communication distance
• Two plans: leader, other• Query plans disseminated to all nodes
Query PlanQuery Plan
In-networkaggregation
NetworkInterface
Sensorscan
Towards the leader
Select
AggregateOperator
NetworkInterface
Towards the gatewayQPO
QPL
Data from local sensor
Partially aggregateddata from other sensors
Partially aggregated results
AggregatedResults
ExampleExample
• Query Q:– Monitor office temperature– Generate notification to administrator when
temperature over threshold
• Optimize query• Query Plan QP generated, leader
identified, computation plans generated• Query plans disseminated• Query proxy actions initiated
• Query Q:– Monitor office temperature– Generate notification to administrator when
temperature over threshold
• Optimize query• Query Plan QP generated, leader
identified, computation plans generated• Query plans disseminated• Query proxy actions initiated
Example (cont’d)Example (cont’d)
• Sensors collect temperature• Leader aggregates sensors readings,
performs AVG• Aggregate value compared to initial
condition of query Q • If AVG > threshold
– Value sent to gateway– Administrator notified
• Otherwise, sensors continue
• Sensors collect temperature• Leader aggregates sensors readings,
performs AVG• Aggregate value compared to initial
condition of query Q • If AVG > threshold
– Value sent to gateway– Administrator notified
• Otherwise, sensors continue
Another ExampleAnother Example
TinyDB: An Acquisitional Query Processing System for Sensor NetworksSAMUEL R. MADDEN, MICHAEL J. FRANKLIN, JOSEPH M. HELLERSTEIN, and WEI HONGACM Transactions on Database Systems, Vol. 30, No. 1, March 2005, Pages 122–173.
Related ProjectsRelated Projects• CoSense - Xerox PARC• SCADDS - UCLA• WebDust - Rutgers• Agent-based Tasking of Massive Sensor
Networks - Univ of MD• Reactive Sensor Networks - Penn State• TinyOS - Berkeley• Telegraph - Berkeley• Location-Centric Distributed Computation
and Signal Processing - Wisconsin
• CoSense - Xerox PARC• SCADDS - UCLA• WebDust - Rutgers• Agent-based Tasking of Massive Sensor
Networks - Univ of MD• Reactive Sensor Networks - Penn State• TinyOS - Berkeley• Telegraph - Berkeley• Location-Centric Distributed Computation
and Signal Processing - Wisconsin
Wrap-UpWrap-Up
• Cougar is one possible architecture for a sensor network
• Performs in-network computation• Decreases energy consumption• One leader per query plan• Attempt to merge similar queries• Propagate results to system if
condition met
• Cougar is one possible architecture for a sensor network
• Performs in-network computation• Decreases energy consumption• One leader per query plan• Attempt to merge similar queries• Propagate results to system if
condition met
Questions?Questions?