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?