Mobile Databases

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• Mobile Databases

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Outline

• Introduction
• Mobile DB Architecture Wiredwireless or MANET.
• Mobile DB strategies query processing, location
  based services, caching, replication.
• Mobile data dissemination, mobile security

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Mobile computing
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Mobile computing

• Portable (handheld) computing devices and
  wireless communications (GSM, CDMA, GPRS, 802.11
  a/b/g, WiMax)
• Can access data/resources from anywhere, anytime
• Example
• IPhone access emails, browse web.
• Location based services, e.g. query gas stations,
  restaurants, hotels, etc.
• Remote log in your computer from your PDA.
• Increasing popularity

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Mobile DB

• Mobile database data management technology
  enabling use of databases on mobile computing
  environment
• Data available anywhere independent of
  availability of fixed network
• Can access public data using internet browser
• Can access private data through distributed DB
• Data on mobile and fixed hosts sharable in
  seamless way
• More complex techniques needed to support this

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Example of Mobile DB
2 Query Location
3 Answer
4 Answer
1 Query Location Information
Query list the addresses of the gas stations
near me
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Challenges In Mobile Computing

• Origins in distributed systems
• Problems more challenging
• Asymmetric communication bandwidth
• Limited and intermittent connectivity
• Limited life of power supply of mobile units
• Changing topology of network
• Mobile database assumes traditional database
  properties, e.g. consistency, atomic

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Challenges In Mobile ComputingE.g. Handoff issues
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Outline

• Introduction
• Mobile DB Architecture Wiredwireless or MANET.
• Mobile DB strategies query processing, location
  based services, caching, replication.
• Mobile data dissemination, mobile security

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Mobile Computing Architecture

• Mobile units MU or Mobile Hosts MH
• Fixed hosts FH on fixed network
• Base Station BS serves as gateway to fixed and
  wireless network
• Geographic mobility domain divided into cells
• Mobile host wireless connection to BS of cell
• Movement of mobile units unrestricted
• Must maintain info for access contiguity

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Mobile Computing Architecture
a wireless cell
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DB
DB
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Mobile DB-WiredWireless

• When Mobile DB mix of fixed and wireless network
• Fixed FH location, high capacity, reliability,
  low connection cost
• Wireless support dynamic network topology, low
  capacity, reliability, high connection cost

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Mobile DB fixed network

• Suppose all of DB on fixed network, clients are
  mobile
• How is this different from distributed database
  that is non-mobile?
• Difficult to maintain sustained connection to
  server
• Database server typically is stateless,
  especially under broadcast systems
• Mobile units often cannot maintain a sustained
  network connection

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Mobile DB-MANET

• Mobile adhoc networks
• MUs do not need to communicate via a fixed
  network
• In MANET, MU responsible for routing own data,
  acting as BS
• Must be able to handle changes in network topology

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Example Mobile DB-MANET
Example Battlefield Ad hoc DB information system.
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Mobile DB-MANET

• No central control
• Peer-to-peer P2P
• Difficult for transaction processing and data
  consistency
• Example applications
• Battlefield information sharing
• Multi-user games
• Mobile ad-hoc sensor network
• Disaster relief team communication

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Outline

• Introduction
• Mobile DB Architecture Wiredwireless or MANET.
• Mobile DB strategies query processing, location
  based services, caching, replication.
• Mobile data dissemination, mobile security

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Mobile strategies

• Provide data cache on mobile host
• Cache replicas of frequently accessed data
• Work offline
• Reduce power consumption
• Client may be unreachable
• What if data cached updated during disconnection?

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Mobile strategies

• Resources of MU can be limited
• Mobile hosts personalized
• Bring in fraction of data need to access
• MU has low security
• Mobile DBs high degree of unavailability

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Mobile Data Management Issues

• Mobile environment requires new strategies for
• Querying location dependency
• Concurrency control ensuring updates consistent
• Processing transactions
• Data dissemination
• Recovery/fault tolerance
• Security

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Query processing

• Queries depend on geographic location, e.g. Get
  addresses of the hotels near me.
• Query optimization more difficult because of
  mobility and resource changes of MU
• MU may be in transit or may cross cell boundaries

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Location-based services

• Location dependent cache information may become
  stale
• Frequently updated location dependent queries
• Apply spatial queries to refresh cache problem

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Example Cloak scheme by Mokbel

• The user location is represented as a cloaked
  region that includes the accurate user location
• K-anonymity there are K users in the cloaked
  region.
• The area of the cloaked region achieves a
  trade-off between the user privacy and the
  service quality

A cloaked region that provides 4-anonymity
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Cloak scheme Architecture
2 Query cloaked Spatial Region
3 Answer
Trusted third party that cloaks/anonymizes the
exact location information.
1 Query Location Information
4 Answer
Query list the addresses of the gas stations
near me
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Caching

• To compensate for high latency and unreliable
  connectivity
• Frequently accessed data is cached
• Can work offline if necessary
• Dozing energy conserving state, unreachable
  state
• Can add proxies for unreachable components
• Proxies keep track of updates to cache

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Caching Data
Caching Data which data sets should be cached?
To where? When to replace? Objectives reduce
access latency, reduce network transmissions,
increase data availability
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Data replication

• Replication strategy answers the questions
• Who perform replica updates
• When propagate the replica updates
• Which sites need updated replicas
• Replication propagation time synchronous,
  asynchronous.

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Synchronous Replication

• Updates are propagated to all sites immediately
• Locks replicas while updating
• No inconsistencies are possible so no need for
  reconciliation
• Reads at connected sites get current data
• Reads at disconnected sites may get stale data
• Problems deadlocks, high message overhead,
  impractical for frequently-disconnected net.

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Asynchronous Replication

• Propagates replica updates after the updating
  transaction commits.
• Updates first occur on local data.
• Other sites update only after transaction
  commits.
• Problems may read stale replicas.

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Outline

• Introduction
• Mobile DB Architecture Wiredwireless or MANET.
• Mobile DB strategies query processing, location
  based services, caching, replication.
• Mobile data dissemination, mobile security

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Data Dissemination -Broadcasting

• Assumptions
• Requests are read-only (Most are)
• Because of latency, server can handle fewer
  clients in same amount of time
• Broadcasting acceptable solution
• Scalable single broadcast of data item can
  satisfy all outstanding requests for data item

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Data Dissemination -Broadcasting

• Broadcast-based data dissemination approaches
• Push-based data broadcasting
• Pull-based data broadcasting

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Data Dissemination -Broadcasting

• How does a mobile client access the database
• Widely accepted approach is to broadcast the data
  Data on Air

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Data Dissemination -Broadcasting

• Data contents within a file or database are
  repeatedly broadcast through the broadcast
  channel
• channel becomes a disk
• clients can retrieve data as it goes by
• expected wait time for a data item is the same

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Data Dissemination BroadcastingFlat broadcasting
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Data Dissemination BroadcastingData Disks

• broadcast data in different frequencies according
  to their relevant importance
• multi-level memory hierarchy
• hot data are broadcast more frequently then cold
  data
• Data with similar access frequency are grouped
  into disks

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Data Dissemination -Broadcasting
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Data Dissemination -Broadcasting

• also called adaptive approaches
• data items are broadcast on-demand
• only requested data will appear as data on air
• How do decide which page to broadcast next?

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Data Dissemination -Broadcasting

• Data broadcasting is prioritized according to
  some metrics
• Most common algorithms are
• First come Fist Served (FCFS) broadcasts the
  pages in the order they are requested.
• Most Requests First (MRF) broadcasts the page
  with maximum number of pending requests.
• Longest Wait First (LWF) selects the page that
  has the largest total waiting time, i.e., the sum
  of the time that all pending request for the item
  have been waiting. (RW is approximation)

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Security

• Mobile data less secure than data at fixed
  location
• Data is more volatile
• Must manage and authorize access to critical data
• Not desirable to share data with unauthorized
  users
• Many security issues not specific to DBSs
• Peers may need help of unauthorized peers to
  transfer data
• Encrypt data

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Example Attacks for Vehicular Network
Localization Services
Fake node E
A
Attacker B
B I am between (E, F)
B I am between (E, F)
Attacker B
Fake node F
C

• Report wrong locations

• Fake non-existing nodes

A

• Impersonate other node

Attacker B
B I am node D
C
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Recovery and Fault tolerance

• Site, media, transaction and communication
  failures
• Voluntary shutdown not a site failure
• Transaction failures can occur during handoff

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• Lei Tang
• Ph.D. student
• Research in Mobile Computing