Low Cost Commit Protocols for Mobile Computing Environments

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Low Cost Commit Protocols for Mobile Computing
Environments
Marc Perron Baochun Bai
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Introduction

• Introduction to Mobile Database Environment
• Commit Protocols
• Classical Two-Phase Commit
• Mobile Two-Phase Commit
• Optimistic Concurrency Control with Update Time
  Stamp
• Conclusion Which is better

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Mobile database environment

• Mobile networks are composed of cells.
• Similar to C/S environment.
• Base station (Mobile Support Station)
• Mobile unit
• Low bandwidth
• High error rate

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Mobile database environment

• Asymmetry in communication
• Downstream (server-to-client) bandwidth is high.
  Using broadcast disk method to deliver data to
  client.
• Upstream (client-to-server) bandwidth is low.
• High disconnection rate
• Power limitation

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Commit Protocols

• 2-Phase Commit (2PC)
• Two phases Prepare Phase and Commit Phase
• Logs are maintained on the base stations
• Presumed Commit (PC)
• Optimized 2PC
• Reduces the number of exchanged messages and the
  number of log

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Commit Protocols (Cont.)

• Early Prepare (EP)
• Eliminates a round messages by putting a
  transaction into ready state as soon as it
  finishes processing the work part of the
  transaction.
• WoundCertifier
• Certification report with ReadSet and WriteSet of
  active transactions is broadcasting by server.

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Commit Protocols (Cont.)

• Batched Transaction
• Executes the entire transaction on the mobile
  unit using cached copies of data.
• Optimistic Concurrency Control with Update Time
  Stamp (OCC-UTS)
• Each data item has a timestamp.
• Invalidation report broadcast by server.
• Two versions (with or without local cache).

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Commit Protocols

• Focus on two commit protocols
• Classical 2-Phase Commit
• Presumed Commit
• Presumed Abort
• Optimistic Concurrency Control with Update Time
  Stamp

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Classical 2PC

• In Mobile Environment, behaves essentially the
  same as in non-mobile distributed environments
• A few differences arrise due to mobile nature
• Mobile Client logs maintained on fixed network
• Behaviour slightly modified to reduce messages
  sent over wireless link

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Classical 2PC

• Phase 1
• Co-ordinator sends 'prepare' message to all
  participants to prepare them to commit the
  transaction
• Phase 2
• If all participants respond successfully to the
  prepare message, the co-ordinator globaly commits
  the transaction

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Classical 2PC

• Non-Mobile Database Environment
• Co-ordinator The process at the site where the
  transaction originated. The execution controlled
  by this process
• Participant Processes at other sites
  participating in executing the transaction

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Classical 2PC

• In Mobile Computing Environment
• Mobile unit always co-ordinator
• Large number of messages over wireless link
• Not all participants directly accessable from
  mobile units

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M-2PC (Mobile-2PC)

• Modifications to 2PC for mobile environment
• Base station replaces mobile unit as transaction
  co-ordinator
• Control of transaction handed off with state
  information

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M-2PC

• Participant State Diagram (MU)

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M-2PC

• State Diagram for co-ordinator/participant (FN)

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Optimistic Concurrency Control with Update Time
Stamp

• On transaction commit, server verifies that
  execution is serializable.
• Two forms of validation
• Backward Check if commiting transaction
  invalidated by commit of another.
• Forward Check if commiting transaction conflicts
  with any other active transaction.

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OCC-UTS

• Data has time stamp associated with it
• Time stamps used to determine if transaction
  attempting commit is serializable
• Mobile client checks transactional cache
  consistency using invalidation reports from server

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OCC-UTS

• Upon receiving commit operation
• Mobile client sends RequestToCommit message
• Mobile client listens to CommitList and AbortList
  to determine outcome of transaction

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OCC-UTS

• Server's algorithm
• Server keeps list of updated data items for
  transactions committed between tsi-?L and tsi.
• ? number of invalidation broadcast windows
• L Length of broadcast window
• tsi most recent time stamp on a data item

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OCC-UTS

• Server receives RequestToCommit message
• adds message to outsanding RequestToCommit queue
• Takes a message m from queue
• If at least one data item in m has timestamp
  older than most recent, transaction is aborted
• Otherwise transaction commited

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Conclusion

• OCC-UTS takes advantage of broadcast disks
• Requires fewer upstream messages on a wireless
  link
• Less complex than M-2PC, transactions performed
  on local cached data
• We conclude that OCC-UTS better than M-2PC for
  mobile computing environment

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Summary

• Low Cost Commit Protocols for Mobile Computing
  Environments
• Introduction
• Commit Protocols
• Two Phase Commit
• Mobile-Two Phase Commit
• Optimistic Concurrency Control with Update
  Timestamp
• Conclusion