Low cost commit protocols for mobile computing environments
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Low Cost Commit Protocols for Mobile Computing Environments. Marc Perron & Baochun Bai. Introduction. Introduction to Mobile Database Environment Commit Protocols Classical Two-Phase Commit Mobile Two-Phase Commit Optimistic Concurrency Control with Update Time Stamp

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Presentation Transcript

Introduction l.jpg
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


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