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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 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
mobile database environment
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
mobile database environment4
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
commit protocols
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
commit protocols cont
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.
commit protocols cont7
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).
commit protocols8
Commit Protocols

Focus on two commit protocols:

  • Classical 2-Phase Commit
    • Presumed Commit
    • Presumed Abort
  • Optimistic Concurrency Control with Update Time Stamp
classical 2pc
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
classical 2pc10
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
classical 2pc11
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
classical 2pc12
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
m 2pc mobile 2pc
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
m 2pc

Participant State Diagram (MU)

m 2pc15

State Diagram for co-ordinator/participant (FN)

optimistic concurrency control with update time stamp
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.
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
occ uts18

Upon receiving commit operation:

  • Mobile client sends RequestToCommit message
  • Mobile client listens to CommitList and AbortList to determine outcome of transaction
occ uts19

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
occ uts20

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
  • 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

Low Cost Commit Protocols for Mobile Computing Environments


Commit Protocols

Two Phase Commit

Mobile-Two Phase Commit

Optimistic Concurrency Control with Update Timestamp