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Low Cost Commit Protocols for Mobile Computing Environments

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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Low Cost Commit Protocols for Mobile Computing Environments

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  1. Low Cost Commit Protocols for Mobile Computing Environments Marc Perron & Baochun Bai

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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.

  7. 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).

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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. M-2PC Participant State Diagram (MU)

  15. M-2PC State Diagram for co-ordinator/participant (FN)

  16. 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.

  17. 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

  18. OCC-UTS Upon receiving commit operation: • Mobile client sends RequestToCommit message • Mobile client listens to CommitList and AbortList to determine outcome of transaction

  19. 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

  20. 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

  21. 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

  22. 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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