1 / 22

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

kiara-miranda
Download Presentation

Low Cost Commit Protocols for Mobile Computing Environments

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related