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 Conclusion : Which is better

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

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

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

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

OCC-UTS Server's algorithm  Server keeps list of updated data items for transactions committed between t si -  L and t si.   : number of invalidation broadcast windows  L: Length of broadcast window  t si : most recent time stamp on a data item

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

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

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