Low Cost Commit Protocols for Low Cost Commit Protocols for Mobile Computing Mobile Computing

EnvironmentsEnvironments

Marc Perron & Baochun Bai

IntroductionIntroduction

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 environmentMobile 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 environmentMobile 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 ProtocolsCommit 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 (Cont.)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 ProtocolsCommit Protocols

Focus on two commit protocols: Classical 2-Phase Commit

Presumed Commit Presumed Abort

Optimistic Concurrency Control with Update Time Stamp

Classical 2PCClassical 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 2PCClassical 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 2PCClassical 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 2PCClassical 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-2PCM-2PC

Participant State Diagram (MU)

M-2PCM-2PC

State Diagram for co-ordinator/participant (FN)

Optimistic Concurrency Control Optimistic Concurrency Control with Update Time Stampwith 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-UTSOCC-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-UTSOCC-UTS

Upon receiving commit operation: Mobile client sends RequestToCommit message

Mobile client listens to CommitList and AbortList to determine outcome of transaction

OCC-UTSOCC-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

OCC-UTSOCC-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

ConclusionConclusion

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

SummarySummary

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