1. FT-PPTC: An Efficient and Fault-Tolerant Commit Protocol for Mobile Environments
Brahim Ayari, Abdelmajid Khelil and Neeraj Suri
Department of Computer Science
TU Darmstadt, Germany
{brahim, khelil,

2. Motivation Mobile transactions Mobile participants Atomic commit is a
Mobile inventory
M-commerce
Mobile gaming
Mobile participants
Atomic commit is a
fundamental operation
Wired Network

3. Motivation Heterogeneity Perturbations Wireless networks Mobile nodes
Slowest process determines
resource blocking time
Perturbations
Disconnections
Node/Comm failures
Increases resource
blocking time
or abort rate
Wired Network
WLAN
GPRS
UMTS

4. Objectives Ensure strict atomicity
Minimize the blocking time of resources in the wired network
Minimize transaction abort rate (heterogeneity and fault-tolerance)
Reduce (wireless) message complexity
System Model
Transaction Model
Fault Model
Our Approach
Related Work
Evaluation
Summary

5. System Model Mobile Hosts (MH) vs. Fixed Hosts (FH) Heterogeneity in
MHs may not have stable storage
Heterogeneity in
Wireless network (WLAN,
UMTS, GPRS ...)
 Delay and bandwidth
Mobile nodes (laptops, PDAs,
cellular phones …)
 Processing capability
Autonomy of (mobile) nodes
Management and sharing of data
Execution/decision to commit or abort the transaction

6. Transaction Model Mobile transaction: Coordinator (CO) role
Distributed transaction where at least one MH participates in its execution
Set of execution fragments
Coordinator (CO) role
Distributes execution fragments to corresponding nodes
Takes the final decision either to commit or abort
Stores information about execution state
Implemented by a FH
2PC in the fixed network

7. Fault Model Communication failures Node failures Message loss
Network disconnection
Node failures
Transient MH failures: SW/HW faults, power loss, user-triggered failures (put-off) …
Permanent MH failures: Loss, damage …
CO failures: Crash recovery model
FH failures: Crash recovery model

8. Related Work Unilateral Commit for Mobile (UCM) [PDCS’00]
Supports mobile participants
Strict assumptions (two-phase locking for all participants)
Mobile Two Phase Commit (M-2PC) [ADC’05]
Mobile participants are assumed to be connected from initializing the transaction until finishing the execution of their fragments
Transaction Commit On Timeout (TCOT) [TOC’02]
Uses timeouts to reduce the number of messages exchanged
Provides only semantic atomicity and not strict atomicity
Supports only one mobile participant
Entail high resource blocking time
Handle (small) subset of failures

9. Fault-Tolerant Pre-Phase Transaction Commit (FT-PPTC)
Main idea:
►Decouple commit of mobile participants from fixed participants
►Initial mobile commit, delegate problem to fixed part of network;
 Reduction of resource blocking time on fixed participants
 Re-use established protocols (commit, recovery, fault-tolerance)
CO distributes fragments first for mobile participants and waits for their execution results (Pre-Phase)
CO waits for a mobile participant for the timeouts that the participant initializes and updates (execution and shipping timeouts)
 CO waits for the slowest mobile participant and proceeds with the following phase iff the pre-phase succeeds
Handles network and node heterogeneity
Each mobile participant is represented in the fixed network by a proxy called mobile host agent (MH-Agent)
 Handles failures: Crash, disconnection, wireless message loss …

10. Send corresponding execution fragment
The FT-PPTC Protocol
Initiator
Coordinator
MH-Agent
Mobile Participant
Fixed Participant
Begin
Send transaction
& own timeouts
Forward fragment
Send corresponding execution fragment
Send timeouts
Forward timeouts
Hetero-
geneity ~
Pre-Commit Phase ~
Send updates
Send YES Vote
Send updates
Fixed Participant
Execution fragment
Ack
2PC
Prepare
Core 2PC Phase
Vote
Decision
Decision
Decision
Ack
Ack
Ack
End
Release
resources
Release
resources
Release
resources

11. FT-PPTC: Failure Resilience Scenarios
Initiator
Coordinator
MH-Agent
Mobile Participant
Begin
Send transaction
and timeouts
Forward fragment
Send corresponding execution fragment
Send timeouts
Transient
Disconnect.
Forward timeouts ~
Pre-Commit Phase ~
Extend timeouts
Forward extended timeouts
Send updates ~
Permanent
failures
Send updates
Send YES Vote
Fixed Participant
Execution fragment
Ack
2PC
Prepare
Core 2PC Phase
Vote
Decision
Decision
Decision
Ack
Ack
Ack
End
Release
resources
Release
resources
Release
resources

12. Related Work Unilateral Commit for Mobile (UCM) [PDCS’00]
Supports mobile participants
Strict assumptions (two-phase locking for all participants)
Mobile Two Phase Commit (M-2PC) [ADC’05]
Mobile participants are assumed to be connected from initializing the transaction until finishing the execution of their fragments
Transaction Commit On Timeout (TCOT) [TOC’02]
Uses timeouts to reduce the number of messages exchanged
Provides only semantic atomicity and not strict atomicity
Supports only one mobile participant
Entail high resource blocking time
Handle (small) subset of failures

13. Comparison to Related Work
Comparison of the message complexity in failure-free case:
mp : #mobile participants sp : #fixed participants e : #timeout extensions
Coverage of failures:
Protocol
Atomicity
# Phases
Wireless Message Complexity
Overall Message Complexity
FT-PPTC
strict 2
(2 + 1)mp + e
(3mp + e) + (4sp + 2mp + e)
M-2PC
(2 + 1)mp - 1
(4mp - 1) + 4sp
TCOT
semantic 1
2mp – 1 + e
(2mp - 1) + 2sp + eall
UCM
(1 + 1)mp
2mp + 2sp
Protocol
Transient MH-failure
Permanent
MH-failure
Coordinator failure
Message loss
Delay of messages
Network disconnections
FT-PPTC x
M-2PC
TCOT
UCM

14. Simulation Settings Simulator: SimJava 2.0 (University of Edinburgh)
Load model: Each MH initiates a mobile transaction
Parameter
Value
# fixed participants 4
# mobile participants
[1, 25]
Average execution time of one fragment (MH)
5 ms
Average execution time of one fragment (FH)
2 ms
Transmission delay over wireless link
10 ms
Transmission delay over wired link
Average number of fragments per MT 5

15. Simulation Results Throughput Resource Blocking Time
 FT-PPTC is comparable to 2PC
 FT-PPTC is scalable

16. Conclusions and Future Work
FT-PPTC: Fault-tolerant atomic commit protocol for mobile transactions
Decouples commit of mobile participants from fixed participants
 Minimizes blocking time of resources on fixed part of the network
 Provides resilience to both communication and node failures
Future work
Design efficient fault-tolerant recovery mechanisms
Comprehensive sensitivity analysis
Consider ad-hoc communication between the mobile devices

17. Questions ? Brahim Ayari, Abdelmajid Khelil and Neeraj Suri
Department of Computer Science
TU Darmstadt, Germany
{brahim, khelil,