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, suri}@informatik.tu-darmstadt.de

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

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

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

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

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

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

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

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 …

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

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

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

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

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

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

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

Questions ? Brahim Ayari, Abdelmajid Khelil and Neeraj Suri Department of Computer Science TU Darmstadt, Germany {brahim, khelil, suri}@informatik.tu-darmstadt.de