ISADS'03 Message Logging and Recovery in Wireless CORBA Using Access Bridge Michael R. Lyu The Chinese Univ. of Hong Kong

u Introduction u Wireless CORBA u Fault Tolerance Architecture u Simulation and Evaluation u Conclusions Outline

Introduction u Mobile Computing u Permanent failures Physical damage u Transient failures Mobile hosts Wireless links Environmental conditions u Fault Tolerant CORBA Entity replication

Wireless CORBA Architecture Visited Domain Home Domain Terminal Domain Access Bridge Static Host Terminal Bridge GIOP Tunnel ab 1 ab 2 mh 1 GTP Messages

Wireless CORBA Architecture Visited Domain ab 1 ab 2 Access Bridge Static Host Home Domain Home Location Agent Terminal Domain Terminal Bridge GIOP Tunnel mh 1 Terminal Domain Terminal Bridge GIOP Tunnel mh 1 Terminal Domain Terminal Bridge Access Bridge

Device, Wireless & Mobile Issues u Device Issues Slow processor Small memory Small disk space Low power supply Physical damage Applying mobile host as stable storage  A large number of system messages or a large size of information carried in one message   Checkpoints and logs collection u Wireless Issues High bit error rate Little bandwidth Long transfer delay u Mobile Issue Handoff  Applying Access Bridge as stable storage  Uncoordinated checkpointing Pessimistic message logging

Checkpointing Strategy in MH u Uncoordinated Checkpointing takes its checkpoints independently no coordinated messages or additional control information u Trigger local timer number of received messages u Stable Storage currently associated AB

Message Logging Mechanisms in AB u All messages are transmitted through or to AB u Quasi-sender-based messages intended to MHs from other hosts (ABs or static hosts) needs the sequence number of corresponding acknowledgement u Receiver-based only sequence number of messages sent from MHs

Fault Tolerance Architecture Client Object Terminal Bridge Recovery Mechanism ORB Platform Mobile Host Recovery Mechanism Logging Mechanism Platform Access Bridge Mobile Side Fixed Side Mobile Support Station ORB Recovery Mechanism Logging Mechanism ORB Platform Static Server GIOP Tunnel Multicast Messages Object Replica Recovery Mechanis m

Message Types u CheckpointData u CheckpointDataReply u FetchCheckpoint u FetchCheckpointReply u PurgeCheckpoint

Unavailable Mobile Host u Mobile host disconnection u Mobile host crash u Access bridge crash

Message Status in Access Bridge u Received default status u Sent relayed to its target but before receiving the acknowledgement u Processed after receiving an acknowledgement

Transient Mobile Host Disconnection Access Bridge Mobile Side GTP GTP Adaptation Layer Transport Layer Fixed Side TCP IIOP Connection ID mapping u Fixed side keeps IIOP connections with remote servers for a predefined time period

Mobile Host Handoff Access Bridge 1 Access Bridge 2 Access Bridge 3 Home Location Agent Handoff Location Update

Mobile Host Handoff Home Location Agent Access Bridge 1 Access Bridge 2 Access Bridge 3 Handoff Location Update

Mobile Host Crash Home Location Agent Access Bridge 1 Access Bridge 2 Access Bridge 3

Mobile Host Recovery Home Location Agent Access Bridge 1 Access Bridge 2 Access Bridge 3 Collect last checkpoint and successive message logs Sorted by Ack. SN Reconnect Messages Replay

Log Collection Strategy u The HLA triggers the transfer of checkpoint and message logs u The collected logs are to be saved in the current AB u The timer is adaptive It will extend the time period if an MH moves frequently it will shorten the time period if the MH maintains connection with an AB for a long time u Checkpointing terminates log collection process

Access Bridge Crash u Select Previous or intended AB as stable storage replicates the execution context and message logs in an AB to its previous or intended AB for each MH u Engage handoff as a recovery mechanism creates connectivity with other ABs just as a normal handoff from the previous or intended AB creates connectivity with the previous or intended AB without additional handoff steps u The HLA removes the failed AB from the MH’s track

Garbage Collection u Initiated with log collection by HLA u Initiated by AB selects some MHs as the targets of garbage collection enforces MHs to take checkpoints

u Introduction u Wireless CORBA u Fault Tolerance Architecture u Simulation and Evaluation u Conclusions Outline

Program Execution Time

Handoff Effect

Conclusions u Describes a message logging and failure recovery protocol in wireless CORBA u Employs both quasi-sender-based and receiver-based message logging methods u Tolerates mobile host disconnection, mobile host crash, and access bridge crash u Engages handoff mechanism as a means to recover from access bridge crash

Thank You