1. Fault Tolerance in CORBA and Wireless CORBA Chen Xinyu 18/9/2002

2. Outline s Introduction to CORBA and Wireless CORBA s What is Fault Tolerance s Fault Tolerant CORBA s Fault Tolerance in Wireless CORBA s Conclusion s Future Work

3. What is CORBA s Common Object Request Broker Architecture A Distributed Object Computing (DOC) open standard –Compare to platform/language specific alternatives –e.g., Java RMI, Microsoft’s DCOM A language-neutral environment A middleware infrastructure specification s Administered by the Object Management Group a.k.a., the OMG

4. Wireless CORBA Architecture s Encapsulates, forwards or ignores incoming GIOP messages s Decapsulates and forwards messages from the GIOP tunnel s Generates mobility events s Lists available services s Similar to the Access Bridge s Does not provide forwarding s Generates mobility events s Does not list services s Abstract transport-independent tunnel for GIOP messages s Concrete tunnels for TCP/IP, UDP/IP and WAP. s Only one GIOP tunnel s Keeps track of the associated access bridges s Redirects requests for services on the terminal Source: Telecom Wireless CORBA, OMG Doucment dtc/01-06-02

5. Wireless CORBA CORBA Key: TCP/IP Network CORBA objects may be invoked anywhere along the “end to end” path IIOP GTP Tunnel GIOP GTP Access Point

6. Fault, Error and Failure FaultErrorFailure Fault tolerant mechanisms an anomalous condition occurring in the system hardware or software the part of the system state that is liable to lead to a failure occurs when the delivered service of a system or a component deviates from its specification Fault tolerance is the ability of a system to continue providing its specified service despite component failure

7. Fault Tolerant CORBA Architecture Source: Bell Labs Research

8. Object Replication Styles s Passive Replication Passive Replication Only one replica processes each request, other replicas are available as backups Lower memory and processing costs Slower recovery from faults Duplicate message detection during recovery from faults s Active Replication Active Replication Several replicas process each request Faster recovery from faults State transfer to initialize new replicas

9. ORB Passive Replication Client invokes a method of Server A Server B Reliable totally ordered multicast Primary replica Primary replica Only primary replica of Server A executes the method Reply returned from primary replica of Server B to primary replica of Server A Only primary replica of Server B executes the method Reliable totally ordered multicast for state transfer Object Source: Eternal Systems, Inc

10. Active Replication Object ORB Client invokes a method of Server A Server B Reliable totally ordered multicast STOP Duplicate invocations suppressed Reliable totally ordered multicasts for requests and replies Object Duplicate replies suppressed STOP Source: Eternal Systems, Inc

11. Device, Wireless & Mobile Issues s 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 a message   Applying Access Bridge as Stable Storage  Uncoordinated checkpointing Pessimistic message logging  Checkpoints and Logs collection s Mobile Issue Handoff s Wireless Issues High bit error rate Little bandwidth Long transfer delay

12. Recovery Scheme s Uncoordinated checkpointing time predefined number of messages s Pessimistic message logging no extra communication overhead s Independent rollback recovery only failed objects rollback

13. Fault Tolerance Architecture Client Object Terminal Bridge Recovery Mechanism ORB Platform Mobile Host Recovery Mechanism Logging Mechanism Platform Access Bridge Mobile Side Fixed Side Access Bridge ORB Recovery Mechanism Logging Mechanism ORB Platform Remote Server GIOP Tunnel Multicast Messages Server Replica

14. Checkpoint and Logs Collection Strategies s Pessimistic checkpoint and logs are transferred during handoff generates heavy volume of data transfer s Lazy creates a linked list of Access Bridges complicated recovery s Frequency-based the number of handoffs s Distance-based the distance between mobile host and the Access Bridge carrying its latest checkpoint

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

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

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

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

19. Conclusion s Fault Tolerant CORBA is based on Object Replication s Fault tolerance in Wireless CORBA is based on Rollback-Recovery Protocol s Checkpoint and message logs collection is important in Wireless CORBA

20. Future Work s Low-cost Checkpointing Algorithm forces a minimum number of objects to take checkpoints minimizes the number of synchronization messages makes checkpointing nonblocking s Failure Detection in Wireless Environment

21. Question and Answer

22. Thank You