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Reliable Communication

Reliable Communication. Smita Hiremath CSC 8530. Reliable Client-Server Communication. Point-to-Point communication Established by TCP Masks omission failure, crash failure not masked. Cont…. RPC Semantic in the presence of Failure The client is unable to locate the server

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Reliable Communication

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  1. Reliable Communication Smita Hiremath CSC 8530

  2. Reliable Client-Server Communication Point-to-Point communication • Established by TCP • Masks omission failure, crash failure not masked

  3. Cont… RPC Semantic in the presence of Failure • The client is unable to locate the server • The request message from the client to the server is lost • The server crashes after receiving a request • The reply message from the server to the client is lost • The client crashes after sending the request.

  4. Cont… Client Cannot Locate the Server • Server might be down • Server evolves and new version of interface is installed Lost Request Messages • Easiest one to deal • Client stub start the timer when sending the request.

  5. Cont… • Server Crashes • A server in client-server communication • Normal case • Crash after execution • Crash before execution

  6. Cont… • At least once semantics – Guarantees that the RPC has been carried out at least once, but possibly more. • At most once semantics – Guarantees that the RPC has been carried out at most once, but possibly none at all • Guarantees nothing. RPC may have been carried out anywhere from zero to a large number of times

  7. Cont… Lost Reply Message • Some requests can be repeated (idempotent) • Assign each request a sequence number Client Crashes • Computations are orphans • Extermination – the log is kept on the disk or some other medium that survives crash. • Reincarnation – divide the time up into sequential numbered epochs.

  8. Cont… • Gentle reincarnation – computations are killed if the owner cannot be found. • Expiration – RPC is given standard amount of time.

  9. Reliable Group Communication Basic Reliable-Multicasting • Message sent to a process group should be delivered to each member of that group. • Agreement exists on who is the a member of the group.

  10. Reliable-Multicasting scheme A simple solution to reliable multicasting when all receivers are known and are assumed not to fail • Message transmission • Reporting feedback

  11. Scalability in Reliable Multicasting Two different approaches • Nonhierarchical Feedback Control • Hierarchical Feedback Control

  12. Nonhierarchical Feedback Control • Scalable Reliable Multicasting (SRM) • Never acknowledges successful delivery of multicast messages. • If the receiver missed a message, it multicasts its feedback to the rest of the group, which allows another group member to suppress its own feedback. • Only a single feedback message will reach the sender, which in turn subsequently retransmits the message.

  13. Nonhierarchical Feedback Control • Several receivers have scheduled a request for retransmission, but the first retransmission request leads to the suppression of others.

  14. Nonhierarchical Feedback Control • Scale reasonably well • Problems with this approach • Ensuring that only one request for retransmission is returned to the sender requires accurate scheduling of feedback messages at each receiver. • Feedback also interrupts those processes to which the message has been successfully delivered.

  15. Hierarchical Feedback Control • Scalability for very large groups of receivers. • The group of receivers are partitioned into number of subgroups, which are organized into trees. • Each subgroup appoints a local coordinator, which is responsible for handling retransmission • Local coordinator will have its own history buffer.

  16. Hierarchical Feedback Control The essence of hierarchical reliable multicasting. • Each local coordinator forwards the message to its children. • A local coordinator handles retransmission requests.

  17. Atomic Multicast • Atomic multicast problem – message must be delivered to either all processes or none at all.

  18. Virtual Synchrony • The logical organization of a distributed system to distinguish between message receipt and message delivery

  19. Cont… The principle of virtual synchronous multicast.

  20. Message Ordering Four different orderings: • Unordered multicasts • FIFO-ordered multicasts • Causally-ordered multicasts • Totally-ordered multicasts

  21. Cont… Three communicating processes in the same group. The ordering of events per process is shown along the vertical axis.

  22. Cont… Four processes in the same group with two different senders, and a possible delivery order of messages under FIFO-ordered multicasting

  23. Cont… Six different versions of virtually synchronous reliable multicasting.

  24. Implementing Virtual Synchrony • Process 4 notices that process 7 has crashed, sends a view change • Process 6 sends out all its unstable messages, followed by a flush message • Process 6 installs the new view when it has received a flush message from everyone else

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