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Algorithm for Virtually Synchronous Group Communication

Algorithm for Virtually Synchronous Group Communication. Idit Keidar, Roger Khazan, Nancy Lynch MIT Lab for Computer Science Theory of Distributed Systems Group. Virtual Synchorny. Application. Virtual Synchrony. Multicast Service. Membership Service. Virtual Synchrony.

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Algorithm for Virtually Synchronous Group Communication

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  1. Algorithm for Virtually Synchronous Group Communication Idit Keidar, Roger Khazan, Nancy Lynch MIT Lab for Computer Science Theory of Distributed Systems Group

  2. Virtual Synchorny Application Virtual Synchrony Multicast Service Membership Service

  3. Virtual Synchrony • Synchronization of Messages and Views: • Powerful abstraction for replication • Semantics: VS [Birman, Joseph 87], EVS, SVS Procs that go together through same views, deliver same sets of messages.

  4. Example: Virtual Synchrony

  5. Project Goals High-qualitydesign of a VS GCS for WANs • Mathematical-quality (precise, formal, well-documented) • Useful semantcis • Efficient algorithm • Scalable architecture • Modular design • Specification • Algorithm • Proof • Performance Analysis

  6. Publications • ICDCS’00: Intern. Conf. On Distributed Comp. • Submited to SICOMP (SIAM Journal of Comp.) • ICSE’00: Intern. Conf. On Software Engineering • Invited to ACM TOSEM

  7. Virtual Synchrony: How To? • Before moving into new view: • Need to know which synch msgs to use, since there may be several view proposals Exchange synch messages (“flush”) to agree which msgs to deliver in old view.

  8. Example: Synchronization Msgs

  9. Problematic Scenario

  10. Existing Solutions • Limit Reconfiguration • Do not allow joins during reconfiguration • When someone wants to join: • first, deliver view without joiner; • then, start new reconfiguration. • Use common id to identify synch msgs for same view proposal

  11. Limited Reconfiguration

  12. Problems with Existing Solutions • Limited Reconfiguration • Obsolete views delivered to application • Creates overhead • Limits usefulness of virtual synchrony • Use of common id to identify synch msgs • Pre-agreement or dissemination is required • Costly, especially in WANs

  13. Our Idea • Don’t limit reconfiguration • Issue locally unique id per process for each view proposal • Tag synch msgs with these local ids • View includes vector of latest local ids • View is a triple: e.g., < 4, {p, q, r}, [8, 9, 3] > • Procs use sync msgs identified by view • Hence, procs use right sync msgs

  14. Our Algorithm Allows Joiners

  15. No Common Sync Ids Required

  16. Transient Failure

  17. Implementation • VS library (C++), linked with application • Use [KSMD,00] membership service implemented in C++, socket interface with members • Reliable FIFO layer (made in Hebrew University), uses IP multicast and recovers lost messages, library --- linked with VS

  18. Group Communication -- Useful “Building Block” • Group Abstraction • processes interact in a group • dynamic: fail/join/partition/merge • Reliable Group Multicast • Group Membership -- generates “views” • tell each process who it is connected to • Systems: Ensemble, Horus, Isis, Newtop, Psync, Sphynx, Relacs, Totem, Transis

  19. Example: Group Communication

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