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-quality design 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