1. Transis Efficient Message Ordering in Dynamic Networks PODC 1996 talk slides Idit Keidar and Danny Dolev The Hebrew University Transis Project

2. Transis The System Model Asynchronous Network partitions and site failures Transport layer (Transis) provides: –Locally ordered multicast within connected components –Membership

3. Transis An Algorithm for Totally Ordering Messages A connected majority can always make progress –Previous protocols sometimes block All processes may initiate messages, that may eventually be ordered A majority orders messages within two rounds

4. Transis COReL - Consistent Object Replication Layer COReL Transis Application physical network multicast messages totally ordered messages locally ordered messages send messages membership

5. Transis Reliable Multicast Logging all messages ACKing logged messages Retransmission upon recovery Gossip-like dissemination

6. Transis Message Ordering Transis orders messages within each network component –This order is not atomic COReL uses this order within a primary component: Members of a primary component P totally order a message once it was ACKed by all the members of P

7. Transis The Colors Model Red: –No knowledge of order –Messages from minority Yellow: –Message from primary Green: –Totally ordered –All members of primary marked as yellow (ACKed) Message Queue (MQ)

8. Transis Knowledge Levels in MQ prq 5 4 3 2 1 6 5 4 3 2 1 4 3 2 1 2 1 x s Process s partitioned from p, q and r

9. Transis Invariants of MQ FIFO and Causal Question is ordered before answer No changes in green Agreed green prefix Yellow If p marked m as green in P, and q knows of P: q has m as yellow or green q and p have the same messages before m

10. Transis Recovery: (upon membership change) Retransmission of missing messages Green and yellow messages from the last primary component are ordered before concurrent red messages –Agreeing upon MQ –Preserving the invariants If majority: establish new primary

11. Transis Establishing a New Primary Recovery Establish Commit Attempt Majority? All Attempted? All Committed? If a membership change occurs - restart the Recovery

12. Transis During Recovery - Need to Know the Last Primary q, r and s become connected {p,r}q, 4 3 2 1 6 5 4 3 2 1 4 3 2 1 {s} 2 1 x y 2 1 x y 4 3 {q, r, s} BEFORE RECOVERYAFTER Last Primary

13. Transis Numbering the Primary How can we determine which primary is latest? Use two variables: –LAST COMMITTED PRIMARY Used to know which is later Unique number –LAST ATTEMPTED PRIMARY Guarantees uniqueness of LAST COMMITTED PRIMARY

14. Transis A Majority Can Always Make Progress When a majority becomes connected: All members recover state of last primary The members establish a new primary The members of an established primary order messages ACKed by all of them

15. Transis Conclusions COReL orders messages within connected primary components in two communication rounds Extends local order determined by Transis within primary components Messages from minorities diffuse through the system and eventually become ordered

16. Transis Thanks to... Yair Amir, Dalia Malki, Catriel Beeri