Hi folks,
Cassandra provides *linearizable consistency (CAS, Compare-and-Set) by using Paxos 4 round-trips as following* *1. **Prepare/promise* *2. **Read/result* *3. **Propose/accept* *4. **Commit/acknowledgment * Assume we have an application for resistering new account, I want to make sure I only allow exactly one user to claim a given account. For example, we do not allow two users having the same username. Assuming we have a cluster consist of 5 nodes N1, N2, N3, N4, and N5. We have two concurrent clients C1 and C2. We have replication factor 3 and the partitioner has determined the primary and the replicas nodes of the INSERT example are N3, N4, and N5. The scenario happens in following order: 1. C1 connects to coordinator N1 and sends INSERT V1 (assume V1 is username, not resister before) 2. N1 sends PREPARE message with ballot 1 (highest ballot have seen) to N3, N4 and N5. Note that this prepare for C1 and V1. 3. N3, N4 and N5 send a PROMISE message to N1, to not promise any with older than ballot 1. 4. N1 sends READ message to N3, N4 and N5 to read V1. 5. N3, N4 and N5 send RESULT message to N1, informing that V1 not exist which results in N1 will go forward to next round. 6. Now C2 connects to coordinator N2 and sends INSERT V1. 7. N2 sends PREPARE message with ballot 2 (highest ballot after re-prepare because first time, N2 does not know about ballot 1, then eventual it solves and have ballot 2) to N3, N4 and N5. Note that this prepare for C2 and V1. 8. N3, N4 and N5 send a PROMISE message to N2, to not promise any with older than ballot 2. 9. N2 sends READ message to N3, N4 and N5 to read V1. 10. N3, N4 and N5 send RESULT message to N2, informing that V1 not exist which results in N2 will go forward to next round. 11. Now N1 send PROPOSE message to N3, N4 and N5 (ballot 1, V1). 12. N3, N4 and N5 send ACCEPT message to N1. 13. N2 send PROPOSE message to N3, N4 and N5 (ballot 2, V1). 14. N3, N4 and N5 send ACCEPT message to N2. 15. N1 send COMMIT message to N3, N4 and N5 (ballot 1). 16. N3, N4 and N5 send ACK message to N1. 17. N2 send COMMIT message to N3, N4 and N5 (ballot 2). 18. N3, N4 and N5 send ACK message to N2. As result, both V1 from client C1 and V1 from client C2 have written to replicas N3, N4, and N5. Which I think it does not achieve the goal of *linearizable consistency and CAS. * *Is that true and such scenario could be occurred?* I look forward to hearing from you. Regards,
