On Aug 15, 2011, at 18:18 , Tuure Laurinolli wrote: > > On Aug 12, 2011, at 20:40 , Tuure Laurinolli wrote: >> >>> Updates will however propagate from the master to other slaves eventually so >>> a write from one slave is not immediately visible on all other slaves. >>>>> It sounds like eventual consistency from master to other slaves. if so, I >>>>> am interested in finding out details about Neo4j HA member nodes voting >>>>> quorum & arbitrater setup (assuming using zookeeper) >> >> Looking at the code, it seems that the transaction is first prepare()'d on >> the slave, then the prepared log shipped to the master, applied and >> committed there, and the master txid shipped back and used to commit the >> transaction on slave. However, the locks seem to be held (both on slave and >> master) until the slave finishes committing or rolling back, so no >> visibility problems should occur. > > Further, the Transaction that MasterClient/MasterServer/MasterImpl creates on > server-side is apparently only ever really used to hold locks. It is always > rolled back (finishTransaction() in MasterImpl). This leads me to wonder if > the following scenario is possible: > > - Slave A replicates T to master, master commits it and gets > ready to return txnId, client X reads it from master, master > crashes > - Is it guaranteed that slave A commits the txn locally > before a new master is elected (since a new master elected > at this point won't have T, and thus client X would have > read an update that never completed successfully > > Clearly slave A cannot commit the transaction, and its client gets some sort > of error. Also, since master crashed, a new master will be elected and the > transaction will never have existed in the new cluster. Yet client X managed > to read the result, which will be wrong. Also, in this case, when master is > restarted, can it rejoin the cluster, since its state has diverged from that > of the cluster?
Hm, actually client X can't read anything touched by T from master, since slave A will have taken write locks on things it modifies, and the write locks are associated with T that never finishes in this example. Still, master's state will diverge from cluster state. _______________________________________________ Neo4j mailing list [email protected] https://lists.neo4j.org/mailman/listinfo/user
