2011/8/15 Tuure Laurinolli <tuure.laurino...@portalify.com> > > 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? > > If a machine starts up and sees that it has diverged from the cluster (represented by the current master) it will ask the master for a fresh database and start from there... keeping the diverged db around in another folder.
> > _______________________________________________ > Neo4j mailing list > User@lists.neo4j.org > https://lists.neo4j.org/mailman/listinfo/user > -- Mattias Persson, [matt...@neotechnology.com] Hacker, Neo Technology www.neotechnology.com _______________________________________________ Neo4j mailing list User@lists.neo4j.org https://lists.neo4j.org/mailman/listinfo/user
