>> if he has p1, then p1 will be committed to be more precise, this is only a yes if the new leader can deliver p1 to quorum; otherwise we'll be back to case 1. basically what ben and alex emphasized about the key point (deliver to quorum / quorum ack).
On Thu, Sep 5, 2019 at 1:26 PM Alexander Shraer <shra...@gmail.com> wrote: > > the global state is neither COMMITTED nor DROPPED. > > Just like in Paxos, if a quorum ACKs (it does not matter whether L1 > received the acks before crashing) then its guaranteed not to be lost. > If less then a quorum acked, them its unknown until recovery happens in > which case it could be committed or dropped, depends on what L2 knows. > The global state isn't known to any process though. > > > so for a client, if write query takes too much time, the client may > receive Timeout Exception, and it must query servers again to know whether > previous write is SUCCESS or FAIL？ > Yes. ZK doesn't currently have a good way of finding this out, > https://issues.apache.org/jira/browse/ZOOKEEPER-22 > > > Alex > > > > On Wed, Sep 4, 2019 at 10:37 PM 121476...@qq.com <121476...@qq.com> wrote: > > > thank you, Michael. seems i got the idea. > > in case2, when L1 fails before receiving a quorum's ACKs, the global > state > > is neither COMMITTED nor DROPPED. > > Until a new leader elected and syncs to his followers, if he has p1,then > > p1 will be committed; if he has not p1, then p1 will be dropped. > > so for a client, if write query takes too much time, the client may > > receive Timeout Exception, and it must query servers again to know > whether > > previous write is SUCCESS or FAIL？ > > > > > > > > 121476...@qq.com > > > > From: Michael Han > > Date: 2019-09-04 02:26 > > To: user > > Subject: Re: a misunderstanding of ZAB > > +1 with what Alex has said. > > > > The commit case is easy to understand. For skip case, think this example: > > > > old quorum: F1 F2 F3 F4 F5, with F1 as L1. L1 has p on F1 and F2. > > new quorum: F1 F2 F3 F4 F5, with F3 as L2. It's possible, because > although > > F1 and F2 has latest zxid, they could be partitioned away and F3 F4 F5 > are > > enough to form quorum to elect a new leader. > > > > Now partition healed, the commit of p on F1 and F2 should be dropped (in > > ZK, this is what "TRUNC" sync is for). > > > > >> L2 become new leader, he should skip p1. > > > > If your L2 is F2 here, p1 will not be skipped, since p1 is available on > F2 > > the new leader. > > > > On Tue, Sep 3, 2019 at 10:35 AM Alexander Shraer <shra...@gmail.com> > > wrote: > > > > > In case2, it is possible that p1 is committed or dropped. It depends on > > > whether L2 knows about p1. > > > Note that L2 needs the support of a quorum to become leader, and in ZK > > > since there is no state copy from followers to leader, the leader > > candidate > > > needs to have the longest log. > > > So, if L2's log includes p1 it will be committed otherwise it will be > > > dropped. > > > > > > In case1 L2's log necessarily includes p1 since it is present at a > quorum > > > and without having it in the log its not possible to have a log more > > > up-to-date than that of a quorum / get the support of a quorum to > become > > > leader. > > > > > > Alex > > > > > > > > > On Tue, Sep 3, 2019 at 4:52 AM Norbert Kalmar > > <nkal...@cloudera.com.invalid > > > > > > > wrote: > > > > > > > Hi, > > > > > > > > That's a good question. So if I understand correctly, you are asking > > what > > > > happens if there is a new Leader Election in ZooKeeper, what is the > > "last > > > > seen zxid". I checked the ZAB protocol, it is not entirely clear for > me > > > as > > > > well, but my understanding is that the last seen zxid is the last > > > > transaction, which is read from txnlogs in case of a recovery. > > Honestly, > > > > there's nothing else this could be read from. So if it hasn't been > > > > committed to the datatree (and that exists in memory anyway, at least > > > until > > > > a snapshot is taken), it is still the last txn that is logged by one > of > > > the > > > > followers, so he will win the Leader Election, and the followers will > > get > > > > this txn as well. > > > > Anyone agree/disagree? :) > > > > > > > > Regards, > > > > Norbert > > > > > > > > On Mon, Sep 2, 2019 at 4:50 AM 121476...@qq.com <121476...@qq.com> > > > wrote: > > > > > > > > > hi, i'm a new to zookeeper, and this problem confuses me for nearly > > two > > > > > months... > > > > > papers tell me that zab must satisfy: > > > > > A message delivered by one sever must be delivered on quorum. > > > > > A message skipped must always be skipped. > > > > > Then consider two cases below, L is short for leader, F is short > for > > > > > follower, p is short for proposal. > > > > > Case1: > > > > > L send p1 to F2 F3 F4 F5. > > > > > F2 F3 ack p1, reach a quorum. > > > > > L1 is about to send commit but failed... > > > > > L2 become new leader, he should commit. > > > > > > > > > > Case2: > > > > > L1 send p1 to F2 F3 F4 F5. > > > > > Only F2 ack p1, not reach a quorum. > > > > > Then L1 failed... > > > > > L2 become new leader, he should skip p1. > > > > > > > > > > i think L2 should handle the cases in election&recovery phase. but > > how > > > L2 > > > > > can know the global state and decide if commit p1 or skip p1？ > > > > > if anyone helps, i will be much appreciate. > > > > > > > > > > > > > > > > > > > > 121476...@qq.com > > > > > > > > > > > > > > >
