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From Alexander Shraer <shra...@gmail.com>
Subject Re: Doubts about libzookeeper
Date Tue, 04 Aug 2015 18:20:45 GMT
It seems that since the delete may be in-flight (between server 1 and
leader, or still being proposed by the leader)
when the client connects to server 2, doing a sync right a way may not help
since the operation hasn't been committed yet. Perhaps the client should
wait some multiple of synclimit time (3x ?) before invoking the sync to
allow the delete to commit or disappear for sure. This is all related to
https://issues.apache.org/jira/browse/ZOOKEEPER-22, which is still open
unfortunately...

On Tue, Aug 4, 2015 at 10:15 AM, Camille Fournier <camille@apache.org>
wrote:

> True, I'm not sure when the xid increments. If that is the case, you can
> force a sync before the read of the path, to prevent reading stale data. So
> that would be the solve for that edge case although it's an expensive
> solve.
>
> C
>
> On Tue, Aug 4, 2015 at 12:52 PM, Alexander Shraer <shralex@gmail.com>
> wrote:
>
> > Hi Camille,
> >
> > if the client received a response for the delete then sure it shouldn't
> be
> > able to connect
> > to servers that didn't see it. But if it disconnected before seeing the
> > response the example seems possible to me.
> > I haven't checked the code to see when exactly the transaction number is
> > incremented at
> > the client, so I may be wrong, but suppose for example that zkserver-1
> > crashes before
> > sending the delete request to the leader. Then, the request is gone
> > forever. If you don't let the client
> > connect to another server that hasn't seen the delete, the client will
> > never be able to connect.
> > So it seems quite possible that it connects, then the request is executed
> > (if zkserver-1 hasn't crashed
> > after all) and the znode disappears.
> >
> > Alex
> >
> >
> > On Tue, Aug 4, 2015 at 8:33 AM, Camille Fournier <camille@apache.org>
> > wrote:
> >
> > > ZooKeeper provides a session-coherent single system image guarantee.
> Any
> > > request from the same session will see the results of all of its
> writes,
> > > regardless of which server it connects to. See:
> > >
> > >
> >
> http://zookeeper.apache.org/doc/r3.4.6/zookeeperProgrammers.html#ch_zkGuarantees
> > >
> > > So, if your session deletes, and the delete is successfully processed
> by
> > > the quorum, you will not see the path that you have deleted no matter
> > what
> > > server your session connects to. I believe in practice that this means
> > that
> > > the ZK servers that might be behind your session (say server 2 is
> lagging
> > > behind a few commits) will refuse to allow your session to connect to
> it,
> > > so that you will not see stale data.
> > >
> > > This means that the example Lokesh gave:
> > >
> > > "1. Quorum leader has forwarded request to zkserver-2 for "delete
> /path".
> > > 2. If your client connects to "zkserver-2" after step 1 is executed
> (get
> > > /path). Then your "/path" will not be available.
> > > 3. If your client connects to "zkserver-2" before step1 is executed
> (get
> > > /path) then your "/path" would be available and after some time your
> path
> > > would not be available (after zkserver-2 is synched with the leader)"
> > >
> > > Cannot happen, so long as you are in the same session.
> > >
> > > C
> > >
> > > On Tue, Aug 4, 2015 at 6:49 AM, Lokesh Shrivastava <
> > > lokesh.shrivastava@gmail.com> wrote:
> > >
> > > > I think it depends on whether your request reaches zkserver-1 and
> > whether
> > > > it is able to send the request to quorum leader. Considering that
> > "delete
> > > > /path" request has reached the quorum leader then following may
> happen
> > > >
> > > > 1. Quorum leader has forwarded request to zkserver-2 for "delete
> > /path".
> > > > 2. If your client connects to "zkserver-2" after step 1 is executed
> > (get
> > > > /path). Then your "/path" will not be available.
> > > > 3. If your client connects to "zkserver-2" before step1 is executed
> > (get
> > > > /path) then your "/path" would be available and after some time your
> > path
> > > > would not be available (after zkserver-2 is synched with the leader)
> > > >
> > > > Others can correct me if this is not how it works.
> > > >
> > > > Thanks.
> > > > Lokesh
> > > >
> > > > On 4 August 2015 at 12:09, liangdong01@baidu.com <
> > liangdong01@baidu.com>
> > > > wrote:
> > > >
> > > > > Hi,
> > > > >      I'm thinking about a program desgin with libzookeeper, here
is
> > my
> > > > > doubts:
> > > > >
> > > > >     1) first, I connnect to zkserver-1, and there exists the path
> > > > "/path".
> > > > >     2) I sends "delete /path", the request reaches(may not, i don't
> > > know
> > > > > about that) zkserver-1 and dont't know whether this effected, and
> > then
> > > > lost
> > > > > connection before response returns.
> > > > >     3) reconnect the same session to zkserver-2,  and I sends "get
> > > > /path".
> > > > >
> > > > >     which one will the "get /path" return possibly :
> > > > >     1, "not exists"
> > > > >     2, "exists" and "always exists"
> > > > >     3, "exists" and "not exists" afterwards
> > > > >
> > > > >     my biggist problem is wether the 3) will occur or not, thanks!
> > > > >
> > > > >
> > > > >
> > > > >
> > > > > liangdong01@baidu.com
> > > > >
> > > >
> > >
> >
>

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