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Date: Tue, 4 Aug 2015 17:05:22 -0400
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Subject: Re: Doubts about libzookeeper
From: Camille Fournier <camille@apache.org>
To: "user@zookeeper.apache.org" <user@zookeeper.apache.org>
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That's true. I spent some time trying to think about when and how that
would be possible, and didn't get very far. We have guarantees about how
far out of sync a quorum member can be before it's booted, so I would think
that there's some way to timebound this potentially to prevent it, a la
your suggestion about 3X synctime.

C


On Tue, Aug 4, 2015 at 4:58 PM, Alexander Shraer <shralex@gmail.com> wrote:

> Yes, I checked and you're right. It gets queued at the leader until all
> previously proposed requests at the leader
> are committed. But still if the request is only on its way between server 1
> and the leader sync won't immediately help, right ?
>
>
> On Tue, Aug 4, 2015 at 11:39 AM, Camille Fournier <camille@apache.org>
> wrote:
>
> > I thought that sync forced a flush of the queued events on a quorum
> member
> > before completing/got it in the path of events from the leader, so that
> it
> > won't return until all of the pending leader events before it have been
> > seen by this quorum member. Is that not correct?
> >
> > On Tue, Aug 4, 2015 at 2:20 PM, Alexander Shraer <shralex@gmail.com>
> > wrote:
> >
> > > 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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