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From: Alexander Shraer <shralex@gmail.com>
Date: Tue, 4 Aug 2015 14:11:10 -0700
Message-ID: 
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Subject: Re: Doubts about libzookeeper
To: "user@zookeeper.apache.org" <user@zookeeper.apache.org>
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maybe 1 or 2 synctime, is enough given what you said about syncs - after 1
synctime
we know that either server1 disconnected (and will have to bootstrap its
state from the leader
if it ever reconnects) or the request got to the leader. But since synctime
may not be measured
exactly from our request submission it maybe that 2 synctime are needed.
Would need to look
deeper into pings and synctime to tell for sure.

On Tue, Aug 4, 2015 at 2:05 PM, Camille Fournier <camille@apache.org> wrote:

> 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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