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From Enrico Olivelli <eolive...@gmail.com>
Subject Re: create or setData in transaction?
Date Wed, 14 Aug 2019 16:22:39 GMT
Il mer 14 ago 2019, 17:55 Zili Chen <wander4096@gmail.com> ha scritto:

> FYI, etcd provides a transaction mechanism which supports if-else like
> commits.[1]
>

As we have setData with version check usually you don't need a
'transaction' with some kind of rowlevel lock.

Maybe I am missing something in your case.

Enrico



> Best,
> tison.
>
> [1] https://godoc.org/github.com/coreos/etcd/clientv3#OpTxn
>
>
> Zili Chen <wander4096@gmail.com> 于2019年8月14日周三 下午11:52写道:
>
> > Hi Andor,
> >
> > Thanks for your attention.
> >
> > The problem is that in concurrent scenario zk.setData() could still
> failed
> > if there is another thread delete the node. I know with proper lock
> > strategy
> > and ownership separation this can be avoid but it's said that ZooKeeper
> has
> > a transaction mechanism so I'd like to see whether I can make use of it.
> >
> > There is where I turn to
> >
> > zk.multi(Op.check(path1), Op.setData(path2, data)); // path1 == or !=
> > path2 is irrelevant
> >
> > when the existence of a mark node(path1) guarded a condition and I want
> to
> > make
> > sure that setData successes only if the mark node exist. If I check the
> > existence
> > first and commit setData, a remove to the node could break the guard. In
> > other
> > words, I want to tell ZooKeeper that the check and setData should be
> > successful
> > committed or fail to be committed atomically.
> >
> > Best,
> > tison.
> >
> >
> > Andor Molnar <andor@apache.org> 于2019年8月14日周三 下午11:12写道:
> >
> >> Hi Zili,
> >>
> >> There’s no such functionality in ZooKeeper as far as I’m concerned. I
> >> think your multi example (zk.multi(Op.check(path), Op.setData(path,
> data)))
> >> is already a usage pattern which multi is not designed to support.
> >>
> >> Why do you need to do this in “transactions” (there’s no transaction in
> >> ZK)?
> >>
> >> In Java you can do:
> >>
> >> try {
> >>   zk.create();
> >> } catch (NodeExistsException e) {
> >>   // swallow exception
> >> }
> >> zk.setData();
> >> …
> >>
> >> Regards,
> >> Andor
> >>
> >>
> >>
> >>
> >> > On 2019. Aug 6., at 14:44, Zili Chen <wander4096@gmail.com> wrote:
> >> >
> >> > Hi Enrico,
> >> >
> >> > Thanks for your reply.
> >> >
> >> >> In this case usually you use conditional setData, using the 'version'
> >> of
> >> >> thr znode
> >> >
> >> >
> >> > what if the caller has no idea on whether the node exist?(see also
> >> > my if-else pseudo-code above.)
> >> >
> >> > IIRC if we call `setData` on a non-exist path a NoNodeException
> >> > will be thrown.
> >> > Best,
> >> > tison.
> >> >
> >> >
> >> > Enrico Olivelli <eolivelli@gmail.com> 于2019年8月6日周二 下午8:27写道:
> >> >
> >> >> Il mar 6 ago 2019, 13:47 Zili Chen <wander4096@gmail.com> ha
> scritto:
> >> >>
> >> >>> Any ideas?
> >> >>>
> >> >>>
> >> >>> Zili Chen <wander4096@gmail.com> 于2019年7月29日周一
上午11:12写道:
> >> >>>
> >> >>>> Hi ZooKeepers,
> >> >>>>
> >> >>>> Currently our transaction mechanism supports doing
> >> >>>> create/setData/checkExist/delete in transaction. However, taking
> this
> >> >>>> scenario into consideration, we want to put data in path "/path"
> but
> >> >>>> don't know whether the znode exists or not. Let's say we program
as
> >> >>>> below
> >> >>>>
> >> >>>> if (zk.exist(path)) {
> >> >>>>  zk.setData(path, data);
> >> >>>> } else {
> >> >>>>  zk.create(path, data);
> >> >>>> }
> >> >>>
> >> >>
> >> >> Do you need to perform other ops in the same transaction?
> >> >> In this case usually you use conditional setData, using the 'version'
> >> of
> >> >> thr znode
> >> >>
> >> >>
> >> >> Enrico
> >> >>
> >> >>>
> >> >>>> if we want to do the check and "put" in transaction, it would
be
> like
> >> >>>>
> >> >>>> zk.multi(Op.check(path), Op.setData(path, data));
> >> >>>>
> >> >>>> but we cannot add a "else" branch. ZooKeeper's transaction
would
> all
> >> >>>> success or fail.
> >> >>>>
> >> >>>> Is there any way to do an "if-else" transaction?
> >> >>>>
> >> >>>> Best,
> >> >>>> tison.
> >> >>>>
> >> >>>
> >> >>
> >>
> >>
>

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