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From Avery Ching <ach...@yahoo-inc.com>
Subject Re: zoo_set() version question
Date Fri, 12 Dec 2008 20:41:54 GMT
I am working on clusterlib (YST project) and we are using zookeeper for a
backing store of cluster abstractions (i.e. Applications, groups, nodes,
data distributions, etc.).

I believe that this is an optimization use case for using zoo_set() with
versions that are != -1.


1.  I do a successful zoo_set() with version V, where V != -1.
2.  Make more changes to my data.
3.  I want to do another zoo_set() but I don't know what version number to

Since I don't know the version number I have to use either zoo_exists() or
zoo_get().  Even though zoo_exists() gets me the version number I don't know
whether that version is the one that I wrote.  Therefore I need to read the
znode (zoo_get()) and compare with the data that I did set before I make
changes to the data (step 2) ) that I will zoo_set in step 3).  Note that I
am never sure if I was the last writer.

If there is a way to get the used version from zoo_set(), then I do not need
to use zoo_exists() or zoo_get() in between zoo_set() calls (steps 1) and 3)
).  They will still fail, of course, if the version is wrong.  And the other
benefit is that I will know that I was the last writer if I get the version
from my last successful zoo_set() and then do zoo_exists() and the versions


On 12/12/08 11:53 AM, "Patrick Hunt" <phunt@apache.org> wrote:

> Could you explain your use case? There is no way to get the version as
> part of a zoo_set, however there may be some alternative that we can
> suggest if we knew more about the problem you are trying to solve.
> Patrick
> Avery Ching wrote:
>> Patrick,
>> Thanks for responding.
>> I agree that I can use zoo_exists and zoo_get to get the version of the
>> znode as it exists currently.
>> The problem I am trying to solve is that getting the version from struct
>> Stat in either zoo_exists or zoo_get may not be the same version that my
>> last successful zoo_set used.  I would like to get the version that denotes
>> my last successful zoo_set() operation to a particular znode.
>> I understand that the data and version to the znode may change immediately
>> one or multiple times after my zoo_set() and this is fine, but I would still
>> like to know the znode's versions of the data I set.
>> Avery
>> On 12/12/08 11:11 AM, "Patrick Hunt" <phunt@apache.org> wrote:
>>> Avery Ching wrote:
>>>> If zoo_set() completes successfully with version != -1, can we assume that
>>>> version -> version + 1 for this znode?  If not, is there a way for the
>>>> to get the version of the successfully completed zoo_set() operation?
>>> You shouldn't rely on this, it may work, but it's not part of the
>>> contract. Also, nothing says that some other client won't change the
>>> node immediately after you change it.
>>> You can access the version using zoo_exists or zoo_get - specifically
>>> the "struct Stat stat" argument of either of those methods contains a
>>> "version" member.
>>> Patrick

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