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Date: Fri, 18 May 2012 13:55:06 -0400
From: Joe Gamache <gamache@cabotresearch.com>
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To: user@zookeeper.apache.org
Subject: Re: Multi question on using previous ops results
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Can you say again on a higher level what you really want to do?


Hmmm.  Probably not, but I will still try :)

I have a one process that is trying to create some "work" that will be 
represented via a ZNode.   A large number of other processes are looking 
for work.   The node that represents the work will be sequential.  Most 
importantly, it will be incomplete until the structure below it is 
completed.   Thus, I would like to be able to create that node and lock 
it atomically OR create the entire structure below it atomically.   I 
would like to avoid hacking in another parallel node structure solely 
for the purpose of locking (as there will be many 1000's of these created)

All I am really wondering is can Zookeeper do this?   If not, I can find 
alternate ways around - they just seem less elegant to me.


On 5/18/2012 12:46 PM, Ted Dunning wrote:
> No.  It still isn't clear what benefit is of putting the version number of
> one field into another.
>
> You seem to be focussing in on the minutiae of your proposed method rather
> than on the goal that you really want to solve.
>
> A problem that seems similar to what you want is to form a graph where A
> refers to B refers to C refers to A.  The desired option would be update
> the graph atomically so that B now points to D which points to A.  The
> intended use of multi here would be to do the standard read-modify-write
> sort of cycle where you read the contents and versions of the items to be
> updated or referenced in one operation and then in the next you do an
> update which is conditioned on version with items which must remain static
> having a version check.  You repeat this cycle as many times as it takes,
> presumably backing off between attempts.
>
> This has a theoretical possibility of fast updaters starving slow updaters
> but this has never even been a possibility in any system I have seen,
> largely because update rates are typically pretty low.
>
> Can you say again on a higher level what you really want to do?
>
> On Fri, May 18, 2012 at 11:43 AM, Joe Gamache<gamache@cabotresearch.com>wrote:
>
>> ...
>> Ideally, I would like to be able to replace the "0" in various "setData"
>> calls, not sure how to do that exactly, but the above would be a decent
>> start.  [If a node already existed and I was calling the Op.setData method
>> and wanted to follow that with a Op.check call, I believe that I currently
>> have no way of knowing the version number....]
>>
>> Does this make more sense than the gibberish I was trying below?
>>
>> thanks,
>>
>> Joe
>>
>>
>>
>> On 5/16/2012 8:06 PM, Joe Gamache wrote:
>>
>>> No thought about two znodes being synchronized at all.
>>>
>>> The higher level intent of the first (code) example is: how do you get
>>> the version number for a set operation and use that value in the set
>>> atomically?  For the second one it is: how to I create and lock a znode
>>> atomically?   [maybe the Lock znode is attached to the node I am creating,
>>> maybe in another structure?  I don't really care, just can I do it
>>> atomically?]
>>>
>>> Another way to do what I would like would be to create a znode structure
>>> that is not "attached" to the existing one at all, then once I have it the
>>> way it should be, attach it to an existing node.  However, I am quite
>>> certain this is not possible....
>>>
>>>
>>>
>>> On 5/16/2012 7:21 PM, Ted Dunning wrote:
>>>
>>>> It looks like you are assuming that the versions of two znodes are
>>>> synchronized. That seems pretty dangerous.
>>>>
>>>> What is the higher level intent here?  Would it better to simply build a
>>>> multi to update both znodes?
>>>>
>>>> Sent from my iPhone
>>>>
>>>> On May 16, 2012, at 11:59 AM, Joe Gamache<gamache@cabotresearch.**com<gamache@cabotresearch.com>>
>>>>   wrote:
>>>>
>>>>   I need to do something like the following:
>>>>> Stat stats = nodeToSet.getStat();
>>>>> if (stats != null) {
>>>>>     int version = stats.getVersion();
>>>>>     stats = client.getZk().setData(path, data, version);
>>>>>
>>>>> (data was preset in code not shown).   Since I have multiple processes
>>>>> all vying to do things, in the worst case (which happens too frequently),
>>>>> two processes execute the above at basically the same time, which causes
>>>>> errors.   I thought I might be able to use Multi to do this atomically, but
>>>>> I cannot figure out how to use the result (the version number above) from
>>>>> one Op in a later Op.  Is this possible?
>>>>>
>>>>> This was the simplest example I could find, but this happens often -
>>>>> usually with Paths.  For instance, sometimes I create a zookeeper node and
>>>>> want a "LOCK" node underneath it.  If I follow the "recipe" and use an
>>>>> ephemeral sequential node as the lock, how can I refer to sequence in a
>>>>> Path in subsequent Op added to a "multi" or is there any way to do it
>>>>> atomically?  Sorry that was so very wordy.  What I was really trying to
>>>>> ask, is how do I create and lock a node atomically?
>>>>>
>>>>> thanks for any help, insight!
>>>>>
>>>>> joe
>>>>>
>>>>>