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From Ted Dunning <ted.dunn...@gmail.com>
Subject Re: Multi question on using previous ops results
Date Sat, 19 May 2012 00:03:34 GMT
On Fri, May 18, 2012 at 1:55 PM, Joe Gamache <gamache@cabotresearch.com>wrote:

> ...
>
> 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.


That is easy.  Just create the lower structures first.  If the upper
structure needs to have an indicator that it is under construction, just
set a bit inside it using the atomic update method.  Or create a separate
node to do leader election.


> Thus, I would like to be able to create that node and lock it atomically
> OR create the entire structure below it atomically.


Remember ZK doesn't do locks.

You can create the node with the "under-construction" bit set.   Or you can
use a multi to create the parent and children simultaneously.   You can
create the parent with references to a version in another node by getting
the version of the parent, getting the versions of the children, updating
the parent with the version specified.  Retry with backoff on failure.

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)
>

As you like.  This can be handled pretty easily.  Setting watches on the
release of a node is a bit trickier, but can be handled as well.



>
> 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.
>

It can.

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