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From Ted Dunning <ted.dunn...@gmail.com>
Subject Re: Accessing previous versions of znode data
Date Fri, 01 Jul 2011 16:01:07 GMT
By nature, you can't go back in time with ZK.  Some clients may be at an
earlier point time, but each client will see a strictly ordered (and not
very old) version of the world.

If you want to preserve old copies, you need to do that explicitly.  One
think that would make this easier is the multi command (self-serving plug,
here).  This was committed to trunk yesterday.

What this would let you do is to atomically create a saved version of the
config, delete the oldest version and update the live version.

If you don't mind a slightly more complex client protocol, you can just add
new configs in sequentially named files and then update a "currentConfig"
znode that has the name of the current configuration.  If you test the
zkVersion on the currentConfig znode before starting the process, you can
even be safe in the presence of conflicting simultaneous updates.  Reaping
really old versions can happen safely after the new config is in place.

Bottom line is that ZK can support you in your need, but you have to
explicitly perform the actions to give you the semantics that you want.

On Fri, Jul 1, 2011 at 8:21 AM, Burgess, Jason <
Jason_Burgess2@cable.comcast.com> wrote:

> Greetings!
> After doing several searches on this list, as well as going through the
> documentation and code, I've come to the conclusion that it is not possible
> to access the data associated with a specific version number for a
> particular znode. Am I correct?
> For those wondering, here is my use case:
> We are looking to store configuration in the znode (usually less than 10KB)
> and using Watches to notify the entities that are being configured that a
> new configuration is available. However, we may run into an instance where a
> faulty configuration is stored on the znode, and we would like to go back
> through the previous versions until we reach a valid configuration, or we
> have no more configs.
> Thanks!
> jason

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