zookeeper-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Burgess, Jason" <Jason_Burge...@cable.comcast.com>
Subject Re: Accessing previous versions of znode data
Date Fri, 01 Jul 2011 17:09:51 GMT
Ted, Ketan,

Thanks for the input!

jason

On 7/1/11 10:01 AM, "Ted Dunning" <ted.dunning@gmail.com> wrote:

>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
>>


Mime
View raw message