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Zookeeper >> mail # user >> Backups

Hi Ted, Znodes for leader election, group membership, etc, can all be  
recreated, so why should I back them up instead of recreating the  
znodes? In fact, one might bring back a previous snapshot of the  
system that reflects an incorrect system state.

In the case that one stores data that can't be recovered by other  
means, I understand the need, but then we have the durability problem  
that I mentioned and you apparently agreed. Also, ZooKeeper is a  
replicated service, so why can't you simply rely upon the replication  
strategy that ZooKeeper provides to you already? Again, I'm trying to  
understand the use cases here.


On Jan 19, 2012, at 7:11 PM, Ted Dunning wrote:

> A backup can still be useful.  It is a common property that a database
> backup is known to be slightly out of date.
> Such a backup can still be very useful.  In many systems, the most  
> common
> cause of error is simple human intervention.  This especially  
> applies to
> file systems and databases, but can still apply to ZK if an admin
> carelessly tries to clean up part of the namespace and accidentally  
> cleans
> up all of it.  This should be much less common with ZK because manual
> adjustments are so much less a part of standard operation, but they  
> can
> still occur.  In these cases, an out-of-date backup may be enormously
> valuable.
> If somebody wants a precise backup from a particular moment in time,  
> the
> best option is to use the snapshot capabilities exposed by various  
> file
> systems.  Traditional NAS vendors all support this.  At a lower cost  
> and
> complexity point, you can get this from MapR clusters exposed as NFS  
> or by
> a ZFS file system.  This option also allows you to keep multiple  
> snapshots
> from points in the past.
> What Jordan is doing would allow backups without special storage  
> devices
> and, with good backup of the log, would allow nearly current  
> recovery in
> the event of catastrophic loss.  Yes, this loses some durability,  
> but it is
> still very desirable.
> On Thu, Jan 19, 2012 at 11:07 AM, Flavio Junqueira <fpj@yahoo-
> inc.com>wrote:
>> Since you started this thread, I've been thinking about the idea of
>> backing up, and I'm not sure I understand the motivation and if it  
>> is ok to
>> violate safety properties.
>> Given that ZooKeeper is used for coordination, I would think that  
>> in many
>> cases all its state can be reconstructed in an algorithmic manner.  
>> Perhaps
>> the use case for a backup would be the one in which it is being  
>> used as a
>> database, for example, to keep the metadata of a file system.  
>> Periodic
>> backups or even keeping an observer, however, won't guarantee that  
>> if you
>> bring the system up using that backup you'll have all committed  
>> operations.
>> The state of the leader reflects all committed operations, but one  
>> needs to
>> have the latest state of the transaction log to not miss an update.
>> But, it is true that I'm assuming that you can't miss updates. If  
>> you can
>> miss updates, then that's a different story. By missing updates  
>> we'll be
>> violating durability, which is  a property that ZooKeeper is  
>> supposed to
>> provide, so I'm trying to understand in which cases violating  
>> durability
>> would be acceptable. If it is not acceptable and you still want to  
>> have a
>> backup, then I don't see a way other than shutting down the clients  
>> before
>> you take a backup, which doesn't seem to be what is being proposed  
>> here.
>> -Flavio
>> On Jan 18, 2012, at 1:38 AM, Jordan Zimmerman wrote:
>> Neha - can you send me your email address. Send it to:
>>> On 1/17/12 10:10 AM, "Neha Narkhede" <[EMAIL PROTECTED]>  
>>> wrote:
>>> Jordan,
>>>> I'd be interested in previewing it. Let me know.
>>>> Thanks,
>>>> Neha
>>>> On Mon, Jan 16, 2012 at 5:42 PM, Jordan Zimmerman
>>>> <[EMAIL PROTECTED]> wrote:


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