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Zookeeper >> mail # user >> Backups

You're not talking about data corruption, are you? It is incorrect  
data that has been introduced by a user or application by mistake. Am  
I getting it right?


On Jan 19, 2012, at 8:07 PM, Jordan Zimmerman wrote:

> It's that very replication that creates the need for backups. In  
> there is
> a user error or a bad injection of data, the error will quickly  
> replicate
> to all the instances. There's no way to recover without an external  
> backup.
> -JZ
> On 1/19/12 10:39 AM, "Flavio Junqueira" <[EMAIL PROTECTED]> wrote:
>> 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.
>> Thanks,
>> -Flavio
>> 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 backup is known to be slightly out of date.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  In these cases, an out-of-date backup may be  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

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