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 It is a common property that a database backup is known to be slightly out of date.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 In these cases, an out-of-date backup may be enormously valuable.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 <f...@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: >> jzimmer...@netflix.com >> >> On 1/17/12 10:10 AM, "Neha Narkhede" <neha.narkh...@gmail.com> 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 >>> <jzimmer...@netflix.com> wrote: >>> >>>> We'll be backing up to S3. Wouldn't it be redundant to backup all the >>>> instances? >>>> >>>> -JZ >>>> >>>> P.S. I'm working on a ZooKeeper instance manager that will have >>>> backup/restore and a bunch of other stuff. We'll be open sourcing it. If >>>> anyone is interested in previewing it let me know. >>>> >>>> >>>> On 1/16/12 5:39 PM, "Patrick Hunt" <ph...@apache.org> wrote: >>>> >>>> Why would you limit to the leader? Wouldn't backing up any server (as >>>>> long as it's active) be sufficient? If you search the list it's been >>>>> discussed before, using Observers seemed like a reasonable option as >>>>> well. >>>>> >>>>> Patrick >>>>> >>>>> On Fri, Jan 13, 2012 at 2:29 PM, Jordan Zimmerman >>>>> <jzimmer...@netflix.com> wrote: >>>>> >>>>>> That's easy as the backup app is running on the same machine as the ZK >>>>>> instance. I can use 'stat' to see if "my" instance is the leader. >>>>>> >>>>>> On 1/13/12 2:28 PM, "Camille Fournier" <cami...@apache.org> wrote: >>>>>> >>>>>> You want to have to figure out who the leader is every time you want >>>>>>> to >>>>>>> take a backup? That would be the downside to this strategy I would >>>>>>> think. >>>>>>> >>>>>>> C >>>>>>> >>>>>>> From my phone >>>>>>> On Jan 13, 2012 5:24 PM, "Jordan Zimmerman" <jzimmer...@netflix.com> >>>>>>> wrote: >>>>>>> >>>>>>> As a backup strategy, it seems I would only want to backup snapshots >>>>>>>> from >>>>>>>> the leader. Does that make sense? >>>>>>>> >>>>>>>> -JZ >>>>>>>> >>>>>>>> >>>>>>>> >>>>>> >>>>> >>>> >>> >> > flavio > junqueira > > research scientist > > f...@yahoo-inc.com > direct +34 93-183-8828 > > avinguda diagonal 177, 8th floor, barcelona, 08018, es > phone (408) 349 3300 fax (408) 349 3301 > >
