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Zookeeper, mail # user - Re: Efficient backup and a reasonable restore of an ensemble


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Re: Efficient backup and a reasonable restore of an ensemble
Thawan Kooburat 2013-07-09, 00:53
Just saw that  this is the corresponding use case to the question posted
in dev list.

In order to restore the data to a given point in time correctly, you need
both snapshot and txnlog. This is because zookeeper snapshot is fuzzy and
snapshot alone may not represent a valid state of the server if there are
in-flight requests.

The 4wl command should cause the server to roll the log and take a
snapshot similar to periodic snapshotting operation. Your backup script
need grap the snapshot and corresponding txnlog file from the data dir.

To restore, just shutdown all hosts, clear the data dir, copy over the
snapshot and txnlog, and restart them.
--
Thawan Kooburat

On 7/8/13 3:28 PM, "Sergey Maslyakov" <[EMAIL PROTECTED]> wrote:

>Thank you for your response, Flavio. I apologize, I did not provide a
>clear
>explanation of the use case.
>
>This backup/restore is not intended to be tied to any write event,
>instead,
>it is expected to run as a periodic (daily?) cron job on one of the
>servers, which is not guaranteed to be the leader of the ensemble. There
>is
>no expectation that all recent changes are committed and persisted to
>disk.
>The system can sustain the loss of several hours worth of recent changes
>in
>the event of restore.
>
>As for finding the leader dynamically and performing backup on it, this
>approach could be more difficult as the leader can change time to time and
>I still need to fetch the file to store it in my designated backup
>location. Taking backup on one server and picking it up from a local file
>system looks less error-prone. Even if I went the fancy route and had
>Zookeeper send me the serialized DataTree in response to the 4wl, this
>approach would involve a lot of moving parts.
>
>I have already made a PoC for a new 4wl that invokes takeSnapshot() and
>returns an absolute path to the snapshot it drops on disk. I have already
>protected takeSnapshot() from concurrent invocation, which is likely to
>corrupt the snapshot file on disk. This approach works but I'm thinking to
>take it one step further by providing the desired path name as an argument
>to my new 4lw and to have Zookeeper server drop the snapshot into the
>specified file and report success/failure back. This way I can avoid
>cluttering the data directory and interfering with what Zookeeper finds
>when it scans the data directory.
>
>Approach with having an additional server that would take the leadership
>and populate the ensemble is just a theory. I don't see a clean way of
>making a quorum member the leader of the quorum. Am I overlooking
>something
>simple?
>
>In backup and restore of an ensemble the biggest unknown for me remains
>populating the ensemble with desired data. I can think of two ways:
>
>1. Clear out all servers by stopping them, purge version-2 directories,
>restore a snapshot file on one server that will be brought first, and then
>bring up the rest of the ensemble. This way I somewhat force the first
>server to be the leader because it has data and it will be the only member
>of a quorum with data, provided to the way I start the ensemble. This
>looks
>like a hack, though.
>
>2. Clear out the ensemble and reload it with a dedicated client using the
>provided Zookeeper API.
>
>With the approach of backing up an actual snapshot file, option #1 appears
>to be more practical.
>
>I wish I could start the ensemble with a designate leader that would
>bootstrap the ensemble with data and then the ensemble would go into its
>normal business...
>
>
>
>On Mon, Jul 8, 2013 at 4:30 PM, Flavio Junqueira
><[EMAIL PROTECTED]>wrote:
>
>> One bit that is still a bit confusing to me in your use case is if you
>> need to take a snapshot right after some event in your application.
>>Even if
>> you're able to tell ZooKeeper to take a snapshot, there is no guarantee
>> that it will happen at the exact point you want it if update operations
>> keep coming.
>>
>> If you use your four-letter word approach, then would you search for the