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From Flavio Junqueira <fpjunque...@yahoo.com>
Subject Re: Efficient backup and a reasonable restore of an ensemble
Date Mon, 08 Jul 2013 21:30:18 GMT
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 leader or would you
simply take a snapshot at any server? If it has to go through the leader so that you make
sure to have the most recent committed state, then it might not be a bad idea to have an api
call that tells the leader to take a snapshot at some directory of your choice. Informing
you the name of the snapshot file so that you can copy sounds like an option, but perhaps
it is not as convenient.

The approach of adding another server is not very clear. How do you force it to be the leader?
Keep in mind that if it crashes, then it will lose leadership.

-Flavio 

On Jul 8, 2013, at 8:34 AM, Sergey Maslyakov <evolvah@gmail.com> wrote:

> It looks like the "dev" mailing list is rather inactive. Over the past few
> days I only saw several automated emails from JIRA and this is pretty much
> it. Contrary to this, the "user" mailing list seems to be more alive and
> more populated.
> 
> With this in mind, please allow me to cross-post here the message I sent
> into the "dev" list a few days ago.
> 
> 
> Regards,
> /Sergey
> 
> === forwarded message begins here ===
> 
> Hi!
> 
> I'm facing the problem that has been raised by multiple people but none of
> the discussion threads seem to provide a good answer. I dug in Zookeeper
> source code trying to come up with some possible approaches and I would
> like to get your inputs on those.
> 
> Initial conditions:
> 
> * I have an ensemble of five Zookeeper servers running v3.4.5 code.
> * The size of a committed snapshot file is in vicinity of 1GB.
> * There are about 80 clients connected to the ensemble.
> * Clients a heavily read biased, i.e., they mostly read and rarely write. I
> would say less than 0.1% of queries modify the data.
> 
> Problem statement:
> 
> * Under certain conditions, I may need to revert the data stored in the
> ensemble to an earlier state. For example, one of the clients may ruin the
> application-level data integrity and I need to perform a disaster recovery.
> 
> Things look nice and easy if I'm dealing with a single Zookeeper server. A
> file-level copy of the data and dataLog directories should allow me to
> recover later by stopping Zookeeper, swapping the corrupted data and
> dataLog directories with a backup, and firing Zookeeper back up.
> 
> Now, the ensemble deployment and the leader election algorithm in the
> quorum make things much more difficult. In order to restore from a single
> file-level backup, I need to take the whole ensemble down, wipe out data
> and dataLog directories on all servers, replace these directories with
> backed up content on one of the servers, bring this server up first, and
> then bring up the rest of the ensemble. This [somewhat] guarantees that the
> populated Zookeeper server becomes a member of a majority and populates the
> ensemble. This approach works but it is very involving and, thus,
> error-prone due to a human error.
> 
> Based on a study of Zookeeper source code, I am considering the following
> alternatives. And I seek advice from Zookeeper development community as to
> which approach looks more promising or if there is a better way.
> 
> Approach #1:
> 
> Develop a complementary pair of utilities for export and import of the
> data. Both utilities will act as Zookeeper clients and use the existing
> API. The "export" utility will recursively retrieve data and store it in a
> file. The "import" utility will first purge all data from the ensemble and
> then reload it from the file.
> 
> This approach seems to be the simplest and there are similar tools
> developed already. For example, the Guano Project:
> https://github.com/d2fn/guano
> 
> I don't like two things about it:
> * Poor performance even on a backup for the data store of my size.
> * Possible data consistency issues due to concurrent access by the export
> utility as well as other "normal" clients.
> 
> Approach #2:
> 
> Add another four-letter command that would force rolling up the
> transactions and creating a snapshot. The result of this command would be a
> new snapshot.XXXX file on disk and the name of the file could be reported
> back to the client as a response to the four-letter command. This way, I
> would know which snapshot file to grab for future possible restore. But
> restoring from a snapshot file is almost as involving as the error-prone
> sequence described in the "Initial conditions" above.
> 
> Approach #3:
> 
> Come up with a way to temporarily add a new Zookeeper server into a live
> ensemble, that would overtake (how?) the leader role and push out the
> snapshot that it has into all ensemble members upon restore. This approach
> could be difficult and error-prone to implement because it will require
> hacking the existing election algorithm to designate a leader.
> 
> So, which of the approaches do you think works best for an ensemble and for
> the database size of about 1GB?
> 
> 
> Any advice will be highly appreciated!
> /Sergey


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