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From Sergey Maslyakov <evol...@gmail.com>
Subject Re: Efficient backup and a reasonable restore of an ensemble
Date Tue, 09 Jul 2013 04:25:10 GMT
These are interesting points, Thawan. I'd like to make sure that I get them
right.

1. Are you saying that a snapshot file may not be sufficient to restore
Zookeeper to a consistent state? Does it always require a transaction log
file or is it required to get to the most current state? I was hoping that
a snapshot is self-sufficient to do a restore to recent but not necessarily
most current state. Was I wrong?

2. Do you suggest that the same pair of a snapshot (and a transaction log)
needs to be copied on all servers before they are brought online? The what
about the "epoch" files? Do they need to be purged, preserved, or same one
populated through the whole ensemble?


On Mon, Jul 8, 2013 at 7:53 PM, Thawan Kooburat <thawan@fb.com> wrote:

> 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" <evolvah@gmail.com> 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
> ><fpjunqueira@yahoo.com>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
> >> 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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