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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 16:02:42 GMT
I think I am having difficulties understanding the "fuzzy" concept. Let's
say I started to serialize DataTree into a snapshot file and it took 30
seconds. During these 30 seconds, the server saw 5 transactions that
updated the data. Does this mean that the snapshot that I get on disk at
the end of the 30-second interval will have some of these 5 transactions?
Or will it have none? Or will it have all of them? Or will it be
inconsistent and unreadable by Zookeeper?

Please help me better understand the behavior behind the "fuzzy" term.

For my use case, I am perfectly fine if I get a snapshot with none of these
5 transactions, considering that I will pick them up next time I take a
snapshot.


/Sergey


On Tue, Jul 9, 2013 at 12:08 AM, kishore g <g.kishore@gmail.com> wrote:

> Its not really elaborate, it is very similar to what zookeeper does when it
> starts up. It first reads the latest snapshot file and then the transaction
> logs and applies each and every transaction. What I am suggesting is that
> instead of applying all transactions stop at a transaction i provide.
>
> Having this tool will actually simplify your task, you can go back to any
> point in time. Think of a something like this.
>
> checkpoint A // this can store the last zxid or timestamp from the leader.
> Make changes to zk
> //if things fails
> stop zks
> rollback A//run this on each zk, brings back the cluster to its previous
> state.
> start zks // any order should be fine.
>
>
> Also keep in mind that snapshot is fuzzy only if there are writes happening
> while taking snapshot. If you are sure no writes will happen when you are
> taking the snapshot then you are good. Experts, please correct me if this
> is incorrect.
>
> thanks,
> Kishore G
>
>
> On Mon, Jul 8, 2013 at 9:42 PM, Sergey Maslyakov <evolvah@gmail.com>
> wrote:
>
> > Kishore,
> >
> > This sounds like a very elaborate tool. I was trying to find a simplistic
> > approach but what Thawan said about "fuzzy snapshots" makes me a little
> > afraid that there is no simple solution.
> >
> >
> > On Mon, Jul 8, 2013 at 11:05 PM, kishore g <g.kishore@gmail.com> wrote:
> >
> > > Agree, we already have such a tool. In fact we use it to reconstruct
> the
> > > sequence of events that led to a failure and actually restore the
> system
> > to
> > > a previous stable point and replay the events. Unfortunately this is
> tied
> > > closely with Helix but it should be easy to make this a generic tool.
> > >
> > > Sergey is this something that will be useful in your case.
> > >
> > > Thanks,
> > > Kishore G
> > >
> > >
> > > On Mon, Jul 8, 2013 at 8:09 PM, Thawan Kooburat <thawan@fb.com> wrote:
> > >
> > > > On restore part, I think having a separate utility to manipulate the
> > > > data/snap dir (by truncating the log/removing snapshot to a given
> zxid)
> > > > would be easier than modifying the server.
> > > >
> > > >
> > > > --
> > > > Thawan Kooburat
> > > >
> > > >
> > > >
> > > >
> > > >
> > > > On 7/8/13 6:34 PM, "kishore g" <g.kishore@gmail.com> wrote:
> > > >
> > > > >I think what we are looking at is a  point in time restore
> > > functionality.
> > > > >How about adding a feature that says go back to a specific
> > > zxid/timestamp.
> > > > >This way before doing any change to zookeeper simply note down the
> > > > >timestamp/zxid on leader. If things go wrong after making changes,
> > bring
> > > > >down zookeepers and provide additional parameter of a zxid/timestamp
> > > while
> > > > >restarting. The server can go the exact point and make it current.
> The
> > > > >followers can be started blank.
> > > > >
> > > > >
> > > > >
> > > > >On Mon, Jul 8, 2013 at 5:53 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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