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From: Rakesh R <rakeshr@huawei.com>
To: "user@zookeeper.apache.org" <user@zookeeper.apache.org>
CC: "zookeeper-user@hadoop.apache.org" <zookeeper-user@hadoop.apache.org>
Subject: RE: Consistency in zookeeper
Thread-Topic: Consistency in zookeeper
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Date: Fri, 1 Mar 2013 05:32:31 +0000
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Hi Yasin,

Adding one more point,

ZooKeeper provides different ways of achieving data sync. Like Alex & Vladi=
mir explained, sync() api is one way and it has the overhead of performance=
.

Another approach is to define Watchers. This also will be helpful to keep i=
n sync the data between the clients. Its internally using the asynchronous =
way of notifying different events. Also, its very light-weight and here use=
r/client should define specific watchers to achieve the synchronized view o=
f data.

ZK supports various events like NodeDataChanged, NodeChildrenChanged. Since=
 it is asynchronous, there will be slight latency in recieving the events.

Reference:
http://zookeeper.apache.org/doc/trunk/zookeeperProgrammers.html#ch_zkWatche=
s
Section: =95The data for which the watch was set=20

http://zookeeper.apache.org/doc/r3.2.2/zookeeperTutorial.html#sc_producerCo=
nsumerQueues

-Rakesh
________________________________________
From: Alexander Shraer [shralex@gmail.com]
Sent: Friday, March 01, 2013 5:19 AM
To: user@zookeeper.apache.org
Cc: zookeeper-user@hadoop.apache.org
Subject: Re: Consistency in zookeeper

Hi Yasin,

I assume you mean "linearizability" by "strict consistency".

ZooKeeper provides "sequential consistency". This is weaker than
linearizability but is still very strong, much stronger than "eventual
consistency".
In addition, all update operations are linearizable as they are
sequenced by the leader. With sequential consistency, a reader never
"goes back in time"
even if you read from a different follower every time, you'll never
see version 3 of the data after seeing version 4.

ZooKeeper also provides a sync command. If you invoke a sync command
and then a read, the read is guaranteed to see at least the last write
that
completed before the sync started. So if you always do "sync + read"
instead of just "read", you get linearizability. But you pay in
performance since
these reads will no longer be executed locally on the follower to
which you're connected - they sync is sent to the leader. That's why
ZooKeeper gives
you the option of doing a fast read that is consistent but may
retrieve a slightly old version, or a sync+read that is more
consistent but slower.

Alex

On Thu, Feb 28, 2013 at 3:35 PM, Yasin <yasincelik27@gmail.com> wrote:
> Hello everyone,
>
> From the zookeeper website I understand that zookeeper does not provide
> strict consistency in every instance in time.
> (http://zookeeper.apache.org/doc/trunk/zookeeperProgrammers.html#ch_zkGua=
rantees)
> Have ever anyone considered to make zookeeper strictly consistent at
> anytime. What I mean is that any time a value is updated in zookeeper, an=
y
> client that retrieves the value from any follower should get consistent
> result. Is it feasible to improve the zookeeper core so that zookeeper
> delivers strict consistency not the eventual consistency?
>
> Best
>
> Yasin
>
>
>
> --
> View this message in context: http://zookeeper-user.578899.n2.nabble.com/=
Consistency-in-zookeeper-tp7578531.html
> Sent from the zookeeper-user mailing list archive at Nabble.com.=