-Re: Consistency in zookeeper
Alexander Shraer 2013-03-01, 18:19
its possible, but what it gets you is that the read will see at least
the writes that completed before the sync started.
possibly later writes too. Actually, this is true only with some
timing assumption. As was previously discussed on the
list, in order to really guarantee this property even with leader
failures, the leader would have to broadcast sync commands just like
which it currently doesn't do for some reason.
On Fri, Mar 1, 2013 at 9:49 AM, kishore g <[EMAIL PROTECTED]> wrote:
> Will sync and read really help to achieve what Yasin wants ? is it not
> possible for value to change between sync and read?
> Kishore G
> On Thu, Feb 28, 2013 at 9:32 PM, Rakesh R <[EMAIL PROTECTED]> wrote:
>> Hi Yasin,
>> Adding one more point,
>> ZooKeeper provides different ways of achieving data sync. Like Alex &
>> Vladimir explained, sync() api is one way and it has the overhead of
>> Another approach is to define Watchers. This also will be helpful to keep
>> in sync the data between the clients. Its internally using the asynchronous
>> way of notifying different events. Also, its very light-weight and here
>> user/client should define specific watchers to achieve the synchronized
>> view of data.
>> ZK supports various events like NodeDataChanged, NodeChildrenChanged.
>> Since it is asynchronous, there will be slight latency in recieving the
>> Section: •The data for which the watch was set
>> From: Alexander Shraer [[EMAIL PROTECTED]]
>> Sent: Friday, March 01, 2013 5:19 AM
>> To: [EMAIL PROTECTED]
>> Cc: [EMAIL PROTECTED]
>> 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
>> 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
>> 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.
>> On Thu, Feb 28, 2013 at 3:35 PM, Yasin <[EMAIL PROTECTED]> wrote:
>> > Hello everyone,
>> > From the zookeeper website I understand that zookeeper does not provide
>> > strict consistency in every instance in time.
>> > (
>> > Have ever anyone considered to make zookeeper strictly consistent at
>> > anytime. What I mean is that any time a value is updated in zookeeper,
>> > 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:
>> > Sent from the zookeeper-user mailing list archive at Nabble.com.