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Re: How does cassandra achieve Linearizability?


If you require the best precision you can get, setting up a pair of
stratum 1 ntpd masters in each data center location with a GPS modules
is not terribly complex. Low latency and jitter on servers you manage.
140ms is a long way away network-wise, and I would suggest that was a
poor choice of upstream (probably stratum 2 or 3) source.

As Jonathan mentioned, there's no guarantee from Cassandra, but if you
need as close as you can get, you'll probably need to do it yourself.

(I run several stratum 2 ntpd servers for pool.ntp.org)

-- 
Kind regards,
Michael

On 02/09/2017 06:47 PM, Kant Kodali wrote:
> Hi Justin,
> 
> There are bunch of issues w.r.t to synchronization of clocks when we
> used ntpd. Also the time it took to sync the clocks was approx 140ms
> (don't quote me on it though because it is reported by our devops :) 
> 
> we have multiple clients (for example bunch of micro services are
> reading from Cassandra) I am not sure how one can achieve
> Linearizability by setting timestamps on the clients ? since there is no
> total ordering across multiple clients.
> 
> Thanks!
> 
> 
> On Thu, Feb 9, 2017 at 4:16 PM, Justin Cameron <justin@xxxxxxxxxxxxxxx
> <mailto:justin@xxxxxxxxxxxxxxx>> wrote:
> 
>     Hi Kant,
> 
>     Clock synchronization is important - you should ensure that ntpd is
>     properly configured on all nodes. If your particular use case is
>     especially sensitive to out-of-order mutations it is possible to set
>     timestamps on the client side using the
>     drivers. https://docs.datastax.com/en/developer/java-driver/3.1/manual/query_timestamps/
>     <https://docs.datastax.com/en/developer/java-driver/3.1/manual/query_timestamps/>
> 
>     We use our own NTP cluster to reduce clock drift as much as
>     possible, but public NTP servers are good enough for most
>     uses. https://www.instaclustr.com/blog/2015/11/05/apache-cassandra-synchronization/
>     <https://www.instaclustr.com/blog/2015/11/05/apache-cassandra-synchronization/>
> 
>     Cheers,
>     Justin
> 
>     On Thu, 9 Feb 2017 at 16:09 Kant Kodali <kant@xxxxxxxxxxxx
>     <mailto:kant@xxxxxxxxxxxx>> wrote:
> 
>         How does Cassandra achieve Linearizability with “Last write
>         wins” (conflict resolution methods based on time-of-day clocks) ?
> 
>         Relying on synchronized clocks are almost certainly
>         non-linearizable, because clock timestamps cannot be guaranteed
>         to be consistent with actual event ordering due to clock skew.
>         isn't it?
> 
>         Thanks!
> 
>     -- 
> 
>     Justin Cameron
> 
>     Senior Software Engineer | Instaclustr
> 
> 
> 
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