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From Edouard COLE <Edouard.C...@rgsystem.com>
Subject Re: Is Cassandra really Strong consistency?
Date Sun, 06 Sep 2015 13:01:57 GMT
@ibrahim: When saying "clocks should be synchronized", it includes Cassandra nodes AND clients

NTP is the way to go

Le 6 sept. 2015 à 14:56, Laing, Michael <michael.laing@nytimes.com<mailto:michael.laing@nytimes.com>>
a écrit :

https://en.wikipedia.org/wiki/Network_Time_Protocol

On Sun, Sep 6, 2015 at 8:23 AM, ibrahim El-sanosi <ibrahimsabattt@gmail.com<mailto:ibrahimsabattt@gmail.com>>
wrote:
Assume the Cassandra cluster is located in somewhere in US. Clients that connect from different
part of the world will have different timestamp (if we rely on client timestamp to store write)
or If a coordinator is responsible for generating timestamp during the write, it also may
have different time among replicas, resulting in write conflict can occur and impossible to
resolve.

When you are saying “Clocks should be synchronized”, does Cassandra synchronize the clock
if so how can you refer me to any related article?

Regards,

Ibrahim

On Sun, Sep 6, 2015 at 1:23 PM, Daniel Schulz <danielschulz2005@hotmail.com<mailto:danielschulz2005@hotmail.com>>
wrote:
Cassandra is not changing clock settings; it does use it to omit TTL'ed rows in compaction
phases. So make sure your nodes agree on the very same time using e.g. NTP. It is very crucial
for data integrity on most distributed systems.

________________________________
Date: Sun, 6 Sep 2015 13:10:14 +0100
Subject: Re: Is Cassandra really Strong consistency?
From: ibrahimsabattt@gmail.com<mailto:ibrahimsabattt@gmail.com>
To: user@cassandra.apache.org<mailto:user@cassandra.apache.org>


Do you mean Cassandra does synchronize the clock across all the cluster, if yes how it does
so, or could you refer me to any related article?

Thank you


Ibrahim

On Sun, Sep 6, 2015 at 1:00 PM, Laing, Michael <michael.laing@nytimes.com<mailto:michael.laing@nytimes.com>>
wrote:
I think I saw this before.

Clocks must be synchronized.

On Sun, Sep 6, 2015 at 7:28 AM, ibrahim El-sanosi <ibrahimsabattt@gmail.com<mailto:ibrahimsabattt@gmail.com>>
wrote:

Hi folks,

Assume we have 4-nodes cluster N1, N2, N3, and N4 and replication factor is 3.  When write
CL =ALL and read CL=ONE:

Client c1 sends W1 = [k1,V1] to N1 (a coordinator).  A coordinator (N1) generates timestamp
Mon 05-09-2015 11:30:40,200 (according to its local clock) and assigned it to W1 and sends
the W1 to N2, N3, and N4. After few seconds, Client c2 sends W2 = [K1, V2] to N4 (a coordinator).
A coordinator (N4) generates timestamp Mon 05-09-2015 11:30:38,200 (according to its local
clock, but assume here N4 clock a bit behind, nearly 2 seconds) and assigned it to W2 and
sends the W2 to N2, N3, and N4 (itself).

As we have write CL =ALL and read CL = ONE. Now, Client c2 wants to read K1, connects to a
coordinator N1, a coordinator sends read K1 to N2, picking latest timestamp which is [K1,
V1]:Mon 05-09-2015 11:30:40,200.

So in this scenario, the latest data that wrote to the replicas is [K1, V2] which should be
the correct one, but it reads [K1,V1] because of divert clock.


Can such scenario occur?

Thank you





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