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From: "Hiller, Dean" <Dean.Hiller@nrel.gov>
To: "user@cassandra.apache.org" <user@cassandra.apache.org>
Date: Wed, 19 Sep 2012 06:46:07 -0600
Subject: Re: Data Model - Consistency question
Thread-Topic: Data Model - Consistency question
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Message-ID: <CC7F1983.114F0%Dean.Hiller@nrel.gov>
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Yes, this scenario can occur(even with quorum writes/reads as you are deali=
ng with different rows) as one write may be complete and the other not whil=
e someone else is reading from the cluster.  Generally though, you can do r=
ead repair when you read it in ;).  Ie. See if things are inconsistent when=
 reading it and either 1. Wait and read again or 2. Figure out a way to dis=
play the results correctly based on merging data.  In general #1 on a lot o=
f systems is not a bad approach when you can't merge the data yourself beca=
use the conflicts are not happening to often(maybe < 1% of the time in a lo=
t of cases)

With playOrm if you have partitions, you can still have that relational mod=
el you described if you can figure out a partition strategy AND you can the=
n query on it with joins and such.

Later,
Dean


From: Roshni Rajagopal <roshni_rajagopal@hotmail.com<mailto:roshni_rajagopa=
l@hotmail.com>>
Reply-To: "user@cassandra.apache.org<mailto:user@cassandra.apache.org>" <us=
er@cassandra.apache.org<mailto:user@cassandra.apache.org>>
Date: Wednesday, September 19, 2012 6:16 AM
To: "user@cassandra.apache.org<mailto:user@cassandra.apache.org>" <user@cas=
sandra.apache.org<mailto:user@cassandra.apache.org>>
Subject: Data Model - Consistency question

Hi Folks,

In the relational world, if I needed to model students, courses relationshi=
p, I may have done
a students -master table
a course - master table
a bridge table students-course which gives me the ids to students and the c=
ourses they are taking. This can answer both 'which students take course A'=
, as well as 'which courses are taken by student B'

In the cassandra world, I may design it like this
a static student column family
a static course column family
a student-course column family with student id as key and dynamic list of c=
ourse - ids to answer 'which courses are taken by student B'
a course-student column family with course id as key and dynamic list of st=
udent ids  'which students take course A'

A screen which displays some student entity details as well as all the cour=
ses she is taking will need to refer to 2 column families

Suppose an application inserts a new row in student column family, and a ne=
w row in student-course column family, as transactions or consistency acros=
s column families is not guaranteed, there is a chance that the client rece=
ives information that a student is attending a course from student-course c=
olumn family, but does not exist in the student column family.

If we use Strong consistency from the  reads + writes combination - will th=
is scenario not occur ?
And if we dont, can this scenario occur?

Regards,
Roshni


Regards,
Roshni