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From "Dobrin (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-8649) CAS per (partition key + clustering key) and not only per (partition key)
Date Tue, 20 Jan 2015 15:08:34 GMT
Dobrin created CASSANDRA-8649:
---------------------------------

             Summary: CAS per (partition key + clustering key) and not only per (partition
key)
                 Key: CASSANDRA-8649
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8649
             Project: Cassandra
          Issue Type: New Feature
          Components: Core
            Reporter: Dobrin


Reading the description at http://www.datastax.com/dev/blog/lightweight-transactions-in-cassandra-2-0
:
...
* The columns updated do NOT have to be the same as the columns in the IF clause.
* Lightweight transactions are restricted to a single partition; this is the granularity at
which we keep the internal Paxos state. As a corollary, transactions in different partitions
will never interrupt each other.
...

So my understanding of the above is that if multiple writers for example perform CAS inserts
(INSERT...IF NOT EXISTS) using the same partition key and different clustering keys will interrupt/interfere
with each other?
Is this understanding correct? (my tests seems to confirm it)

For example if I want to model users from different country/city/area and I want to be able
to list all the users from a given country ordered by (city,area) and also I know that a single
cassandra node will be able to store all the users from a given country but I need to partition
users from different countries because a single cassandra node will not be enough:

CREATE COLUMNFAMILY user (
	country text,
	city text,
	area text,
	id text,
	json text,
	version bigint,
	PRIMARY KEY ((country), city, area, id)
);

Where id is the user id and json is a JSON serialized user object (an aggregate) containing
more information about the user. 

I want to be able to CAS insert many users into the same country concurrently using
	INSERT INTO user(country, city, area, id, json, version) VALUES ('x',...) IF NOT EXISTS;
	
and be able to CAS update users from the same country concurrently:
	UPDATE user SET json='{...}',version=18 WHERE country='x' AND city='y' AND area='z' AND id='123'
IF version=17;
	
As I understand this will not be efficient because all the above concurrent statements will
have to be "ordered" by the same paxos instance/state per country 'x'? (and trying it results
in a lot of WriteTimeoutException-s)

If yes - can we made paxos to support IF statements per column/cell?

By cell/column I mean all the underlying persistent state that is behind the compound primary
key (partition key + clustering key) - in the above example
	the state is json and version
	the partition key is the country
	and the clustering key is (city, area, id)

(	
I'm stating it explicitly as I'm not completely sure whether this is a single cell or double
cells underneath at the storage engine, references used:
http://www.datastax.com/dev/blog/cql3-for-cassandra-experts
http://www.datastax.com/dev/blog/does-cql-support-dynamic-columns-wide-rows
)

In other words is it possible to make CAS per (partition key + clustering key) and not only
per (partition key)?



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