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  1. Cassandra
  2. CASSANDRA-8649

CAS per (partition key + clustering key) and not only per (partition key)




      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:

      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:

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




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            dobrin Dobrin
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