Cassandra has a limit of 2 billion cells per partition, but what's a partition?

With the advent of CQL3 the terminology has changed slightly from the old thrift terms.

Basically

Create Table foo (a int , b int, c int, d int, PRIMARY KEY ((a,b),c))

Will make a CQL3 table. The information in a and b is used to make the partition key, this describes which node the information will reside on. This is the 'partiton' talked about in the 2 billion cell limit.

Within that partition the information will be organized by c, known as the clustering key. Together a,b and c, define a unique value of d. In this case the number of cells in a partition would be c * d. So in this example for any given pair of a and b there can only be 2 billion combinations of c and d

So as you model your data you want to ensure that the primary key will vary so that your data will be randomly distributed across Cassandra. Then use clustering keys to ensure that your data is available in the way you want it.

Watch this video for more info on Datmodeling in cassandra The Datamodel is Dead, Long live the datamodel

Edit: One more example from the comments

Create Table foo (a int , b int, c int, d int, e int, f int, PRIMARY KEY ((a,b),c,d))

Partitions will be uniquely identified by a combination of a and b.

Within a partition c and d will be used to order cells within the partition so the layout will look a little like:

(a1,b1) --> [c1,d1 : e1], [c1,d1  :f1], [c1,d2 : e2] ....  

So in this example you can have 2 Billion cells with each cell containing:

• A value of c
• A value of d
• A value of either e or f

So the 2 billion limit refers to the sum of unique tuples of (c,d,e) and (c,d,f).

From : http://www.datastax.com/documentation/cql/3.0/cql/cql_reference/create_table_r.html

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Using a composite partition key¶

A composite partition key is a partition key consisting of multiple columns. You use an extra set of parentheses to enclose columns that make up the composite partition key. The columns within the primary key definition but outside the nested parentheses are clustering columns. These columns form logical sets inside a partition to facilitate retrieval. 


CREATE TABLE Cats (
  block_id uuid,
  breed text,
  color text,
  short_hair boolean,
  PRIMARY KEY ((block_id, breed), color, short_hair)
);

For example, the composite partition key consists of block_id and breed. The clustering columns, color and short_hair, determine the clustering order of the data. Generally, Cassandra will store columns having the same block_id but a different breed on different nodes, and columns having the same block_id and breed on the same node.

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Implication

==> Partition is the smallest unit of replication (which on its own makes sh** no sense. :) )

==> Every combination of block_id and breed is a Partition.

==> On any given machine in cluster, either all or none of the rows with same partition-key will exist.