CODEWITHSUNDEEP
cassandracassandra-3.0distributed-system
Sahil Ummer
Sahil Ummer

Reputation: 625

How does Cassandra Partitioning actually work?

I understand that two tables with same partition columns and values have same token generated. Does that mean that all the cells of this partition in both tables are actually in the same partition ? How does Cassandra store data internally ?

Eg:

Create table table1 (emp_id int PRIMARY KEY, name text, role text);
Create table table2 (emp_id int PRIMARY KEY, name text, role text);​
​​
​​INSERT INTO table1(emp_id, name, role) VALUES (1, 'sahil', 'MTS');
​​INSERT INTO table2(emp_id, name, role) VALUES (1, 'sahil', 'MTS');

SELECT token(emp_id) from table1 where token(emp_id) = token(11596);

 system.token(emp_id)
----------------------
  **7447223576279188802**​

SELECT token(emp_id) from table2 where token(emp_id) = token(1);

 system.token(emp_id)
----------------------
  **7447223576279188802**

​​

Upvotes: 0

Views: 405

Answers (2)

Nadav Har'El
Nadav Har'El

Reputation: 13731

Rows (items of data) that have the same table and the same partition key are said to be in the same partition. The most important consequence of being in the same partition is that data in the same partition is guaranteed to be co-located - handled by the same replica nodes and in ScyllaDB, even by the same CPU. This allows efficiently scanning a partition: All the partition's data can be read from a single node and Cassandra doesn't to go back and forth between replicas to read the various pieces of the partition and combine them. This is also what allows a node that handles the partition's full data to maintain it sorted by the clustering key: A process called compaction is merging different pieces of a sorted partition (these are sstables, or sorted string tables) into a bigger sorted partition.

When you have two different tables in the same keyspace, and use the same partition key in both, they are not stored physically on disk together - because each table has its own set of sstables (files on disk), so in that sense they are not "in the same partition". However, the co-location property which I mentioned earlier still holds (if the two tables are in the same keyspace): Two identically-keyed partitions in the two tables will be stored on exactly the same node. Why is this important/useful? Usually it isn't. One place where this knowledge can become useful is that it can be used in some situations to achieve atomic batch write to both tables at once, utilizing the fact that all replicas will see both writes together, whereas usually two writes to two tables go to different nodes at different times.

Upvotes: 0

Paul
Paul

Reputation: 416

For your example, because both tables have the same partition key, then when identical values are inserted, they will be mapped to the same token. It is on insert that the hash function to the PK is applied to determine what replica will get the data. If you use the Murmur3 partitioner (which is used by default) then you get a consistent token value, i.e. using the same PK and PK value, the result is the same. You can reference this page for understanding: https://docs.datastax.com/en/cassandra-oss/3.x/cassandra/architecture/archDataDistributeHashing.html

Upvotes: 1

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