How do partition keys work?

Question

I am new to Cassandra and I read that the primary key is the same thing as the partition key.

My question is simple, in this case:

CREATE TABLE users (
  user_name varchar PRIMARY KEY,
  password varchar,
  gender varchar,
  session_token varchar,
  state varchar,
  birth_year bigint
);

As the partition key is responsible for data distribution accross your nodes, how will the data be distributed by username in this case?

Answer 1

Actually, The PRIMARY KEY is not the same as the partition key. The partition key is a part of the PRIMARY KEY. And yes, it is the part which determines how a row is distributed across the cluster.

how will the data be distributed by username in this case?

If I CREATE your table, insert some values and query it I can get a bit of a window into the distribution process by using the token function in my SELECT:

> SELECT token(user_name), user_name FROM user2;

 system.token(user_name) | user_name
-------------------------+-----------
    -5077180869401877077 |   Patdard
    -4874582970682694928 |      Robo
     4639906948852899531 |      Bill
     4645660266327417866 |       Bob
     4877648712764681009 | Valentina
     5726383012007749221 |   Helcine
     7724711996172375448 |  Jebediah

(7 rows)

Let's assume that I have 5 nodes. In Cassandra each node is responsible for a primary token range. Let's assume the following:

1)  5534023222112865485 to -9223372036854775808
2) -9223372036854775807 to -5534023222112865485
3) -5534023222112865484 to -1844674407370955162
4) -1844674407370955161 to  1844674407370955161
5)  1844674407370955161 to  5534023222112865484

Note: Ranges computed by running:

python -c 'print [str(((2**64 / 5) * i) - 2**63) for i in range(5)]'

Also depicted this way in MVP Robbie Strickland's Cassandra High Availability.

Cassandra takes the hashed token value of the partition key (user_name in this case) and uses that to determine which node the row show be distributed to. Given the hashed token values above, and the ranges that I have listed out, these are the nodes which each user name should go to:

Node 1: Helcine, Jebediah
Node 3: Patdard, Robo
Node 5: Bill, Bob, Valentina

Depending on your replication factor (RF), Cassandra may also place additional replicas of each row on other nodes.