Apache Cassandra™ 1.2

Consistent hashing

Details about how the consistent hashing mechanism distributes data across a cluster in Cassandra.

Consistent hashing partitions data based on the primary key. For example, if you have the following data:

jim age: 36 car: camaro gender: M
carol age: 37 car: bmw gender: F
johnny age: 12 gender: M  
suzy age: 10 gender: F  

Cassandra assigns a hash value to each primary key:

Primary key Murmur3 hash value
jim -2245462676723223822
carol 7723358927203680754
johnny -6723372854036780875
suzy 1168604627387940318

Each node in the cluster is responsible for a range of data based on the hash value:

Node Murmur3 start range Murmur3 end range
A -9223372036854775808 -4611686018427387903
B -4611686018427387904 -1
C 0 4611686018427387903
D 4611686018427387904 9223372036854775807

Cassandra places the data on each node according to the value of the primary key and the range that the node is responsible for. For example, in a four node cluster, the data in this example is distributed as follows:

Node Start range End range Primary key Hash value
A -9223372036854775808 -4611686018427387903 johnny -6723372854036780875
B -4611686018427387904 -1 jim -2245462676723223822
C 0 4611686018427387903 suzy 1168604627387940318
D 4611686018427387904 9223372036854775807 carol 7723358927203680754

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