Cassandra uses a component known as the snitch to direct read and write operations to the appropriate nodes. When an operation is sent to the cluster, it is the snitch's job (Williams 2012) to determine which nodes in specific data centers or racks can serve the request.
Most snitches that ship with Apache Cassandra are both data center- and rack-aware. That is, they are capable of viewing (predetermined) groups of nodes in the cluster as logical data centers. Additionally, the data centers can be further divided into logical racks.
The term logical is used here because the data centers may or may not be physically separate from each other. That is, nodes in a single physical data center can be split into smaller data centers, and ...
