Cassandra is frequently used in systems spanning physically separate locations. Cassandra provides two levels of grouping that are used to describe the topology of a cluster: data center and rack. A rack is a logical set of nodes in close proximity to each other. A data center is a logical set of racks.
Cassandra comes with a simple default configuration of a single data center ("datacenter1") containing a single rack ("rack1").
Cassandra leverages the information you provide about your cluster's topology to determine where to store data, and how to route queries efficiently. Cassandra stores copies of your data in the data centers you request to maximize availability and partition tolerance, while preferring to route queries to nodes in the local data center to maximize performance.
To support decentralization and partition tolerance, Cassandra uses a gossip protocol that allows each node to keep track of state information about the other nodes in the cluster. The gossiper runs every second on a timer.
Gossip protocols (= epidemic protocols) generally assume a faulty network, are commonly employed in very large, decentralized network systems, and are often used as an automatic mechanism for replication in distributed databases. They take their name from the concept of human gossip, a form of communication in which peers can choose with whom they want to exchange information.
The gossip protocol in Cassandra is primarily implemented by the org.apache.casandra.gms.Gossiper class, which is responsible for managing gossip for the local node. When a server node is started, it registers itself with the gossiper to receive endpoint state information.
Because Cassandra gossip is used for failure detection, the Gossiper class maintains a list of nodes that are alive and dead.