Cassandra places its nodes on a logical ring of tokens, and each node owns one or more token ranges. Cassandra hashes a row's partition key into a token and stores the row on the node that owns that range. This is consistent hashing.
There is no master node, so there is no single point of failure. Nodes share cluster state through the gossip protocol, so each node knows about every other node and can handle any request. Each row is replicated to several nodes for fault tolerance.
Rewriting in plainer words…
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