Cassandra stores several copies of each row on different nodes, so the data survives node failures. The replication factor (RF) sets how many copies exist. With an RF of 3, each row lives on 3 distinct nodes.
Each keyspace has a replication strategy that picks the nodes for the copies. SimpleStrategy is meant for test clusters, and NetworkTopologyStrategy is for production, with one or more datacenters.
Every replica can accept writes, and no copy is the primary. A client can send a request to any node, which acts as the coordinator and forwards it to the replicas that store the data.
Rewriting in plainer words…
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