What does "Fault Tolerance" refer to in Cassandra?

Fault tolerance in Cassandra refers to its capability to continue operating despite node failures. This is a critical feature of distributed databases, especially for systems that are expected to be always available and reliable, serving real-time data needs.

Cassandra achieves fault tolerance through its architecture, which includes features such as data replication and decentralized design. Each piece of data is replicated across multiple nodes in the cluster, meaning that if one node fails, the data is still accessible from another node holding a replica. This redundancy allows the system to maintain high availability and reliability, ensuring uninterrupted service even during hardware malfunctions or network issues.

The other choices address different aspects of the database. Query speed optimization pertains to how efficiently data can be retrieved, balancing data across nodes is about the uniform distribution of data to prevent hotspots, and data normalization involves organizing data efficiently to minimize redundancy. However, none of these directly capture the essence of fault tolerance, which is fundamentally about the system's resilience to failures.