Key-value stores offer different consistency guarantees:
Strong consistency: Read always returns the latest write. Requires coordination. Higher latency.
Eventual consistency: Reads may return stale data. Updates propagate over time. Lower latency.
Read-your-writes: You see your own writes immediately. Others may see stale data.
Tunable consistency: Systems like Cassandra let you choose per-operation. Write to of replicas, read from of . Quorum ensures you see latest.
For session stores, read-your-writes usually suffices. For financial data, you need strong consistency.