Replication protects against data loss:
Synchronous replication: Write to all replicas before acknowledging. Strong durability. Higher latency.
Asynchronous replication: Acknowledge after primary write. Replicas catch up. Lower latency but potential data loss on failure.
Replication factor: S3 stores copies across availability zones. Higher factors cost more but survive more failures.
Consistency: Synchronous gives strong consistency. Asynchronous allows stale reads from replicas.
Choose based on durability requirements and latency budget. Financial data needs synchronous. Logs can use asynchronous.