Redundancy means having more than you need. If one fails, others take over.
Active-passive: Primary handles traffic. Standby waits. Failover on primary failure. Simple but standby is underutilized.
Active-active: Multiple instances handle traffic. Load balanced. Better utilization but harder consistency.
Replicate across availability zones. If AWS us-east-a fails, us-east-b continues. For critical systems, replicate across regions.