1. EC2 vs Lambda
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EC2: Persistent, control over OS, good for long-running, steady-state apps.
Lambda: Event-driven, ephemeral, scales to zero, good for bursty traffic or glue code.
2. High Availability Multi-Region
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Active-Active (complex data sync) or Active-Passive (easier). Use Global Load Balancer (Route53), Replicated DBs (DynamoDB Global Tables, Aurora Global), and stateless app tiers.