How would you make a simple web application highly available in the cloud?

What the interviewer wants to hear

• Run multiple instances across at least two availability zones
• Put a load balancer with health checks in front
• Use an Auto Scaling group to replace failed instances
• Use a managed database with Multi-AZ failover
• Keep app servers stateless; store sessions and files externally

Model answer

The goal is to remove single points of failure. I would run the web tier on at least two instances, or containers, spread across two or three availability zones. An Application Load Balancer sits in front, does health checks and only sends traffic to healthy targets. An Auto Scaling group keeps the desired count and replaces any instance that fails. The app servers must be stateless, so sessions go into something like Redis or DynamoDB and uploaded files go to S3. For the database I would use a managed service like RDS or Aurora with Multi-AZ, so a standby takes over automatically if the primary fails. DNS with Route 53 health checks can add another layer. Finally, I would add monitoring and alarms so the team knows when something degrades, and test failover regularly.

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