Multi-AZ Web Application

High availability is not just about adding more servers — it requires deliberate architecture decisions across every layer of your stack. In this challenge, you will design a web application that maintains full functionality even when an entire AWS Availability Zone goes offline. The architecture spans a multi-AZ VPC with public and private subnets across three AZs, an Application Load Balancer with cross-zone load balancing and health checks, an Auto Scaling Group with proper scaling policies and AZ rebalancing, and an RDS Multi-AZ deployment with automatic failover. You will design the networking layer with route tables, NAT Gateways (one per AZ for resilience), and security groups that follow least privilege. The application tier uses launch templates with user data scripts for consistent instance configuration, and the ASG uses target tracking scaling based on CPU and request count. For the data tier, you will compare RDS Multi-AZ (synchronous replication, automatic failover) versus Aurora (storage-level replication across six copies in three AZs). The design includes Route 53 health checks for DNS failover, CloudWatch alarms for automated incident detection, and a runbook for manual failover procedures. You will calculate the cost difference between two-AZ and three-AZ deployments and justify the trade-off. This challenge teaches multi-AZ resilience patterns, failure domain isolation, and the operational reality of maintaining high availability on AWS.