Explanation

To achieve high availability for critical applications on AWS, deploying across multiple Availability Zones (AZs) is the recommended approach.

Why Multiple Availability Zones?

• Fault Isolation: Each Availability Zone is physically separate with independent power, cooling, and networking
• Automatic Failover: If one AZ fails, the application can continue running in other AZs
• High Availability Architecture: Multi-AZ deployments provide redundancy and resilience

Why Not the Other Options?

• Single Availability Zone: Creates a single point of failure - if that AZ goes down, the entire application goes down
• AWS Direct Connect: This is for dedicated network connections, not for application deployment or high availability
• Reserved Instances: This is a billing/pricing model for cost savings, not a deployment strategy for high availability

Best Practice

For mission-critical applications, AWS recommends deploying across at least 2-3 Availability Zones within the same region to ensure continuous operation even during AZ-level failures.