AWS Certified Solutions Architect – Associate (SAA-C02) — Question 625

A company is running a critical business application on Amazon EC2 instances behind an Application Load Balancer. The EC2 instances run in an Auto Scaling group and access an Amazon RDS DB instance.
The design did not pass an operational review because the EC2 instances and the DB instance are all located in a single Availability Zone. A solutions architect must update the design to use a second Availability Zone.
Which solution will make the application highly available?

Answer options

• A. Provision a subnet in each Availability Zone. Configure the Auto Scaling group to distribute the EC2 instances across both Availability Zones. Configure the DB instance with connections to each network.
• B. Provision two subnets that extend across both Availability Zones. Configure the Auto Scaling group to distribute the EC2 instances across both Availability Zones. Configure the DB instance with connections to each network.
• C. Provision a subnet in each Availability Zone. Configure the Auto Scaling group to distribute the EC2 instances across both Availability Zones. Configure the DB instance for Multi-AZ deployment.
• D. Provision a subnet that extends across both Availability Zones. Configure the Auto Scaling group to distribute the EC2 instances across both Availability Zones. Configure the DB instance for Multi-AZ deployment.

Correct answer: C

Explanation

In AWS, subnets are strictly mapped to a single Availability Zone and cannot span across multiple AZs, which immediately invalidates options B and D. To achieve true high availability for the database layer, Amazon RDS must be configured for a Multi-AZ deployment to enable automatic replication and failover to a standby instance in the second AZ. Option C is the only choice that correctly respects subnet design limitations while ensuring database redundancy.