AWS Certified Solutions Architect – Associate (SAA-C03) — Question 279

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 VPC architecture, subnets are strictly bound to a single Availability Zone and cannot span across multiple AZs, which eliminates options B and D. To make the application tier highly available, the Auto Scaling group should be configured to distribute EC2 instances across subnets in two different AZs. For the database tier, configuring Amazon RDS for Multi-AZ deployment ensures synchronous replication to a standby DB instance in a second AZ, providing automated failover and redundancy.