SAA-C03 practice question 5 of 35

A company deployed a three-tier web application in a single Availability Zone in the us-east-1 Region on a single Amazon EC2 instance. Usage of the…

Choose your answer, then check it against the explanation and the full breakdown below.

Domain Design Resilient Architectures

A company deployed a three-tier web application in a single Availability Zone in the us-east-1 Region on a single Amazon EC2 instance. Usage of the application is growing. A solutions architect needs to ensure that the application can handle the growing amount of traffic. The solutions architect also needs to ensure the application is resilient. Which solution will meet these requirements MOST cost-effectively?

Answer options

Correct Answer:

Explanation

This solution correctly addresses all requirements. An EC2 Auto Scaling group configured across multiple Availability Zones (AZs) provides both high availability and resilience by ensuring the application can withstand the failure of a single AZ. The Application Load Balancer (ALB) distributes incoming traffic across the instances in all configured AZs. Using an Amazon CloudWatch alarm to trigger horizontal scaling (adding or removing EC2 instances) is the most effective method to handle variable traffic loads. This dynamic scaling approach ensures that compute capacity matches demand, thereby meeting the scalability requirement in the most cost-effective manner by avoiding over-provisioning.

Why Incorrect

A. Vertical scaling (changing instance size) is less agile for handling traffic spikes than horizontal scaling and can lead to service interruptions during resizing.

B. Provisioning a fixed number of nine instances from the start is not cost-effective, as it leads to paying for unused capacity during periods of low traffic.

C. Placing all instances in the same Availability Zone fails the resilience requirement, as a single AZ failure would cause a complete application outage.

References

1. AWS EC2 Auto Scaling User Guide: "To improve the availability of your applications

you can configure your Auto Scaling group to launch new instances into multiple Availability Zones. If one Availability Zone becomes unavailable

Amazon EC2 Auto Scaling launches new instances in an unaffected Availability Zone." (Source: AWS Documentation

Amazon EC2 Auto Scaling User Guide

"What is Amazon EC2 Auto Scaling?"

Section: Availability).

2. AWS Elastic Load Balancing User Guide: "When you enable an Availability Zone for your load balancer

Elastic Load Balancing creates a load balancer node in the Availability Zone. ... This increases the fault tolerance of your applications." (Source: AWS Documentation

User Guide for Application Load Balancers

"How Elastic Load Balancing works"

Section: Load balancers and Availability Zones).

3. AWS Well-Architected Framework - Reliability Pillar: "Scale horizontally to increase aggregate workload availability. Replace one large resource with multiple small resources to reduce the impact of a single failure. Distribute requests among the multiple

smaller resources to ensure that they don't share a common point of failure." (Source: AWS Whitepaper

AWS Well-Architected Framework

Reliability Pillar

Design Principles

Page 5).

4. AWS Well-Architected Framework - Cost Optimization Pillar: "Use a demand-based approach to provision resources. For example

with Amazon EC2 Auto Scaling

you can add or remove EC2 instances automatically to match the demand of your workload. This stops you from provisioning for peak load and paying for idle resources." (Source: AWS Whitepaper

AWS Well-Architected Framework

Cost Optimization Pillar

Design Principles

Page 6).

Question 5 of 35

Question breakdown

Tests: Design scalable, resilient compute architecture
Difficulty: Easy

Key idea: A multi-AZ Auto Scaling group behind an ALB with dynamic horizontal scaling provides both elasticity and AZ-level resilience without overprovisioning.

How to reach the answer

Clue: 'resilient' requires spreading instances across multiple AZs.
Clue: 'growing traffic' calls for horizontal scaling tied to demand.
Clue: 'MOST cost-effectively' rules out a large fixed fleet.
Option D is the only one combining multi-AZ, Auto Scaling, and demand-based horizontal scaling.

Each option

AIncorrect. Fixed instances with vertical scaling require stop/resize downtime and do not add capacity elastically.
BIncorrect. Nine fixed instances overprovision capacity and lack an Auto Scaling group to actually add or remove instances.
CIncorrect. All instances in one AZ leaves a single point of failure, and scheduled scaling does not track unpredictable growth.
DCorrect. Multi-AZ Auto Scaling with an ALB and alarm-driven horizontal scaling meets resilience, scale, and cost goals.

Common trap: Option C mentions Auto Scaling and an ALB, which looks complete until you notice the single AZ.

In practice: Nearly every stateless web tier on AWS runs as a multi-AZ Auto Scaling group behind an ALB, usually with target tracking policies.

Remember: Resilient plus scalable equals multi-AZ Auto Scaling group behind a load balancer with dynamic scaling.

Official documentation: What is Amazon EC2 Auto Scaling?

Drafted with AI assistance and checked against the answer key.

Keep practicing

Take the free SAA-C03 practice test

Ten exam-style questions with answers and explanations, plus the exam facts and study guides.

Start the free practice test