SAA-C03 Design Resilient Architectures Practice Question
This SAA-C03 practice question tests your understanding of design resilient architectures. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: auto Scaling groups can launch instances across multiple subnets in different AZs.. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Exhibit
Auto Scaling group configuration:
- Desired capacity: 4
- VPC subnets: subnet-0a11 (us-east-1a) only
- Health check type: ELB
Application Load Balancer configuration:
- Enabled subnets: subnet-0a11 (us-east-1a), subnet-0b22 (us-east-1b)
Incident note:
- A planned test stopped all instances in us-east-1a and the application became unavailable.
Based on the exhibit, a web application must stay available if one Availability Zone fails. What is the best change to improve resilience?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Auto Scaling group configuration:
- Desired capacity: 4
- VPC subnets: subnet-0a11 (us-east-1a) only
- Health check type: ELB
Application Load Balancer configuration:
- Enabled subnets: subnet-0a11 (us-east-1a), subnet-0b22 (us-east-1b)
Incident note:
- A planned test stopped all instances in us-east-1a and the application became unavailable.
A
Increase the desired capacity to 8 instances in the same subnet.
Why wrong: More instances in one Availability Zone do not protect against the Zone itself failing. Capacity increases, but resilience does not.
B
Add a subnet in another Availability Zone to the Auto Scaling group and keep the ALB spanning both AZs.
This places application instances across multiple Availability Zones, which protects the stateless tier from a single-AZ failure. The ALB already spans two AZs, so the missing piece is the Auto Scaling group using subnets in more than one AZ. That allows AWS to replace unhealthy instances and continue serving traffic from the surviving Zone.
C
Replace the Application Load Balancer with a Network Load Balancer.
Why wrong: A different load balancer type does not fix the single-AZ compute placement problem. The application still has no surviving instances in another Zone.
D
Move the instances to a larger instance type with more CPU and memory.
Why wrong: Bigger instances can improve performance, but they do not add Availability Zone redundancy. A larger instance in one AZ is still a single point of failure.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add a subnet in another Availability Zone to the Auto Scaling group and keep the ALB spanning both AZs.
Adding a subnet in another Availability Zone (AZ) to the Auto Scaling group and keeping the ALB spanning both AZs ensures that if one AZ fails, the ALB can route traffic to healthy instances in the other AZ. This is the standard pattern for building multi-AZ resilient architectures with Auto Scaling and ALB, as it eliminates the single point of failure at the AZ level.
Key principle: Auto Scaling groups can launch instances across multiple subnets in different AZs.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Increase the desired capacity to 8 instances in the same subnet.
Why it's wrong here
More instances in one Availability Zone do not protect against the Zone itself failing. Capacity increases, but resilience does not.
✓
Add a subnet in another Availability Zone to the Auto Scaling group and keep the ALB spanning both AZs.
Why this is correct
This places application instances across multiple Availability Zones, which protects the stateless tier from a single-AZ failure. The ALB already spans two AZs, so the missing piece is the Auto Scaling group using subnets in more than one AZ. That allows AWS to replace unhealthy instances and continue serving traffic from the surviving Zone.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Auto Scaling groups can launch instances across multiple subnets in different AZs.
✗
Replace the Application Load Balancer with a Network Load Balancer.
Why it's wrong here
A different load balancer type does not fix the single-AZ compute placement problem. The application still has no surviving instances in another Zone.
✗
Move the instances to a larger instance type with more CPU and memory.
Why it's wrong here
Bigger instances can improve performance, but they do not add Availability Zone redundancy. A larger instance in one AZ is still a single point of failure.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often focus on scaling up (more instances or larger instances) or changing the load balancer type, missing the fundamental requirement of distributing resources across multiple Availability Zones to achieve AZ-level resilience.
Detailed technical explanation
How to think about this question
The ALB is a regional service that inherently supports multiple AZs by creating a separate load balancer node in each enabled AZ. When you add a subnet in another AZ to the Auto Scaling group, the ASG can launch instances in that new AZ, and the ALB will automatically route traffic to healthy targets across both AZs. This design leverages the ALB's cross-zone load balancing capability (enabled by default) and the ASG's ability to distribute instances evenly across AZs, achieving an RTO of minutes during an AZ failure.
KKey Concepts to Remember
Auto Scaling groups can launch instances across multiple subnets in different AZs.
Application Load Balancers (ALBs) are inherently multi-AZ and distribute traffic.
Distributing instances across AZs protects against single-AZ failures.
Subnets define the network range and Availability Zone for EC2 instances.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Auto Scaling groups can launch instances across multiple subnets in different AZs.
Real-world example
How this comes up in practice
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
What to study next
Got this wrong? Here's your next step.
Review auto Scaling groups can launch instances across multiple subnets in different AZs., then practise related SAA-C03 questions on the same topic to reinforce the concept.
Design Resilient Architectures — This question tests Design Resilient Architectures — Auto Scaling groups can launch instances across multiple subnets in different AZs..
What is the correct answer to this question?
The correct answer is: Add a subnet in another Availability Zone to the Auto Scaling group and keep the ALB spanning both AZs. — Adding a subnet in another Availability Zone (AZ) to the Auto Scaling group and keeping the ALB spanning both AZs ensures that if one AZ fails, the ALB can route traffic to healthy instances in the other AZ. This is the standard pattern for building multi-AZ resilient architectures with Auto Scaling and ALB, as it eliminates the single point of failure at the AZ level.
What should I do if I get this SAA-C03 question wrong?
Review auto Scaling groups can launch instances across multiple subnets in different AZs., then practise related SAA-C03 questions on the same topic to reinforce the concept.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Auto Scaling groups can launch instances across multiple subnets in different AZs.
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