SAA-C03 Design Resilient Architectures Practice Question
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?
⚠ Common exam trap
Test-takers frequently think increasing instance count or size improves resilience, but without multi-AZ distribution, all instances remain vulnerable to a single AZ failure.
Answer choices
Why each option matters
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.
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
Scaling the desired capacity to eight instances within the same subnet does not change the fact that the subnet maps to a single Availability Zone. A full AZ outage takes down all eight instances simultaneously, leaving the Application Load Balancer with no healthy targets outside that Zone. Horizontal scaling only improves throughput and per-instance load distribution, not fault tolerance across failure domains. The Auto Scaling group must instead be configured with subnets in two AZs to enable cross-zone instance replacement.
When this WOULD be correct
This option would be correct if the question asked for improving performance or handling increased load within a single AZ, without any requirement for multi-AZ resilience.
- ✓
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.
- ✗
Replace the Application Load Balancer with a Network Load Balancer.
Why it's wrong here
Exchanging the Application Load Balancer for a Network Load Balancer changes the L4 connection handling and static IP address behavior, but the underlying availability problem remains: all compute capacity is pinned to one Availability Zone. An NLB still requires registered targets in at least one healthy subnet, and if that single AZ fails, the NLB has no cross-zone instances to route to. Moreover, the ALB already spans two AZs, so the load balancer is not the bottleneck. The missing redundancy is in the Auto Scaling group's placement of instances, not in the type of load balancing.
When this WOULD be correct
This would be correct if the question required handling millions of requests per second with ultra-low latency, or if the application needed to preserve the source IP address for backend processing, as NLB operates at Layer 4.
- ✗
Move the instances to a larger instance type with more CPU and memory.
Why it's wrong here
Choosing a larger instance type with more CPU and memory addresses performance capacity, not availability. The instance remains a single resource within a single Availability Zone, and if that Zone experiences an outage, no amount of compute sizing keeps it reachable. A larger instance might also increase the blast radius if it fails, because the Auto Scaling group replaces it only after noticing health check failures. Availability requires distributing workloads across independent failure domains, which instance size cannot do.
When this WOULD be correct
This option would be correct if the question asked for improving application performance under high load, such as 'A web application is experiencing high CPU utilization and slow response times. What change would best improve performance?'
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The SAA-C03 exam frequently reuses these exact scenarios with slightly different constraints.
✓Add a subnet in another Availability Zone to the Auto Scaling group and keep the ALB spanning both AZs.Correct answer▾
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.
✗Increase the desired capacity to 8 instances in the same subnet.Wrong answer — click to see why▾
Why this is wrong here
Increasing desired capacity within the same subnet does not protect against an Availability Zone failure; all instances remain in a single AZ, so if that AZ fails, the application becomes unavailable.
★ When this WOULD be the correct answer
This option would be correct if the question asked for improving performance or handling increased load within a single AZ, without any requirement for multi-AZ resilience.
Why candidates choose this
Candidates may think that simply adding more instances improves resilience, not realizing that resilience requires distributing instances across multiple Availability Zones.
✗Replace the Application Load Balancer with a Network Load Balancer.Wrong answer — click to see why▾
Why this is wrong here
Replacing the ALB with a Network Load Balancer does not improve resilience across Availability Zones; it operates at a different layer and does not inherently provide cross-AZ fault tolerance for the application.
★ When this WOULD be the correct answer
This would be correct if the question required handling millions of requests per second with ultra-low latency, or if the application needed to preserve the source IP address for backend processing, as NLB operates at Layer 4.
Why candidates choose this
Candidates may think NLB is more resilient because it is simpler and faster, but they overlook that ALB already supports cross-AZ load balancing and is better suited for HTTP/HTTPS applications.
✗Move the instances to a larger instance type with more CPU and memory.Wrong answer — click to see why▾
Why this is wrong here
Increasing instance size (CPU/memory) does not address the requirement for availability zone failure resilience; it only improves performance, not fault tolerance across AZs.
★ When this WOULD be the correct answer
This option would be correct if the question asked for improving application performance under high load, such as 'A web application is experiencing high CPU utilization and slow response times. What change would best improve performance?'
Why candidates choose this
Candidates may think that more powerful instances inherently make the application more resilient, confusing performance improvement with high availability.
Analysis generated from the official SAA-C03blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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