Multi-AZ Resilience with Auto Scaling and ALB
Exhibit
Current deployment: Application Load Balancer subnets: subnet-a1 (AZ-a), subnet-a2 (AZ-a) Auto Scaling group subnets: subnet-a1 (AZ-a) only Desired capacity: 4 instances Minimum capacity: 4 instances Incident report: 2026-04-18T09:21Z AZ-a experienced a power issue 2026-04-18T09:22Z all targets became unhealthy 2026-04-18T09:25Z service returned HTTP 503 to users
Based on the exhibit, the application should continue serving requests if one Availability Zone fails. Which change best improves resilience with the least operational complexity?
Quick Answer
The core idea being tested is that resilience to an Availability Zone failure requires every layer of the architecture to span multiple AZs, not just one. If the Auto Scaling group is multi-AZ but the ALB only has a subnet in a single AZ, or vice versa, the single-AZ component becomes the point of failure and traffic still stops flowing when that AZ goes down. By adding a subnet from a second AZ to both the ALB and the Auto Scaling group, the load balancer gains healthy targets to route to outside the failed AZ, and the Auto Scaling group gains the ability to launch replacement instances in a surviving AZ to maintain capacity. This is deliberately the simplest fix available: it doesn't require a new load balancing service, a different scaling strategy, or added application logic, it just extends the existing components across AZ boundaries, which is why it satisfies the least-operational-complexity requirement. More elaborate approaches, like adding an entirely separate regional failover, would solve a different and larger problem than what's being asked about here. Whenever a scenario asks how to survive a single AZ failure with minimal added complexity, check whether every component in the request path is actually deployed across at least two AZs, because a resilient design is only as strong as its least redundant piece.
⚠ Common exam trap
Test-takers frequently think increasing capacity in a single AZ (Option A) provides resilience, but it actually concentrates risk in that AZ, while the correct answer requires distributing resources across multiple AZs to achieve true fault tolerance.
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 at least one subnet from a second Availability Zone to both the ALB and the Auto Scaling group.
Adding subnets from a second Availability Zone to both the ALB and the Auto Scaling group distributes the application across multiple AZs. This ensures that if one AZ fails, the ALB can route traffic to healthy targets in the remaining AZ, and the Auto Scaling group can maintain capacity by launching instances in the surviving AZ. This approach directly addresses the requirement to continue serving requests during an AZ failure with minimal operational complexity.
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 in AZ-a so more instances can absorb the failure of that same Availability Zone.
Why it's wrong here
Adding more instances in the same Availability Zone does not protect against an AZ-wide outage because all of those instances fail together.
- ✓
Add at least one subnet from a second Availability Zone to both the ALB and the Auto Scaling group.
Why this is correct
A resilient design needs the load balancer and the Auto Scaling group to span multiple Availability Zones. If one AZ fails, the ALB can still route to healthy targets in the remaining AZs and the Auto Scaling group can replenish capacity there. This is the simplest and most common way to achieve AZ-level fault tolerance.
- ✗
Disable health checks so the ALB stops removing targets during brief infrastructure issues.
Why it's wrong here
Disabling health checks hides failures instead of recovering from them. The load balancer would continue sending traffic to unhealthy targets, which makes the outage worse rather than better.
- ✗
Move the application to a single larger instance type so the fleet has fewer moving parts.
Why it's wrong here
A single larger instance increases concentration risk and does not remove the Availability Zone single point of failure. It reduces the number of instances, but it also reduces redundancy.
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Same concept, more angles
8 more ways this is tested on SAA-C03
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Based on the exhibit, the web application must remain available even if one Availability Zone fails. What is the best change to improve resilience with the least redesign?
medium- A.Increase DesiredCapacity to 4 while keeping all instances in subnet-a1.
- ✓ B.Add subnet-b1 in a different Availability Zone to the Auto Scaling group.
- C.Replace the Application Load Balancer with a Network Load Balancer.
- D.Enable EBS encryption on the launch template volumes.
Why B: Adding subnet-b1 in a different Availability Zone to the Auto Scaling group ensures that EC2 instances are launched across two Availability Zones. If one zone fails, the ALB can route traffic to healthy instances in the other zone, maintaining application availability. This change requires minimal redesign because it only modifies the Auto Scaling group's subnet configuration without altering the load balancer or compute architecture.
Variation 2. Based on the exhibit, a web application must stay available if one Availability Zone fails. What is the best change to improve resilience?
easy- A.Increase the desired capacity to 8 instances in the same subnet.
- ✓ B.Add a subnet in another Availability Zone to the Auto Scaling group and keep the ALB spanning both AZs.
- C.Replace the Application Load Balancer with a Network Load Balancer.
- D.Move the instances to a larger instance type with more CPU and memory.
Why B: 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.
Variation 3. Based on the exhibit, a web application must stay available if one Availability Zone fails. What is the best change to improve resilience?
easy- A.Increase the desired capacity to 8 instances in the same subnet.
- ✓ B.Add a subnet in another Availability Zone to the Auto Scaling group and keep the ALB spanning both AZs.
- C.Replace the Application Load Balancer with a Network Load Balancer.
- D.Move the instances to a larger instance type with more CPU and memory.
Why B: 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.
Variation 4. A trading dashboard runs on EC2 instances behind an Application Load Balancer. The design must tolerate the failure of one Availability Zone. What should the Auto Scaling group configuration include? The design must avoid adding custom operational scripts.
medium- A.A single EC2 instance with detailed monitoring
- ✓ B.Subnets in at least two Availability Zones with health checks enabled
- C.All instances in one larger subnet
- D.A Network Load Balancer in one subnet
Why B: Distributing EC2 instances across at least two Availability Zones (AZs) ensures that the Auto Scaling group can maintain capacity even if one AZ fails. Enabling health checks on the Application Load Balancer (ALB) allows the group to automatically replace unhealthy instances without custom scripts, meeting the fault-tolerance requirement.
Variation 5. A trading dashboard runs on EC2 instances behind an Application Load Balancer. The design must tolerate the failure of one Availability Zone. What should the Auto Scaling group configuration include?
medium- A.A single EC2 instance with detailed monitoring
- ✓ B.Subnets in at least two Availability Zones with health checks enabled
- C.All instances in one larger subnet
- D.A Network Load Balancer in one subnet
Why B: Distributing EC2 instances across subnets in at least two Availability Zones ensures that if one AZ fails, the Auto Scaling group can maintain capacity using instances in the remaining AZ(s). Enabling health checks allows the group to detect and replace unhealthy instances, which is essential for fault tolerance. This configuration meets the requirement to tolerate the failure of one Availability Zone.
Variation 6. A trading dashboard runs on EC2 instances behind an Application Load Balancer. The design must tolerate the failure of one Availability Zone. What should the Auto Scaling group configuration include? The team wants the control to be enforceable during normal operations.
medium- A.A single EC2 instance with detailed monitoring
- ✓ B.Subnets in at least two Availability Zones with health checks enabled
- C.All instances in one larger subnet
- D.A Network Load Balancer in one subnet
Why B: Distributing EC2 instances across at least two Availability Zones (AZs) ensures that if one AZ fails, the Auto Scaling group can maintain capacity in the remaining AZ(s). Enabling health checks allows the group to detect instance failures and automatically replace them, providing fault tolerance. This configuration meets the requirement to tolerate a single AZ failure while remaining enforceable during normal operations.
Variation 7. A team wants a web application to keep serving traffic if one Availability Zone fails. Match each architecture element to the resilience behavior it provides.
medium- ✓ A.Application Load Balancer (ALB): Distributes incoming traffic across healthy targets in multiple Availability Zones.
- ✓ B.Auto Scaling group: Maintains desired instance count across multiple Availability Zones by replacing failed instances.
- ✓ C.Multi-AZ RDS: Synchronously replicates data to a standby instance in a different Availability Zone for automatic failover.
- ✓ D.Amazon Route 53: DNS routing with health checks to redirect traffic away from unhealthy resources in a failed Availability Zone.
- E.Application Load Balancer (ALB): Automatically replaces failed instances across Availability Zones.
- F.Auto Scaling group: Distributes incoming traffic across healthy instances in multiple Availability Zones.
Why A: These pairs match architecture elements with their resilience behaviors for surviving an Availability Zone failure, focusing on AWS services that provide high availability and fault tolerance.
Variation 8. A team wants a web application to keep serving traffic if one Availability Zone fails. Match each architecture element to the resilience behavior it provides.
medium- ✓ A.Elastic Load Balancer: Distributes incoming traffic across multiple targets in multiple Availability Zones
- ✓ B.Auto Scaling Group: Automatically replaces unhealthy instances and maintains desired capacity across Availability Zones
- ✓ C.Amazon RDS Multi-AZ: Provides automatic failover to a standby database instance in a different Availability Zone
- ✓ D.Amazon Route 53: Routes end users to healthy endpoints using DNS health checks and failover
- E.Amazon S3: Automatically fails over to a secondary region in case of an Availability Zone failure
- F.AWS CloudFormation: Distributes application load across multiple Availability Zones
Why A: These pairs match architecture elements with their resilience behaviors for surviving an Availability Zone failure, focusing on AWS services that provide high availability and fault tolerance.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This SAA-C03 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SAA-C03 exam.