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Question 598 of 1,040

Design Resilient Architectures →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is to increase the ASG min size to at least 2 and ensure it uses subnets in at least two Availability Zones. This is correct because the current architecture has a single point of failure: the ASG only spans one subnet, meaning a single-AZ failure would wipe out all EC2 capacity, leaving the ECS service with zero tasks despite the ALB spanning two zones. The core concept is that ECS service high availability across availability zones requires both the compute layer (ASG/EC2) and the load balancer to be multi-AZ; the ALB alone cannot keep the service running if all instances are lost. On the SAA-C03 exam, this tests your understanding of the shared responsibility model for high availability, and a common trap is assuming the ALB’s multi-AZ setup is sufficient. Remember the memory tip: “ALB spans AZs, but tasks need EC2 in those AZs—match your ASG subnets to your ALB subnets for true resilience.”

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: aSGs can distribute EC2 instances across multiple Availability Zones.. 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.

An ECS service runs on EC2 instances and is fronted by an ALB. The ALB spans two Availability Zones, and the ECS service desired count is 2 tasks. The underlying EC2 capacity uses an Auto Scaling group (ASG) with min size set to 1, and the ASG also spans only one subnet in practice. What is the most effective change to meet the requirement that the service continues during a single-AZ instance loss?

Question 1mediummultiple choice

Review the full subnetting walkthrough →

A
Set the ECS deployment configuration to maximum percent 100 so tasks replace instances faster during rollouts.
Why wrong: Deployment timing can help during releases but does not ensure capacity exists in another AZ during failures.
B
Increase ASG min size to at least 2 and ensure the ASG uses subnets in at least two Availability Zones.
Multi-AZ instance capacity ensures tasks have eligible compute in another AZ when one AZ loses instances.
C
Enable ALB connection draining longer than expected so existing connections survive longer during an AZ event.
Why wrong: Connection draining affects in-flight sessions but does not restore capacity for new requests after AZ loss.
D
Reduce task memory reservations to pack both tasks onto a single EC2 instance.
Why wrong: Packing tasks onto fewer instances increases blast radius; it does not improve multi-AZ resilience.

Full breakdown with real-world context →

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

✓

Increase ASG min size to at least 2 and ensure the ASG uses subnets in at least two Availability Zones.

Option B is correct because the current architecture has a single point of failure: the ASG spans only one subnet (one AZ), so if that AZ fails, all EC2 instances are lost, and the ECS service cannot run any tasks. By increasing the ASG min size to at least 2 and ensuring it uses subnets in at least two AZs, the ASG will maintain at least one healthy instance in each AZ, allowing the ECS service to survive a single-AZ outage. This aligns with the AWS Well-Architected Framework's principle of deploying across multiple AZs for high availability.

Key principle: ASGs can distribute EC2 instances across multiple Availability Zones.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
Set the ECS deployment configuration to maximum percent 100 so tasks replace instances faster during rollouts.
Why it's wrong here
Deployment timing can help during releases but does not ensure capacity exists in another AZ during failures.
✓
Increase ASG min size to at least 2 and ensure the ASG uses subnets in at least two Availability Zones.
Why this is correct
Multi-AZ instance capacity ensures tasks have eligible compute in another AZ when one AZ loses instances.
Related concept
ASGs can distribute EC2 instances across multiple Availability Zones.
✗
Enable ALB connection draining longer than expected so existing connections survive longer during an AZ event.
Why it's wrong here
Connection draining affects in-flight sessions but does not restore capacity for new requests after AZ loss.
✗
Reduce task memory reservations to pack both tasks onto a single EC2 instance.
Why it's wrong here
Packing tasks onto fewer instances increases blast radius; it does not improve multi-AZ resilience.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates may focus on the ALB's multi-AZ configuration and overlook that the compute layer (ASG/EC2) is the actual bottleneck, leading them to choose connection draining or deployment settings that do not address the fundamental lack of cross-AZ capacity.

Detailed technical explanation

How to think about this question

Under the hood, the ECS scheduler places tasks based on the EC2 instances available in the ASG; if the ASG only provisions instances in one AZ, the ECS service is effectively single-AZ regardless of the ALB's multi-AZ configuration. The ALB's target group health checks will mark all targets as unhealthy if the AZ fails, but without instances in another AZ, there are no healthy targets to route traffic to. In a real-world scenario, this misconfiguration is common when developers assume the ALB's multi-AZ setup alone provides resilience, but the compute layer must also be distributed across AZs.

KKey Concepts to Remember

ASGs can distribute EC2 instances across multiple Availability Zones.
Multi-AZ ASGs enhance fault tolerance for ECS services on EC2.
ECS tasks require available EC2 capacity in a healthy AZ to be scheduled.
An ASG's subnet configuration dictates the AZs where instances are launched.

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

ASGs can distribute EC2 instances across multiple Availability Zones.

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 aSGs can distribute EC2 instances across multiple Availability Zones., then practise related SAA-C03 questions on the same topic to reinforce the concept.

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FAQ

Questions learners often ask

What does this SAA-C03 question test?

Design Resilient Architectures — This question tests Design Resilient Architectures — ASGs can distribute EC2 instances across multiple Availability Zones..

What is the correct answer to this question?

The correct answer is: Increase ASG min size to at least 2 and ensure the ASG uses subnets in at least two Availability Zones. — Option B is correct because the current architecture has a single point of failure: the ASG spans only one subnet (one AZ), so if that AZ fails, all EC2 instances are lost, and the ECS service cannot run any tasks. By increasing the ASG min size to at least 2 and ensuring it uses subnets in at least two AZs, the ASG will maintain at least one healthy instance in each AZ, allowing the ECS service to survive a single-AZ outage. This aligns with the AWS Well-Architected Framework's principle of deploying across multiple AZs for high availability.

What should I do if I get this SAA-C03 question wrong?

Review aSGs can distribute EC2 instances across multiple Availability Zones., then practise related SAA-C03 questions on the same topic to reinforce the concept.

What is the key concept behind this question?

ASGs can distribute EC2 instances across multiple Availability Zones.

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Same concept, more angles

1 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. An ECS service runs on EC2 instances and is fronted by an ALB. The ALB spans two Availability Zones, and the ECS service desired count is 2 tasks. The underlying EC2 capacity uses an Auto Scaling group (ASG) with min size set to 1, and the ASG also spans only one subnet in practice. What is the most effective change to meet the requirement that the service continues during a single-AZ instance loss?

medium

A.Set the ECS deployment configuration to maximum percent 100 so tasks replace instances faster during rollouts.
✓ B.Increase ASG min size to at least 2 and ensure the ASG uses subnets in at least two Availability Zones.
C.Enable ALB connection draining longer than expected so existing connections survive longer during an AZ event.
D.Reduce task memory reservations to pack both tasks onto a single EC2 instance.

Why B: The current architecture has a single point of failure because the Auto Scaling group (ASG) spans only one subnet (one Availability Zone). If that AZ fails, all EC2 instances are lost, and the ECS service cannot run any tasks. Increasing the ASG min size to at least 2 and configuring it to use subnets in at least two AZs ensures that EC2 instances are distributed across AZs, allowing the ECS service to maintain at least one task in the surviving AZ during a single-AZ failure.

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