Auto Scaling Group Multi-AZ Configuration and Health Checks
A company runs a critical web application on EC2 instances behind an Application Load Balancer. To improve resilience, they want to automatically replace failed instances. Which AWS service should they use?
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
✓
Auto Scaling group with health checks
Auto Scaling groups with health checks automatically replace unhealthy instances based on ELB health check integration. Option D is correct. Option A (EC2 Instance Recovery) recovers instances on the same host but does not replace them if the host fails. Option B (Systems Manager Automation) requires manual or scheduled automation, not automatic health-based replacement. Option C (CloudFormation Stack update) does not provide auto-replacement; it manages infrastructure updates.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
EC2 Instance Recovery
Why it's wrong here
EC2 Instance Recovery, triggered by a CloudWatch alarm on a status check, restarts an impaired instance on the same underlying host. This fails to address the core requirement because it does not move the workload to a new host if the host itself has persistent issues, nor does it replace the instance if the OS or application becomes unresponsive beyond what StatusCheckFailed_System detects. It also does not automatically maintain desired capacity in a fleet scenario, leaving a single point of failure and no application-level health awareness.
- ✗
AWS Systems Manager Automation
Why it's wrong here
AWS Systems Manager Automation is a runbook-based service used for operational tasks such as patching, configuration changes, or running remediation workflows. To replace an EC2 instance, you would need to explicitly invoke an automation document, and there is no built-in continuous monitoring of instance health or automatic triggering based on failed status checks. It can be integrated with EventBridge for some events, but it still does not provide the self-healing, always-on replacement loop that an Auto Scaling group offers for failed instances.
- ✗
CloudFormation Stack update
Why it's wrong here
A CloudFormation stack update is a declarative action that applies template changes to infrastructure resources, and it will only replace an EC2 instance if the template explicitly modifies properties that force a replacement (e.g., changing the AMI ID or InstanceType). It does not continuously monitor the health of running instances, and it cannot detect or respond to an instance failure unless you manually create a stack update with the necessary resource definitions. Therefore, it is not a mechanism for automatic, health-based replacement of a failed instance.
- ✓
Auto Scaling group with health checks
Why this is correct
An Auto Scaling group with health checks automatically replaces failed EC2 instances by monitoring them using EC2 status checks and optionally Elastic Load Balancing health checks. When an instance is marked unhealthy, the ASC terminates it and launches a new instance in an attempt to maintain the desired capacity, and it does so across multiple Availability Zones to increase resilience. This meets the requirement for automatic replacement based on instance health, and because it is a continuous, managed process, it is the correct solution for keeping critical applications available.
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1 more way this is tested on DOP-C02
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Variation 1. A company runs a stateless web application on EC2 instances behind an Application Load Balancer. To improve resilience, which configuration should be used for the EC2 instances?
easy- A.Use one EC2 instance with a larger instance type
- B.Use a single, large EC2 instance in one Availability Zone
- C.Use multiple EC2 instances in one Availability Zone with health checks disabled
- ✓ D.Use multiple EC2 instances across two or more Availability Zones
Why D: D is correct because deploying multiple EC2 instances across two or more Availability Zones (AZs) ensures high availability and fault tolerance. If one AZ fails, the Application Load Balancer (ALB) automatically routes traffic to healthy instances in other AZs, maintaining service continuity. This aligns with the AWS Well-Architected Framework's resilience best practices for stateless applications.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This DOP-C02 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 DOP-C02 exam.