Jeff’s Note #
Unlike generic exam dumps, ADH analyzes this scenario through the lens of a Real-World Site Reliability Engineer (SRE).
For AWS SOA-C02 candidates, the confusion often lies in why EC2 instances behind Application Load Balancers fail health checks despite seemingly correct Auto Scaling group setup. In production, this is about knowing exactly how the ALB health checks interact with instance application protocols and ports. Let’s drill down.
The Certification Drill (Simulated Question) #
Scenario #
FultonTech Solutions runs a web service on Amazon EC2 instances grouped in an Auto Scaling setup. To manage variable user demand, the SRE team configured an Application Load Balancer (ALB) in front of the instances. However, the ALB marks all EC2 instances as unhealthy, causing traffic to fail.
The Requirement: #
Identify the best troubleshooting step the SRE should take to resolve the health check failures and restore normal service.
The Options #
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A) Verify if the Auto Scaling group is configured to span multiple AWS regions.
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B) Verify that the application is running on the protocol and port expected by the load balancer’s health check.
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C) Verify the listener priority configuration in the ALB and adjust if necessary.
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D) Verify the maximum instance count in the Auto Scaling group and adjust it if needed.
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Correct Answer #
B
Quick Insight: The Site Reliability Imperative #
The crux with health check failures is often that the load balancer’s health check path, protocol, or port does not match the application’s actual endpoint settings. Ensuring alignment here prevents false negatives that disrupt traffic flow.
Content Locked: The Expert Analysis #
You’ve identified the answer. But do you know the implementation details that separate a Junior from a Senior?
The Expert’s Analysis #
Correct Answer #
Option B
The Winning Logic #
The health check performed by an ALB verifies the instance’s readiness by sending probes on a specific protocol (HTTP/HTTPS) and port periodically. If the application is not actively listening on the configured protocol/port or the endpoint path returns an error status code, the ALB will mark the instance unhealthy, causing it to be removed from serving traffic.
By verifying and ensuring the application listens on exactly the protocol and port expected by the ALB’s health check—and that the health check path returns an HTTP 200 response—SREs can restore healthy status and traffic flow. This diagnosis is fundamental since the ALB health check is the gatekeeper for instance availability in the target group.
The Trap (Distractor Analysis): #
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Why not A?
Auto Scaling groups do not span multiple regions by design; they operate within a single region. This would not impact health check status at the instance level. -
Why not C?
Listener priority affects routing when multiple rules exist on the ALB’s listener. Misconfigured priorities don’t cause health check failures; they influence request routing behavior. -
Why not D?
The maximum instance count controls scaling capacity but does not affect the health check status. Scaling limits won’t cause instances to be marked unhealthy.
The Technical Blueprint #
# Example CLI command to describe target health status in the target group
aws elbv2 describe-target-health --target-group-arn <target-group-arn>
# Use the output to verify health check failures and the specific error codes returned by instances
The Comparative Analysis #
| Option | Operational Overhead | Automation Level | Impact on Health Checks |
|---|---|---|---|
| A | High (Cross-region config) | Low | None – ASG regional scope applies |
| B | Medium (App config check) | Medium | Directly resolves health check issues |
| C | Low | Low | Affects routing, not health checks |
| D | Low | Medium | Controls scale, no health status impact |
Real-World Application (Practitioner Insight) #
Exam Rule #
For the exam, always verify protocol and port alignment when encountering ALB health check failures.
Real World #
In practice, you might also enable detailed ALB access logs and use curl commands from within the VPC to verify the health check endpoint response manually before adjusting Auto Scaling group parameters.
(CTA) Stop Guessing, Start Mastering #
Disclaimer
This is a study note based on simulated scenarios for the AWS SOA-C02 exam.