DOP-C02 Incident and Event Response Practice Question
A company runs a web application on Amazon EC2 instances behind an Application Load Balancer (ALB). The application experiences intermittent 503 errors. The engineer suspects the ALB is returning these errors because the target instances are unhealthy. Which metric should the engineer monitor to confirm this suspicion?
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
✓
HealthyHostCount
The ALB publishes 'HealthyHostCount' metric showing the number of healthy targets. When this count drops to zero, the ALB cannot forward requests and returns 503 errors. Option A (RequestCount) is incorrect because it measures total requests, not health. Option B (UnhealthyHostCount) is a valid metric but does not directly confirm the suspicion that targets are unhealthy; a decreasing HealthyHostCount is more direct. Option D (TargetResponseTime) measures latency, not health status.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
RequestCount
Why it's wrong here
RequestCount measures the number of requests processed by the target group, but it is a traffic volume metric, not a health state indicator. If all EC2 instances fail health checks, the load balancer stops routing traffic to them, and RequestCount may fall to zero or remain non-zero only for 503 responses generated by the ALB itself. It does not expose the outcome of health check probes, so it cannot directly reveal that the target group has zero healthy instances.
- ✗
UnhealthyHostCount
Why it's wrong here
UnhealthyHostCount is a CloudWatch metric in the AWS/ApplicationELB namespace that tracks the number of registered targets that are failing health checks. When every instance is unhealthy, this metric equals the total number of registered targets, not zero, so it does not signal the zero-healthy condition you are checking for. It is a complementary metric to HealthyHostCount but used to detect the opposite condition, making it an incorrect choice here.
- ✓
HealthyHostCount
Why this is correct
HealthyHostCount directly reflects the number of targets that are passing the configured health checks. For a target group with EC2 instances, it is the authoritative CloudWatch metric for monitoring target health; when all instances fail, this value drops to zero, leaving the ALB with no registered and healthy targets to serve traffic, which causes it to return HTTP 503 Service Unavailable. Therefore, it is the correct metric to identify the described incident.
- ✗
TargetResponseTime
Why it's wrong here
TargetResponseTime records the time taken for a target to send the response headers, but it does not convey whether the target passed its health check. A high response time might indicate performance degradation, yet unhealthy targets are removed from rotation and typically do not receive user requests, so this metric can appear normal or nonexistent even when all hosts are failing probes. It is a performance metric, not a health state indicator, so it cannot be used to confirm zero healthy instances.
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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.