DOP-C02 Monitoring and Logging Practice Question
A DevOps engineer needs to monitor the health of a web application running on EC2 instances behind an Application Load Balancer (ALB). Which TWO metrics from ALB should be monitored to detect application errors? (Choose TWO.)
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
✓
HTTPCode_Target_5XX_Count.
(HTTPCode_Target_5XX_Count) is correct because it counts HTTP 5XX errors returned by the target (EC2 instances), indicating application errors. Option E (TargetResponseTime) is correct because elevated response times can indicate application performance issues or errors, and is a key metric for detecting application problems. Option A (RequestCount) is incorrect because it simply counts total requests, not errors. Option B (HTTPCode_ELB_5XX_Count) is incorrect because it counts 5XX errors generated by the ALB itself (e.g., due to configuration issues), not application errors. Option D (HealthyHostCount) is incorrect because it indicates the number of healthy targets, not application-level errors.
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 is a raw traffic metric that measures the total number of requests routed through the Application Load Balancer, regardless of HTTP status code. A spike or drop in request count does not directly indicate application errors; it only reflects user demand and traffic patterns. For health monitoring, you need a metric that correlates with failures, such as 5xx error counts or latency, not just raw volume.
- ✗
HTTPCode_ELB_5XX_Count.
Why it's wrong here
HTTPCode_ELB_5XX_Count captures HTTP 5xx responses generated by the load balancer itself, not by your application targets. These errors typically arise from load balancer misconfiguration, no registered healthy targets, or internal AWS infrastructure issues. Because it measures the load balancer's health rather than the application's, it will not reveal application-level bugs or dependency failures producing 5xx responses inside your targets.
- ✓
HTTPCode_Target_5XX_Count.
Why this is correct
HTTPCode_Target_5XX_Count is the definitive metric for monitoring application health because it directly counts HTTP 5xx responses returned by the registered targets (EC2 instances, containers, or Lambda). This value reflects actual application failures—unhandled exceptions, internal server errors, or gateway timeouts—making it the most precise indicator of unhealthy application logic. It is the recommended metric to alarm on for detecting when your web application is returning server-side errors to users.
- ✗
HealthyHostCount.
Why it's wrong here
HealthyHostCount reports the number of targets that pass health checks in a target group, providing an availability view at the instance or container level. However, a host can be marked healthy even while it returns 5xx errors on a specific endpoint, because health checks often use a limited path like /health. This metric therefore lacks the sensitivity to application errors; it only tells you if targets are reachable and passing the health check criteria, not whether the application is functioning correctly for all requests.
- ✓
TargetResponseTime.
Why this is correct
TargetResponseTime measures the elapsed time between the load balancer sending a request to a target and receiving the response. While not an explicit error count, a persistently high or increasing response time often indicates application degradation—such as database contention, memory exhaustion, or blocking operations—that may precede or accompany 5xx errors. It is a proactive health indicator because it catches performance issues before they manifest as user-visible errors, complementing direct 5xx counts.
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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.