DVA-C02 Development with AWS Services Practice Question
An application running on Amazon EC2 instances behind an Application Load Balancer (ALB) intermittently returns 503 errors. The ALB health checks are failing for some instances intermittently. The developer checks the instance system logs and finds no application errors. What is the most likely cause of the health check failures?
⚠ Common exam trap
It's easy for candidates to assume health check failures are always due to misconfiguration (security groups, paths, or protocols) and overlook transient resource exhaustion, which is a common cause of intermittent failures in production.
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
✓
The application on the instances is experiencing resource exhaustion (e.g., memory or CPU) which causes it to stop responding to health checks temporarily
Intermittent 503 errors from the ALB combined with intermittent health check failures and no application errors in the system logs strongly point to transient resource exhaustion (CPU or memory) on the EC2 instances. When an instance runs out of memory or CPU, the application process may become unresponsive or be killed by the OS (e.g., OOM killer), causing it to fail health checks temporarily. Once resources are freed (e.g., after a spike subsides), the application resumes responding, which explains the intermittent nature of the failures.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The application on the instances is experiencing resource exhaustion (e.g., memory or CPU) which causes it to stop responding to health checks temporarily
Why this is correct
When an application experiences intermittent resource exhaustion, such as high CPU utilization or memory pressure, it can temporarily become unresponsive to incoming requests, including health checks from an Application Load Balancer (ALB). During these transient spikes, the application might fail to respond within the health check timeout period, causing the ALB to mark the instance as unhealthy. Once resources are freed or the load subsides, the application recovers and starts responding to health checks again, leading to an intermittent pattern of healthy/unhealthy states.
- ✗
The security group for the instances does not allow inbound traffic from the ALB on the health check port
Why it's wrong here
If the security group associated with the EC2 instances does not permit inbound traffic from the Application Load Balancer (ALB) on the configured health check port, all health checks would consistently fail. This is a network-level blockage that prevents any health check probes from reaching the application, resulting in a persistent unhealthy status for all targets. Such a misconfiguration would not manifest as intermittent failures, but rather as a continuous inability for the ALB to establish communication.
- ✗
The health check path is not configured correctly and the default path returns a 404 status
Why it's wrong here
An incorrectly configured health check path, where the specified endpoint consistently returns a 404 Not Found HTTP status code or any other non-200/300 series status, would lead to continuous health check failures. The Application Load Balancer (ALB) would consistently interpret these responses as unhealthy, marking all instances as such. This issue is an application-level configuration problem that would not resolve itself intermittently, ensuring a persistent unhealthy state for the target group.
- ✗
The target group is not configured with the correct protocol
Why it's wrong here
If the target group is configured with an incorrect protocol (e.g., HTTP when the application expects HTTPS, or vice-versa) for its health checks, the Application Load Balancer (ALB) would be unable to establish a successful connection or interpret the application's responses correctly. This fundamental protocol mismatch would cause all health checks to consistently fail, as the ALB and the target instance would be speaking different languages. Such a misconfiguration would result in persistent unhealthy statuses, not intermittent ones.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This DVA-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 DVA-C02 exam.