The answer is the ALB idle timeout set too low, causing connections to close before the application responds. When the idle timeout expires before the backend completes processing, the ALB terminates the connection and returns a 502 Bad Gateway error, even though the ECS tasks are running and underutilized. This explains why health checks fail intermittently—the tasks are slow to respond under peak load, not because they are crashing or scaling incorrectly. On the AWS Certified Developer Associate DVA-C02 exam, this scenario tests your understanding of how ALB timeouts interact with application latency, often appearing as a trap where you might mistakenly blame scaling or security groups. Remember that a 502 with healthy tasks and low CPU usually points to a timeout mismatch, not resource exhaustion. Memory tip: “502 means the ALB got tired of waiting—check your idle timeout before you check your tasks.”
DVA-C02 Troubleshooting and Optimization Practice Question
This DVA-C02 practice question tests your understanding of troubleshooting and optimization. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company runs a critical application on Amazon ECS with Fargate. The application uses an Application Load Balancer (ALB) to distribute traffic. Recently, the company noticed that the ALB returns 502 Bad Gateway errors during peak traffic hours. The developer checks the ECS service metrics and sees that the number of running tasks remains constant, while CPU and memory utilization are below 50%. The ALB target group health checks are failing intermittently for some tasks. What is the MOST likely cause of the 502 errors?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ALB idle timeout is set too low, causing connections to be closed before the application responds.
Option A is correct because if the tasks are overloaded and cannot respond within the idle timeout, the ALB returns 502. The health checks fail because the tasks are slow to respond. Option B is wrong because the tasks are not at capacity and the service does not need to scale. Option C is wrong because the security group is not likely the issue if health checks succeed sometimes. Option D is wrong because the tasks are not failing to start; they are running but slow.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 task definition's startup command is failing, causing the tasks to never become healthy.
Why it's wrong here
Health checks are intermittent, so tasks are starting but may be slow.
✗
The ECS service is not configured with auto scaling to handle the increased traffic.
Why it's wrong here
CPU and memory are below 50%, so scaling is not needed.
✓
The ALB idle timeout is set too low, causing connections to be closed before the application responds.
Why this is correct
A low idle timeout can cause the ALB to close connections prematurely, resulting in 502 errors.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The security group for the ECS tasks is blocking traffic from the ALB.
Why it's wrong here
If blocked, health checks would always fail, not intermittently.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
What to study next
Got this wrong? Here's your next step.
Identify which DVA-C02 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Troubleshooting and Optimization — This question tests Troubleshooting and Optimization — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ALB idle timeout is set too low, causing connections to be closed before the application responds. — Option A is correct because if the tasks are overloaded and cannot respond within the idle timeout, the ALB returns 502. The health checks fail because the tasks are slow to respond. Option B is wrong because the tasks are not at capacity and the service does not need to scale. Option C is wrong because the security group is not likely the issue if health checks succeed sometimes. Option D is wrong because the tasks are not failing to start; they are running but slow.
What should I do if I get this DVA-C02 question wrong?
Identify which DVA-C02 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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