The answer is a short keep-alive timeout on the EC2 instances. When the Application Load Balancer sends a request to a target, it expects the TCP connection to remain open long enough to complete the HTTP transaction; if the EC2 instance closes the connection prematurely due to an aggressive keep-alive timeout, the ALB cannot finish writing the request or reading the response, resulting in a 502 error in the access logs. On the AWS Certified Developer Associate DVA-C02 exam, this scenario tests your understanding of ALB-to-target connection lifecycle and how misconfigured timeouts on the backend can mimic application failures—a common trap is confusing 502 errors with security group blocks (which cause 503s) or health check failures. Remember the mnemonic: “502 means the target quit too soon; keep your keep-alive in tune.”
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 production web application on EC2 instances behind an Application Load Balancer. Users report intermittent 502 errors. The developers find that the ALB access logs show 'target_response_code' of 502 for some requests. What is the MOST likely cause?
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 EC2 instances are closing idle connections prematurely due to a short keep-alive timeout.
Option B is correct because a 502 from ALB indicates the target (EC2) closed the connection before the ALB could finish writing the request or reading the response. Option A is wrong because security groups blocking traffic would result in 503 or timeout. Option C is wrong because ALB health checks failing would cause 503. Option D is wrong because DNS resolution is not involved in ALB-to-target communication.
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 EC2 instances are unable to resolve DNS for the ALB.
Why it's wrong here
EC2 instances do not need to resolve the ALB's DNS; the ALB initiates connections to the instances.
✗
The security group for the EC2 instances is blocking traffic from the ALB.
Why it's wrong here
If blocked, the ALB would return a 503 error, not 502.
✓
The EC2 instances are closing idle connections prematurely due to a short keep-alive timeout.
Why this is correct
If the EC2 instance closes the connection before the ALB finishes sending the request, the ALB returns a 502.
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 ALB health checks are failing and the target group has unhealthy instances.
Why it's wrong here
Unhealthy instances would not receive traffic, and the ALB would return 503.
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
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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 EC2 instances are closing idle connections prematurely due to a short keep-alive timeout. — Option B is correct because a 502 from ALB indicates the target (EC2) closed the connection before the ALB could finish writing the request or reading the response. Option A is wrong because security groups blocking traffic would result in 503 or timeout. Option C is wrong because ALB health checks failing would cause 503. Option D is wrong because DNS resolution is not involved in ALB-to-target communication.
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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