The correct answer is that the application response time exceeds the ALB idle timeout. When an Application Load Balancer establishes a connection to a backend target, it waits for data within the configured idle timeout window, which defaults to 60 seconds. If the application takes longer than this period to send its response, the ALB closes the connection and returns a 503 error to the client, even though the target instances themselves are healthy and passing health checks. On the AWS Certified Developer Associate DVA-C02 exam, this question tests your understanding of how ALB connection management differs from health check monitoring—a common trap is confusing a healthy target group status with the ability to complete long-running requests. Remember that health checks only verify a simple endpoint response, not the full application processing time. Memory tip: think "503 = Slow Server, Not Sick Server"—the target is healthy, it just needs more time to cook.
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 EC2 instances behind an Application Load Balancer. The application experiences intermittent 503 errors. The health checks are configured correctly and the instances pass health checks consistently. 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 application response time exceeds the ALB idle timeout
Option D is correct because if the application takes longer than the idle timeout to respond, the ALB returns a 503. Option A is wrong because the target group might be deregistering instances that are terminating, but this would show as unhealthy. Option B is wrong because security groups denying traffic would cause connection timeouts, not 503. Option C is wrong because the health check path issue would cause health check failures, not intermittent 503.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 response time exceeds the ALB idle timeout
Why this is correct
ALB idle timeout is 60 seconds by default; if the app takes longer, ALB returns 503.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The health check path is incorrect
Why it's wrong here
This would cause health check failures, not intermittent 503 errors.
✗
The target group is deregistering instances prematurely
Why it's wrong here
This would make instances unhealthy, not cause intermittent 503 errors.
✗
A security group is blocking traffic from the ALB to the instances
Why it's wrong here
This would result in failed health checks and no traffic reaching the instances.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related DVA-C02 NAT questions on configuration and troubleshooting.
Troubleshooting and Optimization — This question tests Troubleshooting and Optimization — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The application response time exceeds the ALB idle timeout — Option D is correct because if the application takes longer than the idle timeout to respond, the ALB returns a 503. Option A is wrong because the target group might be deregistering instances that are terminating, but this would show as unhealthy. Option B is wrong because security groups denying traffic would cause connection timeouts, not 503. Option C is wrong because the health check path issue would cause health check failures, not intermittent 503.
What should I do if I get this DVA-C02 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related DVA-C02 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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