The answer is an overly long deregistration delay setting on the target group. This is correct because when instances are being de-registered—whether during a scale-in event or a rolling deployment—the ALB continues routing requests to them for the duration of the delay. If those instances have already stopped accepting new traffic (e.g., their application process has shut down), the ALB receives no response and returns a 503 error, even though health checks still show the targets as healthy. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of how connection draining interacts with lifecycle hooks and deployment strategies; a common trap is assuming healthy health checks guarantee the instance is ready for traffic. Remember the memory tip: “Deregistration delay is the gap between ‘healthy on paper’ and ‘actually serving traffic’”—if that gap is too long, you get 503s with green lights.
DOP-C02 Incident and Event Response Practice Question
This DOP-C02 practice question tests your understanding of incident and event response. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 DevOps team is debugging a production incident where an Application Load Balancer (ALB) is returning 503 errors for some requests. The target group instances are healthy. 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 deregistration delay setting on the target group is too long
Option D is correct because a deregistration delay setting can cause the ALB to continue sending requests to instances that are being de-registered, leading to 503 errors if the instances are no longer accepting traffic. Option A is incorrect because healthy instances mean the health checks pass. Option B is incorrect because a missing security group rule would cause connection timeouts, not 503. Option C is incorrect because cross-zone load balancing does not cause 503 errors.
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 security group for the ALB does not allow inbound traffic on port 443
Why it's wrong here
This would cause connection timeouts, not 503.
✗
Health checks are misconfigured to use an incorrect path
Why it's wrong here
Healthy instances indicate health checks are passing.
✓
The deregistration delay setting on the target group is too long
Why this is correct
A long deregistration delay can cause the ALB to route requests to instances that are draining, resulting in 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.
✗
Cross-zone load balancing is disabled
Why it's wrong here
Disabling cross-zone load balancing does not cause 503 errors.
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 DOP-C02 NAT questions on configuration and troubleshooting.
Incident and Event Response — This question tests Incident and Event Response — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The deregistration delay setting on the target group is too long — Option D is correct because a deregistration delay setting can cause the ALB to continue sending requests to instances that are being de-registered, leading to 503 errors if the instances are no longer accepting traffic. Option A is incorrect because healthy instances mean the health checks pass. Option B is incorrect because a missing security group rule would cause connection timeouts, not 503. Option C is incorrect because cross-zone load balancing does not cause 503 errors.
What should I do if I get this DOP-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 DOP-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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