The answer is that the ALB is rejecting requests with 503 errors because it is scaling up to handle a sudden traffic surge, and during that scaling activity, it lacks sufficient healthy targets to serve the load. This occurs because an Application Load Balancer has a limited capacity per node, and when traffic spikes exceed that capacity, the ALB must provision additional nodes—a process that takes time. While scaling, the target group may have too few instances passing health checks, causing the ALB to return 503 errors intermittently until new EC2 instances are fully registered and healthy. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of ALB scaling behavior and the interplay between CloudFront and ALB logs for root cause analysis. A common trap is to blame CloudFront or the target group’s application health, but the intermittent nature points directly to ALB scaling latency. Memory tip: think “503 = Scaling Out, Still Short” to recall that transient 503s during traffic bursts often indicate the ALB is still catching up.
DOP-C02 Monitoring and Logging Practice Question
This DOP-C02 practice question tests your understanding of monitoring and logging. 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 company runs a web application on EC2 instances behind an Application Load Balancer. They use Amazon CloudFront for content delivery. The DevOps team notices that some requests are returning HTTP 503 errors intermittently. After checking the CloudFront and ALB logs, they find that the errors originate from the ALB. 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 ALB is experiencing a surge in traffic and is scaling up, but during the scaling activity, some requests are rejected.
Option D is correct because when an Application Load Balancer (ALB) experiences a sudden surge in traffic that exceeds its current capacity, it may temporarily reject requests with HTTP 503 errors while it scales up. During the scaling activity, the ALB's target group might not have enough healthy registered targets to handle the load, causing the ALB to return 503 responses until new instances are provisioned and pass health checks. This matches the intermittent nature of the errors described in the scenario.
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 SSL certificate on the ALB is expired.
Why it's wrong here
Expired SSL certificate would result in 502 Bad Gateway errors, not 503.
✗
The security group for the ALB is blocking traffic from CloudFront.
Why it's wrong here
If the security group blocked traffic, the ALB would not receive requests, leading to 504 errors from CloudFront.
✗
CloudFront is configured to forward an HTTP method that the ALB does not support.
Why it's wrong here
Unsupported methods would return 405 Method Not Allowed, not 503.
✓
The ALB is experiencing a surge in traffic and is scaling up, but during the scaling activity, some requests are rejected.
Why this is correct
ALB returns 503 when it cannot handle the request load, often during scaling.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse 503 errors with SSL certificate issues or security group misconfigurations, but the intermittent nature of the errors and the fact that they originate from the ALB (not CloudFront) points directly to capacity scaling limitations rather than configuration errors.
Detailed technical explanation
How to think about this question
Under the hood, an ALB has a 'surge queue length' metric that tracks the number of pending requests waiting to be routed to targets; when this queue exceeds the configured limit (default 1,024 per node), new requests are rejected with 503 errors. The ALB also has a 'spillover' mechanism where requests are dropped if no healthy targets are available, which is common during rapid scaling events when new EC2 instances are still initializing and failing health checks. In real-world scenarios, this often occurs during flash sales or DDoS-like traffic spikes where Auto Scaling group launch times (e.g., 2-5 minutes for instance initialization) exceed the ALB's ability to buffer incoming requests.
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.
TExam Day Tips
→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 exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Monitoring and Logging — This question tests Monitoring and Logging — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ALB is experiencing a surge in traffic and is scaling up, but during the scaling activity, some requests are rejected. — Option D is correct because when an Application Load Balancer (ALB) experiences a sudden surge in traffic that exceeds its current capacity, it may temporarily reject requests with HTTP 503 errors while it scales up. During the scaling activity, the ALB's target group might not have enough healthy registered targets to handle the load, causing the ALB to return 503 responses until new instances are provisioned and pass health checks. This matches the intermittent nature of the errors described in the scenario.
What should I do if I get this DOP-C02 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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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