The answer is a slow database query or external API call, because when the ALB’s TargetResponseTime metric shows high p99 values but the EC2 instances have low CPU and memory, the bottleneck is clearly not compute capacity—it’s an I/O-bound dependency. The application is spending time waiting on a downstream service, such as a database or an external API, which increases response time without consuming significant local resources. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your ability to distinguish between compute-bound and dependency-bound latency; a common trap is to assume high response time always means the application server is overloaded. Remember, low CPU with high latency points to a slow dependency, not a scaling issue. A useful memory tip: “Low CPU, high wait—check the database gate.”
DOP-C02 Monitoring and Logging Practice Question
This DOP-C02 practice question tests your understanding of monitoring and logging. 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 DevOps engineer is troubleshooting a slow web application. The application runs on EC2 instances behind an ALB. The engineer notices that the ALB's TargetResponseTime metric shows high p99 values, but the CPU and memory on the EC2 instances are well below thresholds. 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 is waiting on a slow database query or external API call
High p99 TargetResponseTime on the ALB with low CPU and memory on the EC2 instances indicates that the bottleneck is not compute capacity but rather a dependency external to the application server. The application is likely waiting on a slow database query or external API call, which increases response time without consuming significant local CPU or memory. This is a classic symptom of an I/O-bound or network-bound dependency.
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 Auto Scaling group has too many instances, causing increased network overhead
Why it's wrong here
Incorrect. More instances typically reduce latency.
✗
The ALB is routing requests to instances in different Availability Zones, increasing latency
Why it's wrong here
Incorrect. ALB cross-zone routing is optimized.
✓
The application is waiting on a slow database query or external API call
Why this is correct
Correct. Application latency often comes from external dependencies.
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 idle timeout is set too low, causing connections to be dropped
Why it's wrong here
Incorrect. Low idle timeout would cause errors, not high latency.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume high response times must be caused by compute saturation (CPU/memory) or network issues, but the question deliberately shows low resource utilization to force you to consider external dependencies as the root cause.
Detailed technical explanation
How to think about this question
The ALB TargetResponseTime metric measures the time from when the ALB sends a request to a target to when it receives the complete response. When an application thread blocks on a slow database query (e.g., unindexed query, lock contention) or an external API call (e.g., third-party service latency), the thread remains occupied, increasing response time without raising CPU or memory usage. This pattern is often diagnosed by enabling ALB access logs to identify slow requests or by using distributed tracing (e.g., AWS X-Ray) to pinpoint the downstream dependency.
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 application is waiting on a slow database query or external API call — High p99 TargetResponseTime on the ALB with low CPU and memory on the EC2 instances indicates that the bottleneck is not compute capacity but rather a dependency external to the application server. The application is likely waiting on a slow database query or external API call, which increases response time without consuming significant local CPU or memory. This is a classic symptom of an I/O-bound or network-bound dependency.
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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Question Discussion
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