This CV0-004 practice question tests your understanding of troubleshooting. 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 e-commerce application on AWS. The architecture includes an Application Load Balancer (ALB) in front of an Auto Scaling group of EC2 instances across two Availability Zones. The instances are in a private subnet and use a NAT Gateway for outbound internet access. The application stores session data in an ElastiCache Redis cluster. During a flash sale, users report that the site is extremely slow and some requests time out. Monitoring shows the ALB's latency metric is high, and the number of healthy hosts fluctuates. The CPU utilization on the EC2 instances averages 60% and memory averages 70%. The Redis cluster's CPU utilization is 90%, and its memory usage is 95%. The NAT Gateway's metrics show high BytesOutToSource but no errors. Which of the following is the most likely cause of the performance issue?
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 ElastiCache Redis cluster is overloaded and becoming a bottleneck for session lookups
The ElastiCache Redis cluster is the most likely bottleneck because its CPU utilization is at 90% and memory usage at 95%, indicating it is near capacity. Since the application stores session data in Redis, high latency and timeouts during a flash sale are consistent with an overloaded session store that cannot keep up with request volume, causing the ALB to experience increased latency and healthy host fluctuations as sessions fail to be retrieved or written.
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 NAT Gateway is throttling traffic due to bandwidth limits
Why it's wrong here
Incorrect; no errors and BytesOutToSource is high, indicating no throttling.
✓
The ElastiCache Redis cluster is overloaded and becoming a bottleneck for session lookups
Why this is correct
Correct; high Redis CPU and memory cause slow responses and timeouts.
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 Auto Scaling group is not scaling quickly enough due to cooldown periods
Why it's wrong here
Incorrect; CPU/memory are moderate, so scaling is likely adequate.
✗
The ALB's idle timeout setting is too low, causing premature connection drops
Why it's wrong here
Incorrect; ALB idle timeout would cause disconnections, not high latency.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may focus on the NATGateway or Auto Scaling group because they are common bottlenecks, but the key clue is the Redis cluster's high CPU and memory metrics, which directly correlate with session store performance issues in a stateful application.
Detailed technical explanation
How to think about this question
Redis is single-threaded for command processing, so CPU utilization at 90% indicates it is approaching its request-handling limit, causing queuing delays. Memory usage at 95% can trigger eviction policies (e.g., allkeys-lru) or OOM errors, further degrading performance. In a flash sale, session lookups become a serialized bottleneck, as each request must read/write session data before the application can respond, amplifying latency across all users.
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
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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.
Troubleshooting — This question tests Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ElastiCache Redis cluster is overloaded and becoming a bottleneck for session lookups — The ElastiCache Redis cluster is the most likely bottleneck because its CPU utilization is at 90% and memory usage at 95%, indicating it is near capacity. Since the application stores session data in Redis, high latency and timeouts during a flash sale are consistent with an overloaded session store that cannot keep up with request volume, causing the ALB to experience increased latency and healthy host fluctuations as sessions fail to be retrieved or written.
What should I do if I get this CV0-004 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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