The correct answer involves creating a CloudWatch alarm that uses a math expression to calculate the error rate as error count divided by total request count, comparing it to the 5% threshold. This works because a CloudWatch metric filter on the log group extracts the raw error count and total request count as separate custom metrics, and the alarm’s math expression dynamically computes the ratio, enabling precise threshold monitoring. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding that metric filters produce individual metrics, not calculated rates—you must combine them in an alarm expression. A common trap is assuming CloudWatch Logs can directly create an error rate metric, but it cannot; the filter only captures counts. Memory tip: think “filter for raw counts, alarm for math”—the filter gives you the numbers, the alarm does the division.
DOP-C02 Resilient Cloud Solutions Practice Question
This DOP-C02 practice question tests your understanding of resilient cloud solutions. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 Amazon EC2 instances in an Auto Scaling group. The application generates logs that are sent to Amazon CloudWatch Logs. The DevOps team needs to configure a metric filter to monitor for error patterns and trigger an alarm when the error rate exceeds 5% of total requests over a 5-minute period. Which TWO steps should the team take? (Choose TWO.)
Create a CloudWatch Logs log group for the error metric.
Why wrong: Log group already exists.
B
Create a metric filter on the log group to count occurrences of the error pattern.
This creates a custom metric for error count.
C
Create a CloudWatch Logs subscription filter to stream errors to a Lambda function that calculates the error rate.
Why wrong: More complex than needed; metric filter and alarm suffice.
D
Create a CloudWatch metric for the error count.
Why wrong: Metric filter creates the metric automatically.
E
Create a CloudWatch alarm that uses a math expression to calculate the error rate (error count / total request count) and compare it to the threshold of 5%.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create a metric filter on the log group to count occurrences of the error pattern.
Options A and D are correct. A metric filter on the log group creates a custom metric for the error count. The alarm then uses that metric and the math expression to calculate the error rate. Option B is wrong because creating a log group is not the step; the filter is on an existing log group. Option C is wrong because you create a metric filter, not a metric. Option E is wrong because CloudWatch Logs does not directly create a metric for error rate; you need a filter and alarm.
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.
✗
Create a CloudWatch Logs log group for the error metric.
Why it's wrong here
Log group already exists.
✓
Create a metric filter on the log group to count occurrences of the error pattern.
Create a CloudWatch alarm that uses a math expression to calculate the error rate (error count / total request count) and compare it to the threshold of 5%.
Why this is correct
Math expressions allow calculation of rates.
Related concept
Static NAT maps one inside address to one outside address.
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 cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
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.
Resilient Cloud Solutions — This question tests Resilient Cloud Solutions — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Create a metric filter on the log group to count occurrences of the error pattern. — Options A and D are correct. A metric filter on the log group creates a custom metric for the error count. The alarm then uses that metric and the math expression to calculate the error rate. Option B is wrong because creating a log group is not the step; the filter is on an existing log group. Option C is wrong because you create a metric filter, not a metric. Option E is wrong because CloudWatch Logs does not directly create a metric for error rate; you need a filter and alarm.
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.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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