Question 744 of 247
Configuring CloudWatch Alarm for Error Rate Percentage
An application running on EC2 instances behind an Application Load Balancer (ALB) sends custom metrics to CloudWatch. The team wants to set an alarm that triggers when the error rate exceeds 5% over a 5-minute period. The alarm must evaluate the metric every minute. Which configuration is required?
Quick Answer
The answer is a CloudWatch alarm configured with a period of 60 seconds, the Average statistic, 5 evaluation periods, and 5 datapoints to alarm. This configuration is correct because when configuring a CloudWatch alarm for error rate percentage, the period defines how often the metric is evaluated—here, every 60 seconds—while the combination of evaluation periods and datapoints to alarm both set to 5 ensures the alarm only triggers if the error rate exceeds 5% for five consecutive minutes, confirming a sustained breach rather than a transient spike. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of how period, evaluation periods, and datapoints interact to define alarm sensitivity, with a common trap being to confuse evaluation periods with the total time window or to mistakenly use a Sum statistic for a percentage metric. Remember the memory tip: “Five for five, every minute” — five datapoints over five evaluation periods, each one minute long, gives you a true five-minute sustained error rate check.
⚠ Common exam trap
Watch out — candidates often confuse 'period' with the total evaluation window, selecting period = 300 seconds (option A or B) thinking it covers the 5-minute window, but this fails the requirement to evaluate every minute, and they may also incorrectly choose Sum instead of Average for a percentage metric.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Period = 60 seconds, Statistic = Average, Evaluation Periods = 5, Datapoints to Alarm = 5
The alarm must evaluate the error rate every minute (period = 60 seconds) over a 5-minute window. With evaluation periods = 5 and datapoints to alarm = 5, the alarm requires all five 1-minute datapoints to exceed the 5% threshold, ensuring the error rate is sustained for the full 5-minute period. The Average statistic is appropriate because the error rate is a percentage metric that should be averaged over each period.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Period = 300 seconds, Statistic = Average, Evaluation Periods = 1, Datapoints to Alarm = 1
Why it's wrong here
Alarm would only trigger after 5 minutes, not evaluating every minute.
- ✗
Period = 300 seconds, Statistic = Sum, Evaluation Periods = 1, Datapoints to Alarm = 1
Why it's wrong here
Sum is not appropriate for error rate; period of 300 seconds would only alarm once per 5 minutes.
- ✓
Period = 60 seconds, Statistic = Average, Evaluation Periods = 5, Datapoints to Alarm = 5
Why this is correct
This checks that all 5 datapoints exceed 5% over 5 minutes.
- ✗
Period = 60 seconds, Statistic = Sum, Evaluation Periods = 5, Datapoints to Alarm = 5
Why it's wrong here
Sum is not suitable for error rate; it would sum errors and total requests incorrectly.
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Same concept, more angles
1 more way this is tested on SOA-C02
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company runs a critical web application on EC2 instances behind an Application Load Balancer (ALB). The SysOps administrator needs to be notified if the ALB's error rate exceeds 5% for 5 consecutive minutes. Which solution meets this requirement with the least operational overhead?
hard- A.Enable VPC Flow Logs and analyze them with Amazon Athena to detect error rates.
- ✓ B.Use a CloudWatch alarm on the ALB's 'HTTPCode_ELB_5XX_Count' metric with a math expression to calculate error rate.
- C.Enable CloudTrail for the ALB and create a metric filter for 5xx errors.
- D.Use AWS Config rules to monitor the ALB configuration and trigger a notification on changes.
Why B: CloudWatch can directly monitor the ALB's 'HTTPCode_ELB_5XX_Count' metric and combine it with the 'RequestCount' metric using a math expression to calculate the error rate as a percentage. This approach requires no additional logging or external services, and a CloudWatch alarm can be configured to trigger an SNS notification when the error rate exceeds 5% for 5 consecutive minutes, minimizing operational overhead.
Last reviewed: Jun 24, 2026
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