CloudWatch Alarm Duration Threshold for CPU
A DevOps engineer needs to set up an alert for when the CPU utilization of an EC2 instance exceeds 90% for 5 consecutive minutes. Which CloudWatch features should be used?
⚠ Common exam trap
It's easy for candidates to confuse CloudWatch Logs metric filters (used for custom log-based metrics) with native EC2 metrics, or mistakenly think CloudTrail or S3 logs can monitor system performance, when only CloudWatch alarms on the `CPUUtilization` metric directly satisfy the requirement.
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
✓
CloudWatch alarm on the CPUUtilization metric with a period of 5 minutes and threshold of 90.
CloudWatch alarms can be configured on the `CPUUtilization` metric (a standard EC2 metric emitted every 5 minutes by default) with a threshold of 90 and an evaluation period of 1 (since the period is set to 5 minutes, one evaluation period covers the 5 consecutive minutes). This directly meets the requirement without additional setup.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
CloudWatch Logs with a metric filter on CPU utilization logs.
Why it's wrong here
CloudWatch Logs are for ingesting and querying log data, such as application output or system logs; CPU utilization is not natively emitted as a log record. Even if you used the Unified CloudWatch Agent to collect CPU metrics, the agent publishes them as metric values to the CloudWatch metrics namespace, not as log events for a metric filter. A metric filter would require log streams containing CPU utilization strings, which you would have to generate yourself, making this unnecessarily complex and not the standard design.
- ✗
CloudTrail to monitor EC2 instance CPU usage.
Why it's wrong here
CloudTrail is an auditing service that records API activity in your AWS account, such as EC2 instance launches, terminations, and other control-plane operations. It does not capture guest-OS or hypervisor-level performance counters like CPU utilization, and it has no mechanism to query or alarm on CPU metrics. To monitor CPU usage you need CloudWatch metrics, not a trail of API calls, so this option is fundamentally mismatched to the requirement.
- ✓
CloudWatch alarm on the CPUUtilization metric with a period of 5 minutes and threshold of 90.
Why this is correct
CloudWatch alarms are the native way to react to metric changes: the CPUUtilization metric for EC2 is published every 5 minutes with basic monitoring, so setting a period of 5 minutes ensures the alarm evaluates the average CPU utilization over each interval. A threshold of 90 means the alarm state becomes ALARM when the CPU utilization statistic exceeds 90%, and you can attach an action such as an SNS notification or Auto Scaling policy. This leverages a built-in metric with a well-established alarm evaluation model, making it the correct and simplest approach.
- ✗
Amazon S3 server access logs to check CPU utilization.
Why it's wrong here
S3 server access logs record object-level requests made to an S3 bucket, such as PUT and GET operations, and are stored as log files in another bucket. They contain no EC2 instance metrics and are completely unrelated to CPU utilization or OS performance. Using S3 access logs to monitor CPU would require parsing irrelevant data and provides no useful signal, so it is not a valid method for this use case.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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