Question 1

What is an AWS CloudWatch alarm?

Accepted Answer

A CloudWatch alarm watches a single CloudWatch metric over a specified time period and performs one or more actions based on the metric value relative to a threshold. Actions can include sending an SNS notification, triggering an Auto Scaling policy, or stopping/rebooting an EC2 instance. Alarms have three states: OK, ALARM, and INSUFFICIENT_DATA.

Question 2

What is the difference between evaluation periods and the period?

Accepted Answer

The 'period' is the granularity at which the metric is evaluated (e.g., 300 seconds = 5 minutes). 'Evaluation periods' is how many consecutive periods must breach the threshold before the alarm fires. For example: period=300 and evaluationPeriods=3 means the metric must breach the threshold for 15 consecutive minutes before the alarm triggers. Higher evaluation periods reduce false positives but increase alert latency.

Question 3

What does 'treat missing data' mean in CloudWatch alarms?

Accepted Answer

When CloudWatch cannot find data for a metric during an evaluation period, the 'treat missing data' setting determines behavior: 'missing' leaves the alarm state unchanged (default), 'breaching' treats the missing period as if it breached the threshold, 'notBreaching' treats it as within threshold, and 'ignore' excludes that period from evaluation. Use 'breaching' for metrics that should always report (e.g., heartbeat checks) and 'notBreaching' for optional or sparse metrics.

Question 4

What are CloudWatch metric dimensions?

Accepted Answer

Dimensions are key-value pairs that uniquely identify a CloudWatch metric. For EC2, the InstanceId dimension specifies which instance to monitor. For RDS, it's DBInstanceIdentifier. Lambda uses FunctionName. Dimensions filter a metric to a specific resource — without them, you'd get aggregated metrics across all resources of that type. You can add up to 30 dimensions per metric.

Question 5

How do I use the CloudFormation YAML output?

Accepted Answer

Copy the generated CloudFormation YAML into a .yaml file and deploy it using: aws cloudformation deploy --template-file alarm.yaml --stack-name my-alarms. Or add the AWS::CloudWatch::Alarm resource block into an existing CloudFormation stack. The AlarmActions, OKActions, and InsufficientDataActions fields accept SNS topic ARNs — replace the placeholder ARN with your actual SNS topic ARN for notifications.

AWS CloudWatch Alarm Builder

Quick Presets

Alarm Configuration

Dimensions

Alarm Summary

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