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AWS CloudWatch alarms continuously monitor metrics and compare them against your defined thresholds. An alarm can reside in one of three states:

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Core Configuration Settings

CloudWatch evaluates your metrics based on three key parameters:
When an alarm has fewer datapoints than the specified evaluation periods (for example, at startup or during missing metrics), it transitions to INSUFFICIENT_DATA until it can fully evaluate the threshold.

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How States Transition

Imagine you set up an alarm on an RDS instance to fire when CPU utilization exceeds 70%, with:
  • Period = 5 minutes
  • Evaluation Periods = 3
  1. Period 1: CPU spikes to 80% → only one breach datapoint → state moves to INSUFFICIENT_DATA
  2. Period 2: CPU remains above 70% → second breach datapoint → still INSUFFICIENT_DATA
  3. Period 3: CPU stays above 70% → third breach datapoint → state transitions to ALARM
If the CPU then drops below 70% for three consecutive periods, the alarm returns to OK.
The image illustrates alarm states with a graph showing "OK State" and "ALARM State" based on metric thresholds, accompanied by explanations for each state.

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References

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