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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:
Alarm StateDescription
OKMetric value is within the defined threshold.
ALARMMetric value breaches the threshold for the required number of evaluation periods.
INSUFFICIENT_DATANot enough datapoints yet to determine the state.

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Core Configuration Settings

CloudWatch evaluates your metrics based on three key parameters:
SettingDefinition
ThresholdThe metric limit that triggers the alarm (e.g., > 70% CPU).
PeriodThe duration for each datapoint aggregation (e.g., 5 minutes).
Evaluation PeriodsNumber of consecutive periods that must breach the threshold.
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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