CloudWatch Alarms and Actions

Metrics alone only give you visibility. Alarms add the crucial ability to detect when something goes wrong and respond automatically. A CloudWatch Alarm watches a single metric (or a metric math expression) over a period of time and changes state when the value crosses a threshold you define.

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Alarm States

Every CloudWatch Alarm is always in exactly one of three states:

State	Meaning
OK	The metric is within the acceptable range
ALARM	The metric has breached the threshold
INSUFFICIENT_DATA	Not enough data points to evaluate the alarm

When you first create an alarm, it starts in INSUFFICIENT_DATA until enough data points arrive to make a determination.

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Creating an Alarm

An alarm is defined by five key components:

1. Metric — the metric name, namespace, and dimensions to watch
2. Statistic — how to aggregate data points (Average, Sum, Maximum, p99, etc.)
3. Period — the evaluation window in seconds (e.g. 300 = 5 minutes)
4. Evaluation periods — how many consecutive periods must breach the threshold
5. Threshold — the value and comparison operator (GreaterThanThreshold, LessThanOrEqualToThreshold, etc.)