Alarms are useful for monitoring metrics and setting up threshold to take action.
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Service or application publishes metrics data at a specific frequency; usually every 1 minute or 5 minutes.
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This metrics data is typically summarized over a time window aka Time Period to spot a trend. Each summarized data is a data point. Summarization could be average, count of errors, 90th percentile and so on. Time period could be 5 minutes or 15 minutes and so on.
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Using Alarms, data points are monitored over several time periods (known as alarm evaluation interval) to detect persistent issues and to rule out temporary issues that resolve automatically (for example a temporary spike in load observed during one time period).
You can configure alarms in two ways:A. Consecutive data points need to stay above threshold. If alarm evaluation interval is 3 time periods, then 3 consecutive data points need to be above the threshold
B. M out of N consecutive data points need to stay above threshold. If M=3 and N=3, then it is same as (A). If M=2 and N=3, then alarm will alarm will trigger even if only 2 data points are above the threshold out of three consecutive data points.
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