Explanation: You can create a CloudWatch alarm that watches a single metric. The alarm performs one or more actions based on the value of the metric relative to a threshold over a number of time periods. The action can be an Amazon EC2 action, an Auto Scaling action, or a notification sent to an Amazon SNS topic. An alarm has three possible states: OK–The metric is within the defined threshold ALARM–The metric is outside of the defined threshold INSUFFICIENT_DATA–The alarm has just started, the metric is not available, or not enough data is available for the metric to determine the alarm state Reference:
http://docs.aws.amazon.com/AmazonCloudWatch/latest/DeveloperGuide/A…