Answer: Create a custom metric in CloudWatch and make your instances send data to it using PutMetricData. Then, create an alarm based on this metric

Create a custom metric in CloudWatch and make your instances send data to it using PutMetricData. Then, create an alarm based on this metric - You can create a custom CloudWatch metric for your EC2 Linux instance statistics by creating a script through the AWS Command Line Interface (AWS CLI). Then, you can monitor that metric by pushing it to CloudWatch. You can publish your own metrics to CloudWatch using the AWS CLI or an API. Metrics produced by AWS services are standard resolution by default. When you publish a custom metric, you can define it as either standard resolution or high resolution. When you publish a high-resolution metric, CloudWatch stores it with a resolution of 1 second, and you can read and retrieve it with a period of 1 second, 5 seconds, 10 seconds, 30 seconds, or any multiple of 60 seconds. High-resolution metrics can give you more immediate insight into your application's sub-minute activity. But, every PutMetricData call for a custom metric is charged, so calling PutMetricData more often on a high-resolution metric can lead to higher charges. Incorrect options: Create a custom alarm for your ASG and make your instances trigger the alarm using PutAlarmData API - This solution will not work, your instances must be aware of each other's RAM utilization status, to know when the average RAM would be too high to trigger the alarm. Enable detailed monitoring for EC2 and ASG to get the RAM usage data and create a CloudWatch Alarm on top of it - By enabling detailed monitoring you define the frequency at which the metric data has to be sent to CloudWatch, from 5 minutes to 1-minute frequency window. But, you still need to create and collect the custom metric you wish to track. Migrate your application to AWS Lambda - This option has been added as a distractor. You cannot use Lambda for the given use-case.

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