Amazon CloudWatch is an optional service from Amazon EC2 that you can purchase to monitor usage of your EC2 instances.
CloudWatch provides the statistics necessary for Amazon EC2 to perform automatic scaling of a site. Consequently, when you create a site with ArcGIS Server Cloud Builder on Amazon Web Services, CloudWatch is enabled on all instances. Whenever the AWS Auto Scaling API is used to create instances automatically, the instances created have CloudWatch enabled.
CloudWatch produces statistics and graphs describing the loads on your cloud-based services. You can see these graphs in the AWS Management Console when you display the informational details for one or more EC2 instances.
In the AWS Management Console, you can enable CloudWatch when you are completing the wizard to launch a new instance. CloudWatch can also be configured on Amazon Elastic Load Balancers (ELBs), which you would use if you had many EC2 instances working together in one ArcGIS Server site.