CloudWatch alarm for Amazon EC2 Service Instance Limits

Question

Is it possible to set a CloudWatch alarm for when we are approaching the limit of EC2 instances currently allowed on our account?

For instance, if limit for EC2 instances is currently 250, when instance number 240 is provisioned, I want an alarm to trigger.

Answer 1

If you have an auto scaling group which launches new instances and you want to control it, you can use GroupInServiceInstances which gives you the number of instances running as part of the ASG. Read more here.

Answer 2

If you're reading this in 2023, this is much easier now. You can create a CloudWatch alarm directly from the Service Quotas console.

From Service Quotas and Amazon CloudWatch alarms:

To create a CloudWatch alarm for a quota

1. Sign in to the AWS Management Console and open the Service Quotas console at https://console.aws.amazon.com/servicequotas/home.
2. In the navigation pane, choose AWS services and then select a service.
3. Select a quota that supports CloudWatch alarms. If you actively use the quota, utilization appears beneath the quota description. The CloudWatch alarms section appears at the bottom of the page.
4. In Amazon CloudWatch alarms, choose Create.
5. For Alarm threshold, choose a threshold.
6. For Alarm name, enter a name for the alarm. This name must be unique within the AWS account.
7. Choose Create.
8. To add a notification to the CloudWatch alarm, see https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/ConsoleAlarms.html in the Amazon CloudWatch User Guide.

Answer 3

Yes, you could do this with a Lambda function, a CloudWatch Metric and a CloudWatch alarm.

Your alarm would be configured to alarm on the metric, if it exceeds some threshold (the threshold being your instance limit).

Your Lambda function, would run on a schedule e.g. every 5 mins, and would do the following:

• Use the ec2:DescribeAccountAttributes API to get the account instance limit and cloudwatch:DescribeAlarms to get the current threshold of the alarm. If they differ, the alarm threshold should be updated the the instance limit via the cloudwatch:PutMetricAlarm API.

• Use the ec2:DescribeInstances API and count the number of instances that are running and publish the value to a custom CloudWatch metric with the cloudwatch:PutMetricData API.

If the value published to the metric exceeds the threshold of the alarm, it will fire. The lambda function will keep the alarm threshold configured to the limit of instances and will publish datapoints to the metric based on the number of instances currently running.