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I'm trying to trigger an EC2 auto-scaling based on a CloudWatch alarm. I've created the auto scaling group in EC2 - same region and account - but when I try selecting it from the CloudWatch console, nothing shows up.

How do I get CloudWatch to allow me to select the auto scaling group I've created? enter image description here

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You have to create a scaling policy first. If its a step scaling policy, then it'll ask you to select an alarm when you're creating the policy in the ASG console. So go ahead and create the alarm with no actions, then make the step scaling policy and select this alarm, which will automatically create the alarm action.

If you want to use target tracking, you can't manually define the alarms. 2 are automatically made for you when you create the scaling policy (one for scale in, one for scale out, at different values to prevent oscillation)

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  • That worked. Looks like you need to define the CloudWatch alarm from the auto-scaling policy, not vice versa.
    – David_Springfield
    Dec 3, 2019 at 13:01
  • If you already have a scaling policy, you can make an alarm from the CloudWatch console and add the AutoScaling action from there. When creating a step scaling policy from the AutoScaling console, you do have to assign or create an alarm at that point (not required when creating the scaling policy through the CLI/API/Anything-not-the-console)
    – Shahad
    Dec 5, 2019 at 4:15

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