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I have created an ECS cluster linked to an Autoscaling group with an Application load balancer attached. The Autoscaling deploys the instances, the AMI is correctly choosen (ECS optimized).

for whatever reason the ecs instances are being deployed by the Auto Scaling service but are not registering with the ECS Cluster.

Does anybody have an idea, what could cause the error of not having the instances on my cluster?

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3 Answers 3

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Your EC2 instances just don't have enough permissions to register with ECS cluster:

Important

If you do not launch your container instance with the proper IAM permissions, your Amazon ECS agent cannot connect to your cluster.

Check IAM role that you've assigned to your EC2 instances. It should include appropriate permissions, for example:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "ecs:DeregisterContainerInstance",
        "ecs:RegisterContainerInstance",
        "ecr:GetAuthorizationToken"
      ],
      "Resource": "*"
    }
  ]
}

Or you can use AWS-managed policy named AmazonEC2ContainerServiceforEC2Role and assigned it to your EC2 role.

More information is avalable at https://docs.aws.amazon.com/AmazonECS/latest/developerguide/instance_IAM_role.html.

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  • I have created a role called ecsServiceRole for my IAM user which has the policy attached, but still it is not working.
    – aerioeus
    Aug 3, 2018 at 11:24
  • I was not talking about IAM policy for the user, I was talking about IAM policy for EC2 role. The fact that you have those permissions doesn't mean your EC2 instances have those permissions.
    – Sergey Kovalev
    Aug 3, 2018 at 11:42
  • ok, but as far as know, I can only associate a role while creating the Auto Scaling group, which I did, the role is named ecsInstanceRole and contains the policy. I assume this is the correct way, right?
    – aerioeus
    Aug 3, 2018 at 11:46
  • by the way, is there a way to let the Auto scaling deploy the instances to a specific cluster I have created before or will it generate a cluster like -default - of its own?
    – aerioeus
    Aug 3, 2018 at 11:47
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this one is also necessary to make it work; Amazon ECS needs permissions to register and deregister container instances with your load balancer when tasks are created and stopped

{
  "Version": "2008-10-17",
  "Statement": [
{
     "Sid": "",
     "Effect": "Allow",
     "Principal": {
     "Service": "ecs.amazonaws.com"
    },
    "Action": "sts:AssumeRole"
   }
 ]
}

https://docs.aws.amazon.com/AmazonECS/latest/developerguide/check-service-role.html

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We had an error in our user data script. I found the error in EC2 - my instance -> Monitor and troubleshoot -> Get system log. Our bash script in the user data had -ex flags on i.e. #!/bin/bash -ex, which tells the script to exit immediately as a failure on any errors. The EC2 task would finish starting and look healthy to the Auto Scaling Group, but the cloud-init (EC2 initialization) had terminated and didn't complete, leaving the instance unattached to the ECS cluster. Once I fixed the error in our user data script and launched a new instance, it was able o attach to the ECS cluster as expected.

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    Feb 8, 2022 at 17:22

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