Cloudformation template completes deployment before UserData is finished

Question

In the CloudFormation template I am deploying, I am running a few commands in the UserData block. One of these commands starts up a session for NICE DCV: https://aws.amazon.

Answer 1

To ensure the CloudFormation template waits for the completion of the UserData script, you must do two things:

1. Add a CreationPolicy to the resource you are targeting (virtual machine in my case).

2. Add logic in the script to signal its completion. This custom logic uses the cfn-signal utility, which you might have to install in your instance.

---

Here's how the template looks now:

"Properties": {
    "UserData": {
        "Fn::Base64" : {
            "Fn::Join" : [
            "",
            [
               "curl --silent --show-error --retry 5 https://bootstrap.pypa.io/get-pip.py | sudo python\n",
               "curl https://s3.amazonaws.com/cloudformation-examples/aws-cfn-bootstrap-latest.tar.gz -o /home/ubuntu/aws-cfn-bootstrap.tar.gz\n",
               "tar -xvzf /home/ubuntu/aws-cfn-bootstrap.tar.gz -C /home/ubuntu/\n",
               "rm /home/ubuntu/aws-cfn-bootstrap.tar.gz\n",
               "chmod +x /home/ubuntu/aws-cfn-bootstrap-*/bin/cfn-signal\n",
               "pip install /home/ubuntu/aws-cfn-bootstrap-*\n",
               "dcv create-session --type virtual ",
               " --owner ubuntu",
               " --user ubuntu",
               " my-session, "\n",
               "while ! (dcv list-sessions | grep -q 'my-session'); do sleep 1; done\n"
               "/home/ubuntu/aws-cfn-bootstrap-*/bin/cfn-signal -e $? ",
               " --stack ", { "Ref": "AWS::StackName" },
               " --resource MyInstance" ,
               " --region ", { "Ref" : "AWS::Region" }, "\n"
            ]
            ]
        }
    }
},
"CreationPolicy": {
        "ResourceSignal" : {
            "Count": "1",
            "Timeout": "PT5M"
        }
}

---

Breaking the script down.

This fetches and installs the aws-cfn set of tools, needed for cfn-signal:

"curl --silent --show-error --retry 5 https://bootstrap.pypa.io/get-pip.py | sudo python\n",
"curl https://s3.amazonaws.com/cloudformation-examples/aws-cfn-bootstrap-latest.tar.gz -o /home/ubuntu/aws-cfn-bootstrap.tar.gz\n",
"tar -xvzf /home/ubuntu/aws-cfn-bootstrap.tar.gz -C /home/ubuntu/\n",
"rm /home/ubuntu/aws-cfn-bootstrap.tar.gz\n",
"chmod +x /home/ubuntu/aws-cfn-bootstrap-*/bin/cfn-signal\n",
"pip install /home/ubuntu/aws-cfn-bootstrap-*\n",

This is my custom script, which I want to ensure is completed before the cloud formation finishes deployment:

"dcv create-session --type virtual ",
" --owner ubuntu",
" --user ubuntu",
" my-session, "\n",
"while ! (dcv list-sessions | grep -q 'my-session'); do sleep 1; done\n"

Finally, I use the cfn-signal utility to signal the termination of the script:

"/home/ubuntu/aws-cfn-bootstrap-*/bin/cfn-signal -e $? ",
" --stack ", { "Ref": "AWS::StackName" },
" --resource MyInstance" ,
" --region ", { "Ref" : "AWS::Region" }, "\n"

---

See here for a Windows example.