Cloudformation template completes deployment before UserData is finished

Question

In the CloudFormation template I am deploying, I am running a long running operation in the UserData block.

It looks as follows:

"UserData": {
    "Fn::Base64" : {
        "Fn::Join" : [
            "",
            [
            "/usr/local/bin/mylongrunningscript.sh"
            ]
         ]
    }
}

The contents of the script are:

echo "UserData starting!" > /var/log/mycustomlog.log
sleep 300
echo "UserData finished!" >> /var/log/mycustomlog.log

The issue I am seeing is that I believe the CloudFormation template is completing it's deployment before the UserData script finishes running. I believe this is the case because if I am quick enough and ssh into the instance, I will see something as follows:

$ cat /var/log/mycustomlog.log
UserData starting

which suggests that the UserData didn't finish running

How can I make sure that the UserData code execution is completed before the stack is in the "CreateComplete" status?

Does this answer your question? Cloudformation stack shows as complete before execution of all user data — Maria Ines Parnisari
@MariaInesParnisari Thanks! Yes, that worked. I'll add an answer here because the answer in the other question is not comprehensive. — Paolo

Paolo Paolo · Accepted Answer · 2020-02-25T11:59:22

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

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

"UserData": {
    "Fn::Base64" : {
        "Fn::Join" : [
            "",
            [
            "/usr/local/bin/mylongrunningscript.sh"
            ]
        ]
    }
},
"CreationPolicy": {
    "ResourceSignal" : {
        "Count": "1",
        "Timeout": "PT10M"
    }
}

The cfn-signal utility is used 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.

Cloudformation template completes deployment before UserData is finished

1 Answers