What are the benefits of cfn-init over userdata?

Question

My CloudFormation template has gotten pretty long. One reason is because my AWS::CloudFormation::Init section has gotten pretty huge. This is a very small sample of what I have:

"ConfigDisk": {
    "commands": {
        "01formatFS": {
            "command": "/sbin/mkfs.ext4 /dev/xvdf"
        },
        "02mountFS": {
            "command": "/bin/mount /dev/xvdf /var/lib/jenkins"
        },
        "03changePerms": {
            "command": "/bin/chown jenkins:jenkins /var/lib/jenkins"
        },
        "04updateFStab": {
            "command": "/bin/echo /dev/xvdf /var/lib/jenkins ext4 defaults 1 1 >> /etc/fstab"
        }
    }
},

Wouldn't it be better to just put this into the userdata section as a bunch of commands?

/sbin/mkfs.ext4 /dev/xvdf
/bin/mount /dev/xvdf /var/lib/jenkins
/bin/chown jenkins:jenkins /var/lib/jenkins
/bin/echo /dev/xvdf /var/lib/jenkins ext4 defaults 1 1 >> /etc/fstab

What are the benefits of leaving this in the Init over userdata?

Answer 1

A major benefit of AWS::CloudFormation::Init over UserData is that the former is updatable -- if you modify the AWS::CloudFormation::Init section, CloudFormation will update your EC2 instance in place, whereas if you modify the UserData property of an EC2 resource in your template and update your stack, CloudFormation will replace that EC2 instance.

This is handy, for example, if you want to update the packages you have installed on your EC2 instance without recreating it.