what if a reinstallation is required

       此文为上文的一点补充。

As far as I understand the sole purpose of cloud-init or user-data scripts is to do early initialization of instances. From that perspective, it may not make sense to use it as a way to re-provision or re-configure instances since that’s what tools like Puppet, Chef, Ansiable and Salt are for. Terraform was thought out as a way of creating and destroying infrastructure resources, and resource immutability is all over the place. https://github.com/terraform-providers/terraform-provider-aws/issues/23#issuecomment-308115064

       然而Fedora CoreOS这样的immutable system的加入改变了这一现状，我们不再需要ansible/puppet或是其他的配置管理工具，而terraform，很遗憾，尚未对此作出相应的改变。我们所需求的特性即是：在资源特定元数据改变时，重新初始化资源，而非重新创建。幸运的是，terraform提供了provisioner与null resource，他们可以实现这一行为。

TL;DR

resource "null_resource" "reinstall" {
  triggers = {
    user_data = <provider>_server.server.user_data
  }
  provisioner "local-exec" {
    command = "curl <api call to reinstall the given server>/${<provider>_server.server.id}"
  }
}