Want to do automatic subnet division in Terraform?
Make even a single AWS EC2 host highly available using autoscaling groups. Here's example using ASG, EFS for persistent data, and Terraform for easy automation.
Terraform state is a database kept by Terraform to help it reconcile Terraform resource dependencies, and reconcile between Terraform code and the actual state on the target infrastructure. For example, in the state, Terraform expects to find an entry that matches your AWS resource defining a Route53 record and in your Terraform code. Whether or not the entry exits determines if Terraform will make a change or not.
Here's what I've learned so far using Terraform and its modules. I've just scratched the surface so test and research on your own.