Explanation
[-upgrade] - Opt to upgrade modules and plugins as part of their respective installation steps. See the sections
below for more details. Reference: https://www.terraform.io/cli/commands/init#upgrade

Explanation
If a resource successfully creates but fails during provisioning, Terraform will error and mark the resource as
"tainted". A resource that is tainted has been physically created, but can't be considered safe to use since
provisioning failed. Terraform also does not automatically roll back and destroy the resource during the apply
when the failure happens, because that would go against the execution plan: the execution plan would've said a
resource will be created, but does not say it will ever be deleted.

Explanation
https://www.terraform.io/docs/provisioners/index.html

Explanation
Implicit and Explicit Dependencies
By studying the resource attributes used in interpolation expressions, Terraform can automatically infer when
one resource depends on another. In the example above, the reference to aws_instance.web_server.id creates
an implicit dependency on the aws_instance named web_server.
Terraform uses this dependency information to determine the correct order in which to create the different
resources.
# Example of Implicit Dependency
resource "aws_instance" "web_server" {
ami = "ami-b374d5a5"
instance_type = "t2.micro"
}
resource "aws_eip" "web_server_ip" {
vpc = true
instance = aws_instance.web_server.id
}
In the example above, Terraform knows that the aws_instance must be created before the aws_eip.
Implicit dependencies via interpolation expressions are the primary way to inform Terraform about these
relationships, and should be used whenever possible.
Sometimes there are dependencies between resources that are not visible to Terraform. The depends_on
argument is accepted by any resource and accepts a list of resources to create explicit dependencies for.
For example, perhaps an application we will run on our EC2 instance expects to use a specific Amazon S3
bucket, but that dependency is configured inside the application code and thus not visible to Terraform. In that
case, we can use depends_on to explicitly declare the dependency:
# Example of Explicit Dependency
# New resource for the S3 bucket our application will use.
resource "aws_s3_bucket" "example" {
bucket = "terraform-getting-started-guide"
acl = "private"
}
# Change the aws_instance we declared earlier to now include "depends_on"
resource "aws_instance" "example" {
ami = "ami-2757f631"
instance_type = "t2.micro"
# Tells Terraform that this EC2 instance must be created only after the
# S3 bucket has been created.
depends_on = [aws_s3_bucket.example]
}
https://learn.hashicorp.com/terraform/getting-started/dependencies.html