Resource Dependencies
- When Resource A requires resource B to be present (or already existing) this is called as dependency
- In Terraform terms you have to create resource B before resource A
- To Demonstrate this lets add subnets to VPC. To create subnet resource vpc id is required (vpc has to be existing)
- To create resource dependencies use the following expression
"${<resource-type>.<resource-name>.<attribute-name>}"
"${aws_vpc.myvpc.id}"
- If we apply the above expressions to create two subnets in the vpc
provider "aws" {
access_key = "<accesskey>"
secret_key = "<secret-key>"
region = "us-west-2"
}
resource "aws_vpc" "myvpc" {
cidr_block = "192.168.0.0/16"
tags = {
"Name" = "from-tf"
}
}
resource "aws_subnet" "subnet1" {
cidr_block = "192.168.0.0/24"
vpc_id = "${aws_vpc.myvpc.id}"
availability_zone = "us-west-2a"
tags = {
"Name" = "subnet-1"
}
}
resource "aws_subnet" "subnet2" {
cidr_block = "192.168.1.0/24"
availability_zone = "us-west-2b"
tags = {
"Name" = "subnet2"
}
vpc_id = "${aws_vpc.myvpc.id}"
}
- Now Execute terraform apply to create infra and also observe the folder where terraform.tfstate is created where the information about created resources are stored.
- Now Lets add one more subnet to existing template
provider "aws" {
access_key = "<accesskey>"
secret_key = "<secret-key>"
region = "us-west-2"
}
resource "aws_vpc" "myvpc" {
cidr_block = "192.168.0.0/16"
tags = {
"Name" = "from-tf"
}
}
resource "aws_subnet" "subnet1" {
cidr_block = "192.168.0.0/24"
vpc_id = "${aws_vpc.myvpc.id}"
availability_zone = "us-west-2a"
tags = {
"Name" = "subnet-1"
}
}
resource "aws_subnet" "subnet2" {
cidr_block = "192.168.1.0/24"
availability_zone = "us-west-2b"
tags = {
"Name" = "subnet2"
}
vpc_id = "${aws_vpc.myvpc.id}"
}
resource "aws_subnet" "subnet3" {
cidr_block = "192.168.2.0/24"
availability_zone = "us-west-2c"
tags = {
"Name" = "subnet3"
}
vpc_id = "${aws_vpc.myvpc.id}"
}
-
Now if you execute terraform apply command
- Terraform will get the current status from provider (aws)
- It compares the status of provider with tfstate file, if any differences are found they will be added for execution. (Deleting from aws and executing terraform apply)
- during apply plan will be created and compared to current status, if plan is not matching the current state, then the changes will be added to terraforms execution.
-
Experiment Terraform
- By deleting one subnet from template
- change the tag value for one subnet
- change the avaliability zone for one subnet
