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