Interpolation & Resource References

What is interpolation?

Interpolation means using one resource’s attribute inside another resource. That is how you wire a subnet to a VPC, an instance to a security group, and so on.

Reference syntax

resource_type.local_name.attribute

Examples:

aws_instance.example.id
aws_instance.example.public_ip
aws_s3_bucket.demo.bucket
aws_vpc.main.id

Example — subnet depends on VPC

resource "aws_subnet" "public" {
  vpc_id     = aws_vpc.main.id
  cidr_block = "10.0.1.0/24"
}

aws_vpc.main.id tells Terraform: create the VPC first, then create the subnet with that VPC’s ID.

Dependency flow

VPC
 ↓
Subnet
 ↓
EC2 / Route table association / …

You do not need to declare depends_on for normal attribute references — Terraform builds the graph automatically from expressions like aws_vpc.main.id.