Terraform Resources Building Blocks of Infrastructure

Resources are the most important concept in Terraform. Every virtual machine, every database, every network subnet — all of these are resources in Terraform. This topic explains how to write resource blocks, understand their arguments, reference their attributes, and manage their behavior.

What Is a Resource

A resource represents a single infrastructure object. It could be a physical server, a virtual machine, a DNS record, a storage bucket, or any other piece of infrastructure that a cloud provider can create.

Analogy: Lego Bricks

Think of your entire infrastructure as a Lego model. Each individual Lego brick is a resource. A web server is one brick. The database it connects to is another brick. The network they share is a third brick. Terraform snaps those bricks together exactly as you describe in your code.

The Resource Block Syntax

resource "RESOURCE_TYPE" "LOCAL_NAME" {
  argument1 = value1
  argument2 = value2
}

• RESOURCE_TYPE: Determined by the provider. For AWS it might be aws_instance, aws_s3_bucket, or aws_vpc.
• LOCAL_NAME: A name you choose. It exists only inside Terraform and is never sent to the cloud provider. Use descriptive names like web_server, app_database, or main_network.

A Real Example: AWS EC2 Instance

resource "aws_instance" "web_server" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t3.micro"

  tags = {
    Name        = "My Web Server"
    Environment = "Production"
  }
}

Diagram: Resource Block Anatomy

resource  "aws_instance"   "web_server"  {
   ^           ^                ^
Block       Resource          Your
Type        Type             Local Name
(always     (from the        (you choose)
"resource") provider docs)

  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t3.micro"
      ^                ^
  Required         Required
  Argument         Argument

  tags = { Name = "My Web Server" }
      ^
  Optional Argument (key-value map)
}

Required vs Optional Arguments

Each resource type has its own set of arguments. Some arguments are required — Terraform will refuse to create the resource without them. Other arguments are optional — if you omit them, the provider uses its default value.

You find the full list of arguments for any resource in the provider documentation on registry.terraform.io. For example, searching for aws_instance shows every argument, whether it is required, and what it does.

Referencing Resource Attributes

After Terraform creates a resource, it stores the resource's attributes in the state file. You reference those attributes in other resources using dot notation.

resource "aws_instance" "web_server" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t3.micro"
}

resource "aws_eip" "web_ip" {
  instance = aws_instance.web_server.id
                    ^            ^    ^
              resource type   name   attribute
}

The format is: RESOURCE_TYPE.LOCAL_NAME.ATTRIBUTE

This reference creates an implicit dependency — Terraform automatically creates the EC2 instance before trying to attach the Elastic IP, because the EIP needs the instance ID.

Common Resource Types Across Providers

Nested Blocks Inside Resources

Some resources contain nested configuration blocks. These are not separate resources — they are sub-configurations that belong to the parent resource.

resource "aws_instance" "app_server" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t3.small"

  root_block_device {
    volume_size = 20
    volume_type = "gp3"
    encrypted   = true
  }

  network_interface {
    network_interface_id = aws_network_interface.main.id
    device_index         = 0
  }
}

The root_block_device and network_interface blocks configure aspects of the EC2 instance itself. They are not standalone resources — they live inside the aws_instance resource block.

Key Points

• A resource block defines one infrastructure object — a server, bucket, database, or network element.
• The resource type comes from the provider; the local name is yours to choose.
• Reference another resource's attribute with the pattern resource_type.local_name.attribute.
• Attribute references create implicit dependencies that control the order Terraform creates resources.
• Read provider documentation on the Terraform Registry to find all available arguments for any resource type.