Resource arguments and attributes in Terraform - Step-by-Step Execution

Process Flow - Resource arguments and attributes

Define Resource Block

↓

Set Arguments (input values)

↓

Terraform Processes Resource

↓

Resource Created in Cloud

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Terraform Reads Attributes (output values)

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Use Attributes in Config or Output

Terraform resource blocks take arguments as input to create cloud resources, then provide attributes as output to use elsewhere.

Execution Sample

Terraform

resource "aws_instance" "example" {
  ami           = "ami-123456"
  instance_type = "t2.micro"
}

output "instance_id" {
  value = aws_instance.example.id
}

Defines an AWS instance with arguments, then outputs its ID attribute after creation.

Process Table

Step	Action	Arguments Set	Resource State	Attributes Available
1	Start resource block	None yet	Not created	None
2	Set ami argument	ami = "ami-123456"	Not created	None
3	Set instance_type argument	instance_type = "t2.micro"	Not created	None
4	Terraform creates instance	ami and instance_type set	Instance created in AWS	id, arn, public_ip, etc.
5	Read attribute aws_instance.example.id	Arguments unchanged	Instance exists	id = "i-0abcd1234efgh5678"
6	Output instance_id value	Arguments unchanged	Instance exists	Output = "i-0abcd1234efgh5678"
7	End	No changes	Instance exists	Attributes stable

💡 Resource created and attributes read; output shows instance ID.

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
ami	None	"ami-123456"	"ami-123456"	"ami-123456"	"ami-123456"	"ami-123456"
instance_type	None	None	"t2.micro"	"t2.micro"	"t2.micro"	"t2.micro"
resource_state	Not created	Not created	Not created	Created	Created	Created
id (attribute)	None	None	None	Available	"i-0abcd1234efgh5678"	"i-0abcd1234efgh5678"
output instance_id	None	None	None	None	None	"i-0abcd1234efgh5678"

Key Moments - 3 Insights

Why can't we use resource attributes before the resource is created?

Attributes like 'id' are only known after Terraform creates the resource (see step 4 and 5 in execution_table). Before creation, attributes are not available.

Are resource arguments and attributes the same thing?

No. Arguments are inputs you set to create the resource (steps 2 and 3). Attributes are outputs Terraform reads after creation (steps 5 and 6).

Can attributes be used in other resource blocks?

Yes. Attributes like 'id' can be referenced in other resources or outputs after creation, enabling dependencies and dynamic configs.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does Terraform create the actual cloud resource?

A Step 2

B Step 4

C Step 5

D Step 6

💡 Hint

Check the 'Resource State' column for when it changes to 'Instance created in AWS'.

According to variable_tracker, what is the value of 'id' attribute after step 5?

A "i-0abcd1234efgh5678"

B None

C "ami-123456"

D "t2.micro"

💡 Hint

Look at the 'id (attribute)' row under 'After Step 5' column.

If you change the 'instance_type' argument to 't3.small', which step in execution_table would reflect this change?

A Step 5

B Step 4

C Step 3

D Step 6

💡 Hint

Arguments are set before resource creation; see 'Arguments Set' column.

Concept Snapshot

Terraform resource blocks use arguments as inputs to create cloud resources.
After creation, Terraform provides attributes as outputs.
Arguments define what the resource should be.
Attributes give details about the created resource.
Attributes can be used in outputs or other resources.
Resource creation happens after all arguments are set.

Full Transcript

In Terraform, you define resources using blocks that include arguments. These arguments are like instructions telling Terraform what to create, such as the AMI ID and instance type for an AWS instance. Terraform processes these arguments and creates the resource in the cloud. Once created, Terraform reads attributes from the resource, like its unique ID, which you can use elsewhere in your configuration or outputs. The execution flow starts with setting arguments, then creating the resource, and finally reading attributes. Attributes are not available before creation. This process allows you to manage cloud resources declaratively and use their details dynamically.