Data source vs resource difference in Terraform - Visual Side-by-Side Comparison

Process Flow - Data source vs resource difference

Start Terraform Apply

↓

Check for Data Sources

↓

Read Existing Infrastructure

↓

Check for Resources

↓

Create or Update Infrastructure

↓

Finish Apply

Terraform first reads data sources to get info about existing infrastructure, then creates or updates resources to build new infrastructure.

Execution Sample

Terraform

data "aws_ami" "example" {
  most_recent = true
  owners      = ["amazon"]
}

resource "aws_instance" "example" {
  ami           = data.aws_ami.example.id
  instance_type = "t2.micro"
}

This code reads the latest Amazon Linux AMI (data source) and then creates an EC2 instance (resource) using that AMI.

Process Table

Step	Action	Data Source Read	Resource Created	Result
1	Start terraform apply	No	No	Begin execution
2	Read data source aws_ami.example	Reads latest Amazon Linux AMI ID	No	AMI ID fetched from AWS
3	Create resource aws_instance.example	No	Creates EC2 instance with fetched AMI ID	EC2 instance launched
4	Finish apply	No	No	Infrastructure updated successfully

💡 All data sources read before resources created; apply completes after resource creation

Status Tracker

Variable	Start	After Step 2	After Step 3	Final
data.aws_ami.example.id	undefined	ami-0abcdef1234567890	ami-0abcdef1234567890	ami-0abcdef1234567890
resource.aws_instance.example.id	undefined	undefined	i-0123456789abcdef0	i-0123456789abcdef0

Key Moments - 3 Insights

Why does Terraform read data sources before creating resources?

Terraform reads data sources first to get information about existing infrastructure it needs to reference when creating or updating resources, as shown in step 2 of the execution_table.

Can data sources create or change infrastructure?

No, data sources only read existing infrastructure data; they do not create or modify anything. This is clear because resource creation happens only after data source reading (step 3).

What happens if the data source changes between applies?

Terraform will fetch the latest data each apply, so resources depending on data sources may be updated accordingly, reflecting the new data source values as tracked in variable_tracker.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of data.aws_ami.example.id after step 2?

A ami-0abcdef1234567890

B undefined

C i-0123456789abcdef0

D t2.micro

💡 Hint

Check the 'Data Source Read' column in row for step 2 in execution_table

At which step does Terraform create the EC2 instance resource?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Resource Created' column in execution_table

If the data source returned a different AMI ID, how would variable_tracker change after step 2?

A No variables would change

B resource.aws_instance.example.id would change immediately

C data.aws_ami.example.id would have the new AMI ID value

D Terraform would skip resource creation

💡 Hint

Variable tracker shows data source values after step 2; resource IDs update after resource creation

Concept Snapshot

Terraform applies in two phases:
1. Data sources read existing infrastructure info.
2. Resources create or update infrastructure.
Data sources do NOT create or change anything.
Resources depend on data source info to build new infrastructure.

Full Transcript

Terraform uses data sources to read existing infrastructure details before creating or updating resources. In the example, Terraform first reads the latest Amazon Linux AMI ID using a data source, then creates an EC2 instance resource using that AMI. The execution flow starts with reading data sources, then proceeds to resource creation. Variables track the AMI ID after reading the data source and the EC2 instance ID after resource creation. Beginners often wonder why data sources run first, which is to provide necessary info for resource creation. Data sources do not create or modify infrastructure; only resources do. If data source info changes, Terraform fetches the new data on the next apply, which may affect resource updates.