AMI lookup data source example in Terraform - Step-by-Step Execution

Process Flow - AMI lookup data source example

Start Terraform Apply

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Invoke data source: aws_ami

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Query AWS for AMI matching filters

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Receive AMI details

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Use AMI ID in resource creation

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Create EC2 instance with AMI

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End Terraform Apply

Terraform starts, queries AWS for an AMI matching filters, then uses the found AMI ID to create an EC2 instance.

Execution Sample

Terraform

data "aws_ami" "example" {
  most_recent = true
  filter {
    name   = "name"
    values = ["amzn2-ami-hvm-*-x86_64-gp2"]
  }
  owners = ["amazon"]
}

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

This Terraform code looks up the latest Amazon Linux 2 AMI and launches a t2.micro EC2 instance using it.

Process Table

Step	Action	Input/Filter	AWS Response	Resulting Value
1	Start Terraform apply	-	-	-
2	Invoke aws_ami data source	Filter: name=amzn2-ami-hvm-*-x86_64-gp2, owners=amazon, most_recent=true	List of matching AMIs	Select most recent AMI ID: ami-0abcdef1234567890
3	Assign AMI ID to aws_instance resource	AMI ID from data source	-	ami-0abcdef1234567890
4	Create EC2 instance	AMI ID=ami-0abcdef1234567890, instance_type=t2.micro	Instance created with ID i-0123456789abcdef0	EC2 instance running
5	Terraform apply complete	-	-	-

💡 Terraform apply finishes after EC2 instance is created using the looked-up AMI.

Status Tracker

Variable	Start	After Step 2	After Step 3	Final
data.aws_ami.example.id	undefined	ami-0abcdef1234567890	ami-0abcdef1234567890	ami-0abcdef1234567890
aws_instance.example.ami	undefined	undefined	ami-0abcdef1234567890	ami-0abcdef1234567890
aws_instance.example.instance_type	undefined	undefined	t2.micro	t2.micro

Key Moments - 3 Insights

Why does Terraform query AWS before creating the EC2 instance?

Terraform uses the aws_ami data source to find the latest AMI ID matching the filters before it can assign that AMI ID to the EC2 instance resource, as shown in step 2 and 3 of the execution_table.

What happens if no AMI matches the filter?

Terraform will fail during the data source lookup (step 2) because it cannot find a valid AMI ID to assign to the EC2 instance, stopping the apply process.

Why do we use 'most_recent = true' in the data source?

This ensures Terraform picks the newest AMI matching the filter, so the EC2 instance uses the latest image, as reflected in the AWS response selecting the most recent AMI ID in step 2.

Visual Quiz - 3 Questions

Look at the execution_table, what AMI ID is selected at step 2?

A ami-0fedcba9876543210

B ami-0abcdef1234567890

C ami-1234567890abcdef0

D ami-11111111111111111

💡 Hint

Check the AWS Response and Resulting Value columns at step 2 in the execution_table.

At which step is the EC2 instance created?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look for the action mentioning 'Create EC2 instance' in the execution_table.

If we remove 'most_recent = true', how would the execution_table change?

A Terraform would fail to find any AMI.

B Terraform would select a random AMI from the list.

C Terraform would error because multiple matching AMIs are found.

D Terraform would select the AMI with the smallest ID.

💡 Hint

Without 'most_recent = true', the data source will error if more than one AMI matches the filter in step 2.

Concept Snapshot

Terraform AMI lookup data source:
- Use 'data "aws_ami"' with filters to find AMI
- 'most_recent = true' picks latest AMI
- Use 'data.aws_ami.example.id' in resource
- Ensures EC2 uses correct AMI
- Lookup happens before resource creation

Full Transcript

This example shows how Terraform uses the aws_ami data source to find the latest Amazon Linux 2 AMI by filtering on name and owner. Terraform queries AWS during apply, receives the most recent AMI ID, then uses that ID to create an EC2 instance. The execution table traces each step: starting apply, querying AMI, assigning AMI ID, creating instance, and finishing. Variables track the AMI ID and instance type values as they update. Key moments clarify why the lookup happens first, what if no AMI matches, and why most_recent is important. The quiz tests understanding of AMI selection, creation step, and effect of removing most_recent. This process ensures Terraform dynamically uses the latest AMI without hardcoding IDs.