Immutable infrastructure concept in Terraform - Step-by-Step Execution

Process Flow - Immutable infrastructure concept

Write new config

↓

Create new infrastructure

↓

Switch traffic to new infra

↓

Destroy old infrastructure

↓

End

Immutable infrastructure means creating new resources for changes instead of modifying existing ones, then switching traffic and removing old resources.

Execution Sample

Terraform

resource "aws_instance" "web" {
  ami           = "ami-123456"
  instance_type = "t2.micro"
  lifecycle {
    create_before_destroy = true
  }
}

# Change instance_type to t2.small

This Terraform code creates an AWS instance. Changing instance_type triggers creation of a new instance instead of modifying the old one.

Process Table

Step	Action	Terraform Plan Result	Infrastructure State
1	Initial apply	Create aws_instance.web with t2.micro	One instance running t2.micro
2	Change instance_type to t2.small	Plan to create new aws_instance.web and destroy old one	Old instance running t2.micro, new instance pending
3	Apply changes	New instance created, traffic switched	One instance running t2.small, old instance terminated
4	Final state	No changes	One instance running t2.small

💡 No further changes, infrastructure matches desired state

Status Tracker

Variable	Start	After Change	After Apply	Final
instance_type	t2.micro	t2.small	t2.small	t2.small
instance_id	i-abc123	i-abc123 (old), i-def456 (new)	i-def456	i-def456

Key Moments - 2 Insights

Why does Terraform create a new instance instead of updating the existing one?

Terraform treats changes to instance_type as requiring replacement, so it plans to create a new instance and destroy the old one (see execution_table step 2).

What happens to the old instance during the update?

The old instance stays running until the new one is created and traffic is switched, then Terraform destroys the old instance (see execution_table step 3).

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what does Terraform plan to do?

A Update the existing instance in place

B Do nothing

C Create a new instance and destroy the old one

D Destroy the instance without replacement

💡 Hint

See execution_table row 2 under 'Terraform Plan Result'

According to variable_tracker, what is the instance_type after applying changes?

A t2.micro

B t2.small

C t2.medium

D unchanged

💡 Hint

Check variable_tracker row for instance_type after apply

When does the old instance get terminated?

A After the new instance is created and traffic switched

B At the same time as creating the new instance

C Before creating the new instance

D It never gets terminated

💡 Hint

See execution_table step 3 for infrastructure state changes

Concept Snapshot

Immutable infrastructure means no in-place changes.
Change triggers new resource creation.
Switch traffic to new resource.
Destroy old resource after switch.
Ensures stable, repeatable deployments.

Full Transcript

Immutable infrastructure is a way to manage cloud resources where you never change existing resources directly. Instead, when you want to update something, you create a new resource with the new settings. Then you switch your traffic or usage to the new resource. Finally, you remove the old resource. This approach avoids unexpected changes and makes deployments safer and easier to track. For example, in Terraform, changing the instance type of a server causes Terraform to plan creating a new server and destroying the old one. The old server stays running until the new one is ready, then traffic switches, and the old server is removed. This process ensures your infrastructure is always in a known good state.