Create_before_destroy lifecycle rule in Terraform - Step-by-Step Execution

Process Flow - Create_before_destroy lifecycle rule

Start Apply

↓

Check resource change

↓

Create new resource

↓

Wait for new resource ready

↓

Destroy old resource

↓

Apply complete

Terraform first creates the new resource before destroying the old one to avoid downtime.

Execution Sample

Terraform

resource "aws_instance" "example" {
  lifecycle {
    create_before_destroy = true
  }
  ami           = "ami-12345678"
  instance_type = "t2.micro"
}

This Terraform code creates an AWS instance with the create_before_destroy lifecycle rule to ensure new instance is created before old one is destroyed.

Process Table

Step	Action	Resource State Before	Resource State After	Notes
1	Start apply	Old instance exists	Old instance exists	Terraform plan starts
2	Create new instance	Old instance exists	Old instance + New instance	New instance is created first
3	Wait for new instance ready	Old instance + New instance	Old instance + New instance	Ensures new instance is healthy
4	Destroy old instance	Old instance + New instance	New instance only	Old instance is safely removed
5	Apply complete	New instance only	New instance only	Infrastructure updated without downtime

💡 Old instance destroyed only after new instance is ready, ensuring zero downtime.

Status Tracker

Variable	Start	After Step 2	After Step 4	Final
old_instance	exists	exists	destroyed	destroyed
new_instance	not exists	created	created	created

Key Moments - 2 Insights

Why does Terraform create a new resource before destroying the old one?

Because the create_before_destroy rule tells Terraform to avoid downtime by ensuring the new resource is ready before removing the old one, as shown in steps 2 and 4 of the execution_table.

What happens if the new resource fails to become ready?

Terraform will not destroy the old resource, preventing service interruption. This is implied in step 3 where Terraform waits for readiness before proceeding.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the new instance get created?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Refer to the 'Action' column in execution_table row for Step 2.

According to variable_tracker, what is the state of old_instance after Step 4?

A exists

B created

C destroyed

D not exists

💡 Hint

Check the 'old_instance' row under 'After Step 4' in variable_tracker.

If create_before_destroy was false, what would change in the execution_table?

A Old instance destroyed before new instance created

B New instance created before old instance destroyed

C Both instances created simultaneously

D No instances destroyed

💡 Hint

The lifecycle rule controls creation and destruction order shown in execution_table steps.

Concept Snapshot

Terraform lifecycle create_before_destroy rule:
- Ensures new resource is created before old is destroyed
- Prevents downtime during resource replacement
- Applies during terraform apply phase
- Useful for resources needing continuous availability
- Syntax: lifecycle { create_before_destroy = true }

Full Transcript

This visual execution trace shows how Terraform applies the create_before_destroy lifecycle rule. First, Terraform starts the apply process with the old resource existing. Then it creates the new resource while keeping the old one active. Terraform waits until the new resource is ready and healthy. Only after that does it destroy the old resource. This sequence prevents downtime by ensuring continuous availability. The variable tracker shows the old instance exists initially and is destroyed only after the new instance is created. Key moments clarify why the new resource must be ready before destroying the old one and what happens if readiness fails. The quiz tests understanding of the creation and destruction order and the effect of disabling this rule.