Day-09: Lifecycle management rules in terraform

Source: Dev.to

Lifecycle rules

In Terraform, lifecycle rules control how a resource is created, updated, and destroyed. They help improve security, simplify maintenance, and give you finer control over resources.

Terraform provides six lifecycle arguments:

• ignore_changes
• prevent_destroy
• replace_triggered_by
• create_before_destroy
• precondition
• postcondition

1. create_before_destroy

When a resource needs to be changed, this rule creates the new version before destroying the old one, reducing downtime (zero‑downtime deployment).

// main.tf
resource "aws_instance" "instance" {
  ami           = "ami-0f64121fa59598bf7"
  instance_type = "t3.micro"
  region        = tolist(var.allowed_region)[0]

  tags = var.tags

  lifecycle {
    create_before_destroy = true
  }
}

2. prevent_destroy

Prevents accidental deletion of a resource.

// main.tf
resource "aws_s3_bucket" "bucket" {
  bucket = "${var.username}-bucket-${var.environment}-day-09"

  lifecycle {
    prevent_destroy = true
  }
}

3. ignore_changes

Ignores changes made to the resource outside of Terraform (e.g., manual tag edits).

// main.tf
resource "aws_instance" "instance" {
  ami           = "ami-0f64121fa59598bf7"
  instance_type = "t3.micro"
  region        = tolist(var.allowed_region)[0]

  tags = var.tags

  lifecycle {
    ignore_changes = [tags]
  }
}

4. replace_triggered_by

Forces replacement of the resource when a specified attribute changes.

// main.tf
resource "aws_instance" "instance" {
  ami           = "ami-0f64121fa59598bf7"
  instance_type = "t3.micro"
  region        = tolist(var.allowed_region)[0]

  tags = var.tags

  lifecycle {
    replace_triggered_by = [instance_type]
  }
}

5. precondition

Validates a condition before creating or updating the resource.

// main.tf
resource "aws_instance" "instance" {
  ami           = "ami-0f64121fa59598bf7"
  instance_type = "t3.micro"
  region        = tolist(var.allowed_region)[0]

  tags = var.tags

  lifecycle {
    precondition {
      condition     = var.instance_type == "t3.micro"
      error_message = "Instance type must be t3.micro"
    }
  }
}

6. postcondition

Validates a condition after the resource has been created or updated.

// main.tf
resource "aws_instance" "instance" {
  ami           = "ami-0f64121fa59598bf7"
  instance_type = "t3.micro"
  region        = tolist(var.allowed_region)[0]

  tags = var.tags

  lifecycle {
    postcondition {
      condition     = aws_instance.instance.instance_state == "running"
      error_message = "Instance is not in running state"
    }
  }
}

Best practices

• Use lifecycle rules to manage resources effectively.
• Test lifecycle rules in a non‑production environment before applying them to production.
• Document the lifecycle rules used in the Terraform code for better understanding and maintenance.
• Regularly review and update lifecycle rules as requirements evolve.
• Be cautious with ignore_changes; it can hide important modifications.
• Apply create_before_destroy to critical resources to avoid downtime.

@piyushsachdeva