Provisioners and Lifecycle

Provisioners let you execute scripts on resources after creation, while lifecycle rules control how Terraform creates, updates, and destroys resources. This lesson covers both concepts, including when (and when not) to use provisioners.

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Lifecycle Rules

Lifecycle rules modify the default behaviour of resource creation, update, and destruction:

resource "aws_instance" "web" {
  ami           = var.ami_id
  instance_type = var.instance_type

  lifecycle {
    create_before_destroy = true
  }
}

Available Lifecycle Arguments

Argument	Purpose
create_before_destroy	Create the replacement before destroying the original
prevent_destroy	Prevent accidental destruction of critical resources
ignore_changes	Ignore changes to specific attributes
replace_triggered_by	Force replacement when a referenced value changes
precondition	Validate assumptions before applying
postcondition	Validate outcomes after applying

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create_before_destroy

By default, Terraform destroys the old resource before creating the new one. This argument reverses the order:

resource "aws_instance" "web" {
  ami           = var.ami_id
  instance_type = "t3.micro"

  lifecycle {
    create_before_destroy = true
  }
}

Use case: Zero-downtime deployments — the new instance is ready before the old one is removed.

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prevent_destroy