Day-09: Lifecycle management rules in terraform

Lifecycle rules:
In terraform lifecycle rules are the ones that actually controls how the resource is created/updated/destroyed.

• improve security
• Better maintenance of resources
• control over the resources

There are 6 rules to manage the lifecycle

1. ignore_changes
2. prevent_destroy
3. replace_triggered_by
4. create_before_destroy
5. precondition
6. postcondition

1. Create before destroy:
If there is any change in the resource, this rule ensures that the resource with updated changes is created and then the old resource will be destroyed

• This reduces the downtime while making changes (Zero-downtime deployment)

example:
create an instance before destroying

// main.tf
// Create before destroy to avoid downtime
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:
This rule prevents the resource from being destroyed accidentally.
example:
prevent destroy for s3 bucket

// main.tf
// prevent destroy to avoid accidental deletion
resource "aws_s3_bucket" "bucket" {
    bucket = "${var.username}-bucket-${var.environment}-day-09"

    lifecycle {
      prevent_destroy = true
    }

}

3. Ignore changes:
This rule ignores the changes made to the resource outside terraform.
example:
ignore changes to tags

// main.tf
// Ignore changes to tags
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:
This rule replaces the resource when there is a change in the specified attribute.
example:
replace resource when there is a change in instance_type

// main.tf
// Replace instance when there is a change in instance_type
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:
This rule checks the condition before creating/updating the resource.
example:

// main.tf
// Precondition to check instance type
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:
This rule checks the condition after creating/updating the resource.
example:

// main.tf
// Postcondition to check instance state
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.
• Always test the 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 the lifecycle rules as per the changing requirements and best practices.
• Be cautious while using ignore_changes - it can hide important changes.
• Use create_before_destroy for critical resources to avoid downtime.

@piyushsachdeva