10 Meta Arguments

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📌 Mutable vs Immutable Infrastructure (Terraform)

🧱 Mutable Infrastructure

Info

Can be changed / modify

• Can be changed/modify
• Change existing resources in place
• Terraform shows ~ (update)

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🧊 Immutable Infrastructure

Success

Immutable == cannot be changed

• Immutable == cannot be changed
• Immutable resource has to be — -/+ (destroy & create) cannot be ~ (update)
• Never allow to modify existing resources
• Replace with new ones

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⚙️ Terraform Behavior

Terraform

• Decides replace vs update per resource
• AMI / disk / db name → replacement
• Small config change → in-place update

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🔁 Terraform Lifecycle Rules

📌 What are lifecycle rules?

Note

Lifecycle rules control how Terraform creates, updates, and destroys resources.

Defined inside a resource block using:

lifecycle {
  ...
}

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📊 Terraform Lifecycle Rules

Lifecycle Rule	Effect on Resource	Purpose	Common Use Case
create_before_destroy	Creates new resource first, then deletes old	Avoid downtime during replacement	ALB, ASG, EC2, zero-downtime deployments. Used for immutable deployments
prevent_destroy	Blocks terraform destroy or replacement	Protect critical resources	Must Add On Prod DBs, VPCs, S3 buckets
ignore_changes	Terraform won’t act on listed changes	Ignore specific attribute updates	Tags, autoscaling changes, external updates
replace_triggered_by	Resource replaced when referenced item changes	Force replacement based on dependency	AMI change, config dependency refresh

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Syntex

lifecycle {
  ignore_changes = [tags, user_data]    OR   ignore_changes = all
}

• Forces replacement when another resource changes

lifecycle {
  replace_triggered_by = [aws_ami.new.id]
}

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