Terraform Plan Explainer

What Is a Terraform Plan Explainer

A Terraform plan explainer interprets and summarizes the output of terraform plan—the command that previews infrastructure changes before they are applied. Terraform, by HashiCorp, is the leading infrastructure-as-code (IaC) tool used to provision and manage cloud resources across AWS, Azure, GCP, and hundreds of other providers. The plan output shows exactly what Terraform will create, modify, or destroy, but its raw format can be dense and difficult to parse quickly.

Reading Terraform plans correctly is critical: a misunderstood plan can lead to accidental resource deletion, security group changes that expose services, or cost overruns from oversized instances. A plan explainer breaks down the raw output into clear summaries, highlights risky changes, and identifies the resources affected.

How Terraform Plans Work

When you run terraform plan, Terraform compares your configuration files (.tf) against the current state file (terraform.tfstate) and the actual cloud infrastructure. It then generates an execution plan showing the difference:

Key sections of a plan:

• Resource changes: The core of the plan, showing each resource and its planned changes
• Known after apply: Values that can only be determined after creation (like auto-generated IDs)
• Outputs: Changes to output values that may affect dependent configurations
• Move summary: Resources being refactored to new addresses without destruction

A typical plan output line looks like:

# aws_instance.web will be updated in-place
~ resource "aws_instance" "web" {
    ~ instance_type = "t3.micro" -> "t3.large"
  }

Common Use Cases

• Change review: Understand exactly what infrastructure changes will occur before applying
• Pull request review: Include plan output in PR descriptions for team review of IaC changes
• Cost impact analysis: Identify resource size changes that will affect cloud spending
• Security review: Spot changes to security groups, IAM policies, or encryption settings
• Blast radius assessment: Determine how many and which resources a change affects

Best Practices

1. Always run plan before apply — Never use terraform apply without reviewing the plan first, especially in production
2. Watch for destroy operations — The - and -/+ symbols indicate data loss risk; understand why before proceeding
3. Use -target for scoped changes — When making large changes, plan against specific resources to reduce blast radius
4. Save plans for CI/CD — Use terraform plan -out=plan.tfplan to save plans that can be applied exactly as reviewed
5. Review security-sensitive resources carefully — Changes to aws_security_group, aws_iam_policy, and encryption settings deserve extra scrutiny