13.3 Reading Terraform Source | Reading & Contributing to Open Source

Repository Structure

hashicorp/terraform:

Directory	What's There
`command/`	CLI commands (plan, apply, init)
`internal/terraform/`	Core Terraform engine
`internal/states/`	State management
`internal/plans/`	Plan generation
`internal/providers/`	Provider interface

How Plan/Apply Works

terraform plan
  → command/plan.go
    → internal/terraform/context.go (Plan method)
      → Builds a dependency graph of resources
      → Walks the graph, comparing desired vs current state
      → For each resource: calls provider.PlanResourceChange()
      → Outputs a plan diff

terraform apply
  → Takes the plan
  → Walks the graph again
  → For each resource: calls provider.ApplyResourceChange()
  → Updates state file

How Providers Work

Providers are separate binaries that communicate via gRPC (Module 9!):

// A Terraform provider implements this interface:
type Provider interface {
    GetSchema() providers.GetSchemaResponse
    PlanResourceChange(providers.PlanResourceChangeRequest) providers.PlanResourceChangeResponse
    ApplyResourceChange(providers.ApplyResourceChangeRequest) providers.ApplyResourceChangeResponse
    ReadResource(providers.ReadResourceRequest) providers.ReadResourceResponse
    // ...
}

This is why you can write a Terraform provider in Go — it's just a gRPC server.