038 Project 38: Terraform Cost Estimator

038 Build a Terraform Cost Estimator (Simple)

Estimate monthly change by resource action counts.

Full `main.go`

package main

import (
    "encoding/json"
    "fmt"
    "os"
)

type Plan struct {
    ResourceChanges []struct {
        Type   string `json:"type"`
        Change struct {
            Actions []string `json:"actions"`
        } `json:"change"`
    } `json:"resource_changes"`
}

var cost = map[string]float64{
    "aws_instance": 15.0,
    "aws_db_instance": 120.0,
    "aws_lb": 25.0,
}

func main() {
    if len(os.Args) != 2 {
        fmt.Println("usage: tf-cost <plan.json>")
        os.Exit(2)
    }
    b, err := os.ReadFile(os.Args[1])
    if err != nil {
        panic(err)
    }
    var p Plan
    if err := json.Unmarshal(b, &p); err != nil {
        panic(err)
    }

    var delta float64
    for _, rc := range p.ResourceChanges {
        c := cost[rc.Type]
        if c == 0 {
            continue
        }
        for _, a := range rc.Change.Actions {
            if a == "create" {
                delta += c
            }
            if a == "delete" {
                delta -= c
            }
        }
    }
    fmt.Printf("estimated monthly delta: $%.2f\n", delta)
}

Step-by-Step Explanation

Export Terraform plan in JSON form.
Parse resource_changes into typed structs.
Classify actions such as create/update/delete/replace.
Compute risk, policy, or cost summaries.
Gate CI based on explicit thresholds.

Code Anatomy

Decoder layer parses JSON file.
Analysis layer converts raw actions into signals.
Reporting layer prints summary and sets exit status.

Learning Goals

Build deterministic IaC review tools.
Enforce infrastructure policy as code.
Reduce risky deploys with automated checks.