Automating Terraform Testing: From Unit Tests to End-to-End Validation

Infrastructure as Code (IaC) involves both risk management and the deployment of scalable solutions. Automated testing is the foundation of Terraform's risk mitigation method, guaranteeing that your infrastructure operates as anticipated even when subjected to frequent change.

But what does automated Terraform testing look like? Today, we'll look at how the three pillars of testing—unit, integration, and end-to-end tests—work together to protect your infrastructure. We'll also walk through a basic Python-based example to demonstrate the benefits of automated testing in action.

Understanding the Types of Tests

1. Unit Tests: Testing the Smallest Components

Unit tests validate independent parts of your infrastructure. Consider these to be standalone tests of a single module, resource, or variable. For example:

• Does your vpc module create the correct number of subnets?

• Are your IAM policies correctly configured?

These tests are often mocked, which means that no real resources are deployed. The objective is to guarantee that the logic and settings are valid while avoiding the cost of providing infrastructure.

2. Integration Tests: Ensuring Components Work Together

Integration tests verify how various components of your infrastructure interact. For example:

• Does your VPC properly connect to your database instance?

• Are your security groups and load balancers correctly configured to route traffic?

3. End-to-End Tests: Validating the Full System

End-to-end tests replicate actual workflows or user scenarios. For example:

• Can a web server handle a request and return the expected response?

• Is the application accessible from the internet with the expected security constraints?

This type of test involves deploying a full environment, making it the most comprehensive but also the most resource-intensive.

Using Terratest to Set Up a Basic Automated Test

Let's imagine we want to make sure our Terraform-managed web server responds correctly to HTTP requests. Here's how to create an automated test with Terratest, a Go-based testing framework optimized for infrastructure testing.

Prerequisites

1. Terraform configuration for deploying an EC2 instance running a web server.

2. A Go development environment set up on your local machine.

3. Terratest library installed (go get github.com/gruntwork-io/terratest/modules/...).

package test

import (
    "testing"
    "time"

    "github.com/gruntwork-io/terratest/modules/http-helper"
    "github.com/gruntwork-io/terratest/modules/terraform"
)

func TestWebServerResponse(t *testing.T) {
    t.Parallel()

    terraformOptions := &terraform.Options{
        // Path to the Terraform configuration files
        TerraformDir: "../terraform",
    }

    // Ensure resources are cleaned up at the end of the test
    defer terraform.Destroy(t, terraformOptions)

    // Initialize and apply Terraform configuration
    terraform.InitAndApply(t, terraformOptions)

    // Fetch the public IP of the EC2 instance from Terraform outputs
    publicIp := terraform.Output(t, terraformOptions, "public_ip")

    // Construct the URL to the web server
    url := "http://" + publicIp

    // Define a retryable HTTP test to ensure the server is up and running
    maxRetries := 10
    timeBetweenRetries := 5 * time.Second
    http_helper.HttpGetWithRetry(t, url, nil, 200, "Welcome", maxRetries, timeBetweenRetries)
}

Running the test:

Initialize the Go Environment: Ensure that your Go development environment is properly set up by running:

go mod init test
go mod tidy

Execute the test suite:

go test -v

CI/CD Pipeline for Terraform Testing

Automating these tests further with a CI/CD pipeline ensures that every code change is rigorously validated. A typical pipeline might:

1. Lint your Terraform code with tflint.

2. Validate configurations with terraform validate.

3. Plan and review changes with terraform plan.

4. Run Tests: Automatically execute unit, integration, and end-to-end tests using Terratest.

5. Report Results: Fail the pipeline if any test does not pass.

Wrapping Up

Terratest is the gold standard for testing Terraform code in production. By writing unit, integration, and end-to-end tests, you can catch errors early and validate your infrastructure with confidence. Adding these tests to a CI/CD pipeline ensures that every change is thoroughly vetted, reducing risk and enabling faster iterations.