How To Make Cloud Cost Estimation With Terraform

Author(s): Guillaume Vincent

Originally published on Towards AI.

Automate cloud cost estimation in your pull requests

Cloud computing is great for growing your business quickly. You only pay for what you use. It’s a double-edged sword, you can save a lot or pay a lot. That’s why analyzing your bill every day is an important point. Cloud computing providers offer tools to analyze costs. It is a boring task but necessary.

In this article, we’ll see how to do it in a more fun way. As you know cloud infrastructure is often managed as code. The most commonly used tool is Terraform. It is able to track changes between the code and what exists.

Each change involves costs and we will see how to add this dimension to Terraform with Infracost. In the end, you will know how to measure the cost of a change and integrate it into your code review. The project taken as an example here is presented here

What Is Infracost?

“Infracost shows cloud cost estimates for infrastructure-as-code projects such as Terraform. It helps DevOps, SRE and developers to quickly see a cost breakdown and compare different options upfront.” https://github.com/infracost/infracost

Infracost is a simple executable. So it’s super simple to install and use in a CI/CD pipeline. Here we will use it with Terraform but in the project roadmap, it is planned to use it with other tools like Pulumi or CloudFormation.

Once downloaded, Infracost just needs to be registered, and off you go!

$ infracost register 
Please enter your name and email address to get an API key. See our FAQ (https://www.infracost.io/docs/faq) for more details. 
Name: Guillaume Vincent 
Email: [email protected] 
Thank you Guillaume Vincent! Your API key is: xxxxxxxxxxxxxxxxxxxxxxxxx 
Success: Your API key has been saved to /Users/gvincent/.config/infracost/credentials.yml You can now run infracost breakdown --path=... and point to your Terraform directory or JSON/plan file.

Estimating The Cost

The full breakdown of costs

Infracost can be executed to have the full monthly breakdown of costs :

$ infracost breakdown --path terraform_nlb_static_eips
Detected Terraform directory at terraform_nlb_static_eips
 U+2714 Running terraform init
 U+2714 Running terraform plan
 U+2714 Running terraform show
U+2714 Calculating monthly cost estimate
Project: terraform_nlb_static_eips
Name Monthly Qty Unit Monthly Cost
aws_autoscaling_group.webserver
 └─ aws_launch_configuration.webserver
 ├─ Instance usage (Linux/UNIX, on-demand, t2.micro) 0 hours $0.00
 ├─ EC2 detailed monitoring 0 metrics $0.00
 └─ root_block_device
 └─ Storage (general purpose SSD, gp2) 0 GB-months $0.00
aws_eip.nlb
 └─ IP address (if unused) 730 hours $3.65
aws_lb.this
 ├─ Network load balancer 730 hours $16.42
 └─ Load balancer capacity units Cost depends on usage: $0.006 per LCU-hours
PROJECT TOTAL $20.07

You can play modifying the code and re-run the previous command to have an updated cost estimation of the new version.

The diff monthly costs between the current and planned state

Terraform updates, deletes, or creates resources saves the infrastructure state in a file. When you deploy a new version, Terraform is able to detect what needs to be updated, removed, created following your changes. Infracost relies on this state file to show you the cost impact of the current change from the plan.

To illustrate to you that, I deployed the initial code version using terraform apply. Then I scaled out the current EC2 instance type from t2.micro to m4.large:

$ git --no-pager diff 
diff --git a/launch-configuration.tf b/launch-configuration.tf
index db22265..bcf90c7 100644
--- a/launch-configuration.tf
+++ b/launch-configuration.tf
@@ -1,7 +1,7 @@
 resource "aws_launch_configuration" "webserver" {
 name_prefix = "${local.name_prefix}_webserver"
 image_id = data.aws_ami.ubuntu.image_id
- instance_type = "t2.micro"
+ instance_type = "m4.large"
 security_groups = [
 aws_security_group.public.id]
 user_data = data.template_cloudinit_config.this.rendered

Then I launch the Infracost command with diff argument :

$ infracost diff --path . 
Detected Terraform directory at .
 U+2714 Running terraform plan
 U+2714 Running terraform show
U+2714 Calculating monthly cost estimate
Project: .
~ aws_autoscaling_group.webserver
 +$64.53 ($11.37 -> $75.90)
~ aws_launch_configuration.webserver
- Instance usage (Linux/UNIX, on-demand, t2.micro)
 -$8.47
+ Instance usage (Linux/UNIX, on-demand, m4.large)
 +$73.00
Monthly cost change for .
Amount: +$64.53 ($27.79 -> $92.32)
Percent: +232%

The diff usage is perfect to be added in complement of a code review to justify and argue the cost of a change.

Refine cost estimation with usage file

You can feed Infracost with a usage file to refine the cost estimation :

This file is added to the command arguments :

$ infracost breakdown --path . --format html > report.html

Report

You can generate reports to share cost estimation in different formats (HTML, JSON, etc..) :

Adding Infracost To GitHub Action

You can integrate Infracost with a lot of CI/CD solutions. Here we’re going to focus on Github Action because it is easy to reuse existing actions from the marketplace. In the workflow, we’re going to use the following actions :

• Infracost action
• AWS action to configure credentials

The configuration of the workflow is configured in the terraform git project in.github/workflows/infracost.yml:

I committed the previous instance type change to a branch named dev and opened a pull request. This triggers the steps of the workflow :

When the workflow executed ends, the Infracost output is visible inside the pull request :

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Published via Towards AI