Recently we released a new infracost diff command inspired by git diff. This shows a diff of monthly cloud cost estimates between the current and planned state of Terraform projects. At a high-level this might seems like a simple exercise of subtracting the current state's cost estimate from the planned state, but cloud costs are rarely that simple to deal with. Let's take a look at the following screenshot to understand some of the nuances.

1. The aws_instance is being changed, which reduces the cost by $125/month (from $743 to $618).

2. AWS EC2 has many different cost components, so to explain what caused the above change, we also flag the sub-resource ebs_block_device[0] that changed (the first attached block device). Underneath it, we show the cost component that caused the actual cost diff, Provisioned IOPS SSD Storage (io1); i.e. reducing the size of that volume can save $1500/year. For those who have done this in production, they know it's not a one-click change as you need to create a new EBS volume and copy over the data. What Infracost enables you to do is to quickly tell how much such a change would save you, then decide if it's worth it.

3. A new aws_lambda_function is being added. Since we don't know how much it's going to be used, we can't show a cost estimate. But we can still show you the prices you'll be charged for: $0.20 per 1M requests and a tiny amount per GB-second. This is a usage-based resource, so if you like you can create a yaml file to provide usage estimates and get a cost estimate. It's hard to think in GB-seconds, so we enable you to input the average request duration and we'll do the math to map that to GB-seconds based on the memory_size of your function and any rounding rules that AWS applies.
   

   version: 0.1
   
   resource_usage:
     aws_lambda_function.hello_world:
       monthly_requests: 100000000 # Monthly number of requests.
       request_duration_ms: 250 # Average duration of each request in milliseconds.
   

4. Finally we show a summary at the bottom: the EC2 instance change reduces the cost by 17%, and you can use the above yaml file to do simple what-if analysis on the Lambda costs.

The new infracost diff command is used by our CI/CD integrations and is open source alongside the rest of Infracost. We look forward to hearing what you do with it via GitHub issues or our community Slack!