Hi guys!. How's it doing?. I'm trying to execute an

infracost diff

command following the instructions that we can find in the documentation (https://www.infracost.io/docs/features/cli_commands/#option-2-terraform-plan-json-1), but there's something that doesn't make sense for me. If the terraform plan says that it's going to change 1 resource, why the diff command is trying to estimate all the resources and the result is that the whole deployment is added and none is changed?.

Hi Aléxis, that is odd. The plan json contains the prior state and a list of resource changes.

infracost diff

looks at the resource changes so I’m not sure why it would be showing something different than showing the terraform plan. You’re using the commands the same as in the docs, without any other flags or anything?

terraform init
terraform plan -out tfplan.binary
terraform show -json tfplan.binary > plan.json

infracost diff --path plan.json

Yes, I did it

can you see what’s in the

resource_changes

key of the plan.json? If you have

jq

available you do something like:

plan.json | jq .resource_changes

Let me I check it.

OK, hmmm. Let’s pick one resource that is unexpectedly showing in the diff and see how it shows up in the plan.json under the

planned_values.root_module

and

prior_state.values.root_module

keys.

Those are the commands I'm trying to run:

terraform init && 
terraform plan -out 'tfplan.binary' && 
terraform show -json 'tfplan.binary' > 'pr-dev-onboarding-cloud-infrastructure-tfplan.json' && 
echo 'tf show: pr-dev-onboarding-cloud-infrastructure-tfplan.json' && 
cat 'pr-dev-onboarding-cloud-infrastructure-tfplan.json' && 
INFRACOST_VCS_PROVIDER='github' 
INFRACOST_VCS_REPOSITORY_URL='<https://github.com/xxxxxxxx/xxxxxxxxxxxxxxx>' 
INFRACOST_VCS_PULL_REQUEST_URL='<https://github.com/xxxxxxxxxxxx/xxxxxxxxxxxxxxxxxxxxx/pull/20676>' 
INFRACOST_VCS_BASE_BRANCH='development' 
INFRACOST_VCS_PULL_REQUEST_AUTHOR='Alexis Mosquera' 
INFRACOST_VCS_COMMIT_AUTHOR_EMAIL='<mailto:XXXXXXXXXXXX@XXXXXXXXX.com|XXXXXXXXXXXX@XXXXXXXXX.com>' 
INFRACOST_VCS_PULL_REQUEST_TITLE='feat(infracost): enable multiprojects pr' 
INFRACOST_VCS_BRANCH='PR-20676' 
INFRACOST_VCS_COMMIT_SHA='XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX' 
INFRACOST_VCS_COMMIT_MESSAGE='feat(infracost): test special chars' 
INFRACOST_VCS_COMMIT_TIMESTAMP='1662731045' 
infracost diff --path 'pr-dev-onboarding-cloud-infrastructure-tfplan.json' && 
infracost output --format diff --path 'pr-dev-onboarding-cloud-infrastructure-tfplan.json'

Let me check the keys you requested.

infracost output

expects an infracost json, not the plan json so that is where the error is coming from

Ohh... ok. My bad 😄 . I will fix it and test

No that’s why it’s showing everything as being created.

hmmmm...... So how does tf know what are the changed resources?

yeah for the diff we’re comparing the prior_state vs planned_state. (I was wrong when I said it looks at resource_changes, it only uses that in a special case)

Infracost is calculating according to the 0.2+ version of the json format, but I am using an older version the 0.1. It looks like in the 0.2+ the changes are populated in the .prior_state key

If I generate with tf 1.29 I get the plan json with

{"format_version":"1.1","terraform_version":"1.2.9",

Yes, but if you try with the 0.12.31, you wont get the same infracost results.

Yeah I’m running 0.12.31 and I see the

prior_state

populated and the infracost diff working correctly:

...
  "prior_state": {
    "format_version": "0.1",
    "terraform_version": "0.12.31",
    "values": {
      "outputs": {
        "aws_instance_type": {
          "sensitive": false,
          "value": "t2.nano"
        }
      },
      "root_module": {
        "resources": [
          {
            "address": "aws_instance.web_app",
            "mode": "managed",
            "type": "aws_instance",
            "name": "web_app",
            "provider_name": "aws",
            "schema_version": 1,
            "values": {
              "ami": "ami-079ca844e323047c2",
              "arn": "arn:aws:ec2:us-east-1:186080867442:instance/i-0fd93ce246912900e",

hmm..... Maybe, the issue would be with azure and no with aws.

I still get prior_state with that set

hmmmm.... I'm going to test something else.

After conferring with the team, consensus is that infracost+terraform 0.12.31 should be fine, and it is odd that the prior_state doesn’t show up in plan json. Just to confirm, you’re not using a custom build of terraform or anything like that, right?