Adding diff functionality to JSON output about infracost HOT 11 CLOSED

Running infracost twice (on the master vs current branch, or before vs after the change) is one option. Another option is for infracost to use the output from the terraform plan as the diff of the infrastructure about to be created, and show the cost for that, so infracost is run once. I haven't thought much about the implications of this yet, Atlantis's Checkout Strategy might be relevant to consider.

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@sinabakh correct! Users might appreciate the heads-up and have a little more time to switch.

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@m-yosefpor I'd ignore tfplan as we'll probably remove that (a user pointed out that it's confusing and the json one or the tfdirs one can be used to achieve the same thing anyway). I'd also ignore tfstate for now since that's not supported yet.

If it helps simplify the implementation, the main use-case for this is in CI systems or in a local dev machine, where both version a and b would be using the same tfjson or tfdirs flag since the user is comparing versions of the same thing (i.e. it should be rare to see --tfdir first/path --tfjson second/path). Furthermore, enforcing that version a and b have to be the same "type" doesn't preclude us from supporting mixed formats in the future as we could relax that constraint.

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I was thinking about working on it, but did not get a general idea and what is needed. Could you please elaborate more?

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Notes from discussion

Update JSON output to have the following top level fields:

• existing_resources
• planned_resources
• resource_changes

All these keys have the same field structure as our existing resources field.

Cases to think about:

1. Cost component doesn't change -> existingMonthlyCost == plannedMonthlyCost
2. Existing cost component does change, e.g. instance type is changed.
3. New cost component gets added to existing resource
4. Cost component gets removed from existing resource
5. New resource is added
6. Resource gets removed
7. If only the state is used (--use-tfstate)

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For starters, I will try to change the JSON output and then add the original functionality in a separate MR.
@aliscott Are we ok with these fields names? Or it needs more investigation?

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As a reference:

Terraform uses:

prior_state (when it's a plan file) / values (when it's a state file)
planned_values
resource_changes

Pulumi uses:

oldState
newState
detailedDiff

Options:

Currently I'm thinking this hierarchy:

existingState:
  resources:
    ...
  totalHourlyCost
  totalMonthlyCost

plannedState:
  resources:
    ...
  totalHourlyCost
  totalMonthlyCost

diff:
  resources:
    ...
  totalHourlyCost
  totalMonthlyCost

timeGenerated
resourceSummary

@sinabakh @alikhajeh1 any thoughts?

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@aliscott +1 on those names

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It looks good to me, will start working on it.

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@sinabakh it would be good to have the current top-level JSON keys in the JSON file for the next release 0.7.17, and ensure that infracost report still continues to work (terragrunt users rely on that, I can help you test that if needed), then in 0.8.0 we'll remove the old keys and put in the usual "breaking change" release note.

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@alikhajeh1 So if I understand correctly, we will first add new keys alongside the current ones and then will remove the current ones right? I just started working on this task, thanks for heads up.

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