README.md

Sentinel Example

This example shows how to write cost policies with HashiCorp's Sentinel. For simplicity, this example evaluates policies with the Sentinel CLI (a.k.a. Sentinel Simulator). The point of this example is to show how a policy could be written against the Infracost JSON format, not how to run Sentinel, since that's tied to HashiCorp's cloud platform.

When the policy checks pass, the GitHub Action step called "Check Policies" passes and outputs Policy check passed. in the action logs. When the policy checks fail, that step fails and the action logs show the Sentinel output indicating failing policies.

Create a policy file (e.g. `policy.policy) that checks a global parameter 'breakdown' containing the Infracost JSON:

import "strings"

limitTotalDiff = rule {
    float(breakdown.totalMonthlyCost) < 1500
}

awsInstances = filter breakdown.projects[0].breakdown.resources as _, resource {
	strings.split(resource.name, ".")[0] is "aws_instance"
}

limitInstanceCost = rule {
  all awsInstances as _, instance {
  	float(instance.hourlyCost) <= 2.00
  }
}

instanceBaseCost = func(instance) {
  cost = 0.0
 	for instance.costComponents as cc {
    cost += float(cc.hourlyCost)
  }
  return cost
}

instanceIOPSCost = func(instance) {
  cost = 0.0
 	for instance.subresources as sr {
    for sr.costComponents as cc {
      if cc.name == "Provisioned IOPS" {
        cost += float(cc.hourlyCost)
      }
    }
  }
  return cost
}

limitInstanceIOPSCost = rule {
   all awsInstances as _, instance {
  	instanceIOPSCost(instance) <= instanceBaseCost(instance)
  }
}

main = rule {
    limitTotalDiff and
    limitInstanceCost and
    limitInstanceIOPSCost
}

Then use Sentinel to test infrastructure cost changes against the policy.

name: Sentinel
on: [pull_request]

jobs:
  sentinel:
    name: Sentinel
    runs-on: ubuntu-latest

    steps:
      - uses: actions/checkout@v2
      
      - name: Setup Infracost
        uses: infracost/actions/setup@v1
        with:
          api-key: ${{ secrets.INFRACOST_API_KEY }}

      - name: Setup Sentinel
        run: |
          mkdir /tmp/sentinel
          curl -o /tmp/sentinel/sentinel.zip https://releases.hashicorp.com/sentinel/0.18.4/sentinel_0.18.4_linux_amd64.zip
          unzip -d /tmp/sentinel /tmp/sentinel/sentinel.zip
          echo "/tmp/sentinel" >> $GITHUB_PATH

      - name: Run Infracost
        run: infracost breakdown --path=examples/sentinel/code/plan.json --format=json --out-file=/tmp/infracost.json
        
      - name: Run Sentinel
        run: sentinel apply -global breakdown="$(cat /tmp/infracost.json)" examples/sentinel/policy/policy.policy | tee /tmp/sentinel.out

      - name: Check Policies
        run: |
          result=$(</tmp/sentinel.out)
          if [ "$result" != "Pass - policy.policy" ]; then
            echo -e "::error::Policy check failed:\n$result"
            exit 1
          else
            echo "::info::Policy check passed."
          fi