Test Sentinel policies

You can test your Sentinel policies using the built-in testing suite to check your policies against all scenarios and edge cases. Testing gives you confidence in your policy and ensures predictable run outcomes when HCP Terraform enforces your policies during Terraform operations.

Prerequisites

For this tutorial, you will need:

• The Sentinel CLI
• A GitHub account

Clone example Terraform configuration

In your terminal, clone the example code repository. This repository contains an example Sentinel policy and mock data.

$ git clone https://github.com/hashicorp-education/learn-sentinel-write-policy

Navigate to the directory.

$ cd learn-sentinel-write-policy

Review Sentinel policy

Open the restrict-aws-instances-type-and-tag.sentinel file, which contains the Sentinel policy.

This Sentinel policy defines a main rule that consists of two other rules defining infrastructure requirements:

• The mandatory_instance_tags rule checks that all EC2 instances have a Name tag.
• The instance_types_allowed rule checks that EC2 instances are of type t2.micro, t2.small, or t2.medium.

If your new or modified EC2 instances do not meet all of these criteria, Sentinel will flag the run with a FAIL.

Create passing mock data

The configuration in this repository contains pre-generated Sentinel mock data from a Terraform plan run that satisfies the policy defined. You will use this data as the base for your passing and failing Sentinel test cases.

Copy your known passing mock data to a new file with pass in the filename.

$ cp mock-tfplan-v2.sentinel mock-tfplan-pass-v2.sentinel

Write a passing test case

Sentinel requires that your folder structure match test/<policy>/*.json, where <policy> is the name of your policy file without the file extension.

Create a new folder named test with a subdirectory named restrict-aws-instances-type-and-tag.

$ mkdir -p test/restrict-aws-instances-type-and-tag

Navigate to the subdirectory.

$ cd test/restrict-aws-instances-type-and-tag

Create a new file named pass.hcl, then add the following configuration to the file.

mock "tfplan/v2" {
  module {
    source = "../../mock-tfplan-pass-v2.sentinel"
  }
}

test {
    rules = {
        main = true
    }
}

This passing test case uses the passing mock data and expects both the instance_type_allowed and mandatory_instance_tags rules to pass.

Create failing mock data

To write a failing test case, you need to create a mock import data file with values outside your criteria, to simulate infrastructure that does not comply with the policy.

Change into your root learn-sentinel-write-policy directory.

$ cd ../..

Copy the mock-tfplan-v2.sentinel file to a new file with "fail" in the filename.

$ cp mock-tfplan-v2.sentinel mock-tfplan-fail-v2.sentinel

Open the failing mock data file in your text editor and look for the resource_changes collection.

In the resource_changes.aws_instance.ubuntu.change.after block, change:

• instance_type to t2.large, and
• tags from Name to Number.

resource_changes = {
  "aws_instance.ubuntu": {
    "address":  "aws_instance.ubuntu",
    "change": {
      "actions": [
        "create",
      ],
      "after": {
         ## ...
-        "instance_type":   "t2.micro",
+        "instance_type":   "t2.large",
         ## ...
         "tags": {
-          "Name": "Provisioned by Terraform",
+          "Number": "Provisioned by Terraform",
         },
         ## ...
      },
    }
  }
}

Write a failing test case

Change back into your test/restrict-aws-instances-type-and-tag directory.

$ cd test/restrict-aws-instances-type-and-tag

Create a new file named fail.hcl, then add the following configuration to the file. This configuration references the failing mock data and asserts that it should fail the policy.

mock "tfplan/v2" {
  module {
    source = "../../mock-tfplan-fail-v2.sentinel"
  }
}

test {
    rules = {
        main = false
    }
}

Test your policy in the Sentinel CLI

Change into your root learn-sentinel-policies directory.

$ cd ../..

Now use the test command to run the test cases you defined for your policy. Sentinel automatically loads the test cases in the /test directory and passes the subdirectory that matches the policy name to the test command.

$ sentinel test restrict-aws-instances-type-and-tag.sentinel
PASS - restrict-aws-instances-type-and-tag.sentinel
  PASS - test/restrict-aws-instances-type-and-tag/fail.hcl
  PASS - test/restrict-aws-instances-type-and-tag/pass.hcl

Both your passing and failing tests return PASS because both test cases evaluated to their expected values — the passing scenario evaluated to true and the failing scenario evaluated to false.

Run the test with the -verbose flag to get more information about the test.

$ sentinel test -verbose restrict-aws-instances-type-and-tag.sentinel
Installing test modules for test/restrict-aws-instances-type-and-tag/fail.hcl
Installing test modules for test/restrict-aws-instances-type-and-tag/pass.hcl

PASS - restrict-aws-instances-type-and-tag.sentinel
  PASS - test/restrict-aws-instances-type-and-tag/fail.hcl

    trace:
      restrict-aws-instances-type-and-tag.sentinel:39:1 - Rule "main"
        Description:
          Main rule that requires other rules to be true

        Value:
          false

      restrict-aws-instances-type-and-tag.sentinel:32:1 - Rule "instance_type_allowed"
        Description:
          Rule to restrict instance types

        Value:
          false
  PASS - test/restrict-aws-instances-type-and-tag/pass.hcl

    trace:
      restrict-aws-instances-type-and-tag.sentinel:39:1 - Rule "main"
        Description:
          Main rule that requires other rules to be true

        Value:
          true

      restrict-aws-instances-type-and-tag.sentinel:32:1 - Rule "instance_type_allowed"
        Description:
          Rule to restrict instance types

        Value:
          true

      restrict-aws-instances-type-and-tag.sentinel:23:1 - Rule "mandatory_instance_tags"
        Description:
          Rule to enforce "Name" tag on all instances

        Value:
          true

The -verbose flag returns the steps at which each test passes or fails your criteria.

Next steps

You created test policies and ran test cases using the Sentinel CLI. To learn more about Sentinel, review the following resources:

• Learn how to Generate Mock Policy Data
• Review how to Write a Sentinel Policy
• Learn how to Upload a Sentinel Policy Set to HCP Terraform