Getting Started with OPA

Setting Up OPA Overview

To get started with OPA, you first need to install it on your local machine or within your development environment. OPA can be installed in several ways, depending on your operating system and preferences.

Installation Methods:

• Via Precompiled Binaries:

  • Download the appropriate OPA binary from the OPA GitHub releases page.

  • Extract the binary and move it to a directory in your PATH (e.g., /usr/local/bin on Unix-based systems).

  • Verify the installation by running opa version in your terminal.

• Via Homebrew (macOS):

  • Install OPA using Homebrew by running:

    brew install opa

  • Verify the installation by running opa version.

• Via Docker:

  • OPA is also available as a Docker image, which is ideal for containerized environments. Pull the latest OPA image using:

    docker pull openpolicyagent/opa:latest

  • Run OPA in a container:

    docker run -it openpolicyagent/opa:latest run --server

Introduction to the OPA CLI

The OPA CLI is a powerful tool for interacting with OPA, allowing you to evaluate policies, debug Rego code, and manage OPA instances. The CLI provides various commands to work with policies and data.

Key Commands:

• opa run: Starts the OPA server or evaluates policies directly from the command line.

• opa eval: Evaluates a Rego policy and returns the result. This is useful for testing and debugging.

• opa test: Runs tests against your Rego policies to ensure they behave as expected.

Example Usage:

• To start an interactive OPA shell:

  opa run

  This will drop you into a shell where you can directly interact with OPA, write policies, and evaluate queries.

• To evaluate a simple policy:

  opa eval "data.example.allow"

  This command evaluates the policy defined in data.example.allow and returns the result.

Running Your First OPA Policy

Now that OPA is installed and you’re familiar with the CLI, let’s write and evaluate your first policy. In this example, we’ll create a simple policy that checks if a user is authorized to perform an action.

Step 1: Writing the Policy

Create a new file called example.rego with the following content:

package example

default allow = false

allow {
    input.user == "alice"
    input.action == "read"
}

• Explanation:

  • The policy is defined in the example package.

  • The allow rule determines whether the action is permitted. The default value is false, meaning actions are denied by default.

  • The allow rule only returns true (i.e., allows the action) if the input.user is "alice" and the input.action is "read".

Step 2: Evaluating the Policy

To evaluate this policy, use the following command:

opa eval --input - --data example.rego "data.example.allow" <<EOF
{
  "user": "alice",
  "action": "read"
}
EOF

• Explanation:

  • The --input flag provides the input data in JSON format.

  • The --data flag loads the example.rego file containing the policy.

  • The "data.example.allow" argument specifies the policy to evaluate.

Step 3: Understanding the Output

If the input matches the conditions defined in the policy, OPA will return:

code{
  "result": [
    {
      "expressions": [
        {
          "value": true
        }
      ]
    }
  ]
}

This output indicates that the action is allowed for the user "alice" to perform the "read" action.

If the input does not match, the result would be false, meaning the action is denied.

Debugging and Testing Rego Policies

Debugging and testing are crucial when working with policies to ensure they behave as expected.

• Using opa eval for Debugging:

  • Add trace statements in your Rego code using the trace function to help debug the evaluation process. For example:

    allow {
        trace("Checking if user is alice")
        input.user == "alice"
        input.action == "read"
    }

  • Run the evaluation with the --explain=full flag to get detailed traces of the evaluation.

• Writing Tests:

  • OPA allows you to write tests for your policies using the opa test command. Here’s an example of a test file:

    package example
    
    test_allow_read {
        result := data.example.allow with input as {"user": "alice", "action": "read"}
        result == true
    }
    
    test_deny_write {
        result := data.example.allow with input as {"user": "bob", "action": "write"}
        result == false
    }

  • Run the tests using:

    opa test example.rego

Summary

In this lesson, you’ve learned how to set up OPA, use the OPA CLI, and write and evaluate a simple policy. You’ve also explored how to debug and test Rego policies to ensure they work correctly. This foundational knowledge will be critical as you delve into more complex policies and integrations in future lessons.