Open Policy Agent (OPA) Validation

The Open Policy Agent (OPA) is the open-source industry standard for policy-as-code validation. It provides a general-purpose policy engine to unify policy enforcement across your stacks.

The OPA (pronounced “oh-pa”) language (Rego) is a high-level declarative language for specifying policy as code. Atmos has native support for the OPA decision-making engine to enforce policies across all the components in your stacks (e.g. for microservice configurations).

This is powerful stuff: because you can define many policies, it's possible to apply different policies depending on where a component is defined in the stacks. For example, it could validate differently based on environments or teams.

Use Cases

Use Open Policy Agent (OPA) policies to validate Atmos stacks and component configurations.

• Validate component config (vars, settings, backend, env, overrides and other sections) using JSON Schema

• Check if the component config (including relations between different component variables) is correct to allow or deny component provisioning using OPA/Rego policies

Usage

Atmos validate component command supports --schema-path, --schema-type and --module-paths command line arguments. If the arguments are not provided, Atmos will try to find and use the settings.validation section defined in the component's YAML config.

tip

Refer to atmos validate component CLI command for more information

# Validate 'vpc' component using OPA policy in the 'plat-ue2-prod' stack
atmos validate component vpc -s plat-ue2-prod --schema-path vpc/validate-vpc-component.rego --schema-type opa

# Validate 'vpc' component using OPA policy in the 'plat-ue2-dev' stack with additional module paths 'catalog/constants'
atmos validate component vpc -s plat-ue2-dev --schema-path vpc/validate-vpc-component.rego --schema-type opa --module-paths catalog/constants

# Validate 'vpc' component using OPA policy in the 'plat-ue2-dev' stack with additional module paths 'catalog'
atmos validate component vpc -s plat-ue2-dev --schema-path vpc/validate-vpc-component.rego --schema-type opa --module-paths catalog

# Validate 'vpc' component in the 'plat-ue2-prod' stack
atmos validate component vpc -s plat-ue2-prod

# Validate 'vpc' component in the 'plat-ue2-dev' stack
atmos validate component vpc -s plat-ue2-dev

# Validate 'vpc' component in the 'plat-ue2-dev' stack with a timeout of 15 seconds
atmos validate component vpc -s plat-ue2-dev --timeout 15

Configure Component Validation

In atmos.yaml, add the schemas section:

atmos.yaml

# Validation schemas for OPA for validating atmos stacks and components
schemas:
  # https://www.openpolicyagent.org
  opa:
    # Can also be set using `ATMOS_SCHEMAS_OPA_BASE_PATH` ENV var, or `--schemas-opa-dir` command-line arguments
    # Supports both absolute and relative paths
    base_path: "stacks/schemas/opa"

In the component manifest, add the settings.validation section:

examples/quick-start-advanced/stacks/catalog/vpc/defaults.yaml

Add the following Rego package in the file stacks/schemas/opa/catalog/constants/constants.rego:

examples/quick-start-advanced/stacks/schemas/opa/catalog/constants/constants.rego

Add the following OPA policy in the file stacks/schemas/opa/vpc/validate-vpc-component.rego:

examples/quick-start-advanced/stacks/schemas/opa/vpc/validate-vpc-component.rego

Use One Policy File or Many

Atmos supports OPA policies for components validation in a single Rego file and in multiple Rego files.

As shown in the example above, you can define some Rego constants, modules and helper functions in a separate file stacks/schemas/opa/catalog/constants/constants.rego, and then import them into the main policy file stacks/schemas/opa/vpc/validate-vpc-component.rego.

You also need to specify the module_paths attribute in the component's settings.validation section. The module_paths attribute is an array of filesystem paths (folders or individual files) to the additional modules for schema validation. Each path can be an absolute path or a path relative to schemas.opa.base_path defined in atmos.yaml. If a folder is specified in module_paths, Atmos will recursively process the folder and all its sub-folders and load all Rego files into the OPA engine.

This allows you to separate the common OPA modules, constants and helper functions into a catalog of reusable Rego modules, and to structure your OPA policies to make them DRY.

Examples

Validate VPC Component in Stacks

Run the following commands to validate the component in the stacks:

atmos validate component vpc -s plat-ue2-prod

Mapping public IPs on launch is not allowed in 'prod'. Set 'map_public_ip_on_launch' variable to 'false'

exit status 1

atmos validate component vpc -s plat-ue2-dev

In 'dev', only 2 Availability Zones are allowed
VPC name must be a valid string from 2 to 20 alphanumeric chars

exit status 1

Validate Before Provisioning

Try to run the following commands to provision the component in the stacks:

atmos terraform apply vpc -s plat-ue2-prod
atmos terraform apply vpc -s plat-ue2-dev

Since the OPA validation policies don't pass, Atmos does not allow provisioning the component in the stacks:

atmos validate vpc --stack=plat-ue2-prod

atmos validate vpc --stack=plat-ue2-dev

Advanced Policy Examples

examples/quick-start-advanced/stacks/schemas/opa/vpc/validate-vpc-component.rego

note

• If a regex pattern in the 're_match' function contains a backslash to escape special chars (e.g. '.' or '-'), it must be escaped with another backslash when represented as a regular Go string ('\.', '\-').

• The reason is that backslash is also used to escape special characters in Go strings like newline (\n).

• If you want to match the backslash character itself, you'll need four slashes.

Policy Execution Context

Atmos allows enforcing custom governance rules based on metadata about Atmos commands and provides a powerful policy evaluation mechanism by passing structured metadata to OPA policies at runtime.

This metadata enables fine-grained control over when certain actions (like terraform apply) are allowed or denied, based on the context in which they're executed.

Policy Metadata

When Atmos runs a command, it supplies an input object to OPA policies that contains detailed contextual information, such as:

• cli_args: a list of the command line arguments and flags (e.g., executing the atmos terraform apply command will generate the ["terraform", "apply"] list)
• vars: a map of variables passed to the command, either via the stack config files or CLI flags
• other contextual attributes such as the stack and component names

Policy Execution Context Example

Below is an OPA policy rule to enforce infrastructure governance during command execution. Specifically, this rule blocks the execution of atmos terraform apply if the variable foo is set to the string "foo".

validate-component.rego

# 'package atmos' is required in all Atmos OPA policies
package atmos

# Atmos looks for the 'errors' (array of strings) output from all OPA policies
# If the 'errors' output contains one or more error messages, Atmos considers the policy failed

# Don't allow `terraform apply` if the `foo` variable is set to `foo`
# The `input` map contains the `cli_args` attribute (a list of the command line arguments and flags)
errors[message] {
  count(input.cli_args) >= 2
  input.cli_args[0] == "terraform"
  input.cli_args[1] == "apply"
  input.vars.foo == "foo"
  message = "the component can't be applied if the 'foo' variable is set to 'foo'"
}

The rule checks if:

• The cli_args list has at least two items
• The command (first item in the cli_args list) is terraform
• The subcommand (second item in the cli_args list) is apply
• The variable foo is set to "foo"

If all conditions are true, the rule generates an error message.

The generated error message is added to the errors array. Atmos interprets the presence of any messages in errors as a policy violation and blocks the operation with the following error:

atmos terraform apply component-1 -s nonprod

the component can't be applied if the 'foo' variable is set to 'foo'

exit status 1