The package and individual rules in a module can be annotated with a rich set of metadata.

# METADATA
# title: My rule
# description: A rule that determines if x is allowed.
# authors:
# - John Doe <john@example.com>
# entrypoint: true
allow {
  ...
}

Annotations are grouped within a metadata block, and must be specified as YAML within a comment block that must start with # METADATA. Also, every line in the comment block containing the annotation must start at Column 1 in the module/file, or otherwise, they will be ignored.

Annotations

Scope

Annotations can be defined at the rule or package level. The scope annotation in a metadata block determines how that metadata block will be applied. If the scope field is omitted, it defaults to the scope for the statement that immediately follows the annotation. The scope values that are currently supported are:

• rule - applies to the individual rule statement (within the same file). Default, when metadata block precedes rule.
• document - applies to all of the rules with the same name in the same package (across multiple files)
• package - applies to all of the rules in the package (across multiple files). Default, when metadata block precedes package.
• subpackages - applies to all of the rules in the package and all subpackages (recursively, across multiple files)

Since the document scope annotation applies to all rules with the same name in the same package and the package and subpackages scope annotations apply to all packages with a matching path, metadata blocks with these scopes are applied over all files with applicable package- and rule paths. As there is no ordering across files in the same package, the document, package, and subpackages scope annotations can only be specified once per path. The document scope annotation can be applied to any rule in the set (i.e., ordering does not matter.)

Example

# METADATA
# scope: document
# description: A set of rules that determines if x is allowed.

# METADATA
# title: Allow Ones
allow {
    x == 1
}

# METADATA
# title: Allow Twos
allow {
    x == 2
}

Title

The title annotation is a string value giving a human-readable name to the annotation target.

Example

# METADATA
# title: Allow Ones
allow {
  x == 1
}

# METADATA
# title: Allow Twos
allow {
  x == 2
}

Description

The description annotation is a string value describing the annotation target, such as its purpose.

Example

# METADATA
# description: |
#  The 'allow' rule...
#  Is about allowing things.
#  Not denying them.
allow {
  ...
}

Related Resources

The related_resources annotation is a list of related-resource entries, where each links to some related external resource; such as RFCs and other reading material. A related-resource entry can either be an object or a short-form string holding a single URL.

Object Related-resource Format

When a related-resource entry is presented as an object, it has two fields:

• ref: a URL pointing to the resource (required).
• description: a text describing the resource.

String Related-resource Format

When a related-resource entry is presented as a string, it needs to be a valid URL.

Examples

# METADATA
# related_resources:
# - ref: https://example.com
# ...
# - ref: https://example.com/foo
#   description: A text describing this resource
allow {
  ...
}

# METADATA
# related_resources:
# - https://example.com/foo
# ...
# - https://example.com/bar
allow {
  ...
}

Authors

The authors annotation is a list of author entries, where each entry denotes an author. An author entry can either be an object or a short-form string.

Object Author Format

When an author entry is presented as an object, it has two fields:

• name: the name of the author
• email: the email of the author

At least one of the above fields are required for a valid author entry.

String Author Format

When an author entry is presented as a string, it has the format { name } [ "<" email ">"]; where the name of the author is a sequence of whitespace-separated words. Optionally, the last word may represent an email, if enclosed with <>.

Examples

# METADATA
# authors:
# - name: John Doe
# ...
# - name: Jane Doe
#   email: jane@example.com
allow {
  ...
}

# METADATA
# authors:
# - John Doe
# ...
# - Jane Doe <jane@example.com>
allow {
  ...
}

Organizations

The organizations annotation is a list of string values representing the organizations associated with the annotation target.

Example

# METADATA
# organizations:
# - Acme Corp.
# ...
# - Tyrell Corp.
allow {
  ...
}

Schemas

The schemas annotation is a list of key value pairs, associating schemas to data values. In-depth information on this topic can be found here.

Schema Reference Format

Schema files can be referenced by path, where each path starts with the schema namespace, and trailing components specify the path of the schema file (sans file-ending) relative to the root directory specified by the --schema flag on applicable commands. If the --schema flag is not present, referenced schemas are ignored during type checking.

# METADATA
# schemas:
#   - input: schema.input
#   - data.acl: schema["acl-schema"]
allow {
    access := data.acl["alice"]
    access[_] == input.operation
}

Inlined Schema Format

Schema definitions can be inlined by specifying the schema structure as a YAML or JSON map. Inlined schemas are always used to inform type checking for the eval, check, and test commands; in contrast to by-reference schema annotations, which require the --schema flag to be present in order to be evaluated.

# METADATA
# schemas:
#   - input.x: {type: number}
allow {
    input.x == 42
}

Entrypoint

The entrypoint annotation is a boolean used to mark rules and packages that should be used as entrypoints for a policy. This value is false by default, and can only be used at rule or package scope.

The build and eval CLI commands will automatically pick up annotated entrypoints; you do not have to specify them with --entrypoint.

Custom

The custom annotation is a mapping of user-defined data, mapping string keys to arbitrarily typed values.

Example

# METADATA
# custom:
#  my_int: 42
#  my_string: Some text
#  my_bool: true
#  my_list:
#   - a
#   - b
#  my_map:
#   a: 1
#   b: 2
allow {
  ...
}

Accessing annotations

Rego

In the example below, you can see how to access an annotation from within a policy.

Given the input:

{
    "number": 11,
    "subject": {
        "name": "John doe",
        "role": "customer"
    }
}

The following policy

package example

# METADATA
# title: Deny invalid numbers
# description: Numbers may not be higher than 5
# custom:
#  severity: MEDIUM
output := decision {
	input.number > 5

	annotation := rego.metadata.rule()
	decision := {
		"severity": annotation.custom.severity,
		"message": annotation.description,
	}
}

will output

{
  "output": {
    "message": "Numbers may not be higher than 5",
    "severity": "MEDIUM"
  }
}

If you’d like more examples and information on this, you can see more here under the Rego policy reference.

Inspect command

Annotations can be listed through the inspect command by using the -a flag:

opa inspect -a

Go API

The ast.AnnotationSet is a collection of all ast.Annotations declared in a set of modules. An ast.AnnotationSet can be created from a slice of compiled modules:

var modules []*ast.Module
...
as, err := ast.BuildAnnotationSet(modules)
if err != nil {
    // Handle error.
}

or can be retrieved from an ast.Compiler instance:

var modules []*ast.Module
...
compiler := ast.NewCompiler()
compiler.Compile(modules)
as := compiler.GetAnnotationSet()

The ast.AnnotationSet can be flattened into a slice of ast.AnnotationsRef, which is a complete, sorted list of all annotations, grouped by the path and location of their targeted package or -rule.

flattened := as.Flatten()
for _, entry := range flattened {
    fmt.Printf("%v at %v has annotations %v\n",
        entry.Path,
        entry.Location,
        entry.Annotations)
}

// Output:
// data.foo at foo.rego:5 has annotations {"scope":"subpackages","organizations":["Acme Corp."]}
// data.foo.bar at mod:3 has annotations {"scope":"package","description":"A couple of useful rules"}
// data.foo.bar.p at mod:7 has annotations {"scope":"rule","title":"My Rule P"}
//
// For modules:
// # METADATA
// # scope: subpackages
// # organizations:
// # - Acme Corp.
// package foo
// ---
// # METADATA
// # description: A couple of useful rules
// package foo.bar
// 
// # METADATA
// # title: My Rule P
// p := 7

Given an ast.Rule, the ast.AnnotationSet can return the chain of annotations declared for that rule, and its path ancestry. The returned slice is ordered starting with the annotations for the rule, going outward to the farthest node with declared annotations in the rule’s path ancestry.

var rule *ast.Rule
...
chain := ast.Chain(rule)
for _, link := range chain {
    fmt.Printf("link at %v has annotations %v\n",
        link.Path,
        link.Annotations)
}

// Output:
// data.foo.bar.p at mod:7 has annotations {"scope":"rule","title":"My Rule P"}
// data.foo.bar at mod:3 has annotations {"scope":"package","description":"A couple of useful rules"}
// data.foo at foo.rego:5 has annotations {"scope":"subpackages","organizations":["Acme Corp."]}
//
// For modules:
// # METADATA
// # scope: subpackages
// # organizations:
// # - Acme Corp.
// package foo
// ---
// # METADATA
// # description: A couple of useful rules
// package foo.bar
// 
// # METADATA
// # title: My Rule P
// p := 7