Write Polar Rules

Polar is Oso’s declarative policy language. We used Polar in Model your authorization logic to authorize access to resources, and we already wrote a few rules:

• an allow rule: The top level rule that authorize calls to perform resource-level enforcement.
• a has_role rule: The rule used to tell Oso whether an actor has a particular role on a resource

In this guide, we’ll go into more detail on writing rules yourself.

What’s in a rule

To use Oso, you model your authorization logic as a set of rules. This set of rules is called the policy. Let’s take a look at a basic rule:

allow(actor, action, resource) if
	has_permission(actor, action, resource);

The rule has a name, and parameter list. The name is allow, and the parameters are actor, action, and resource (all variables). This rule has a body that calls another rule, has_permission with the same set of parameters. This allow rule will succeed if has_permission succeeds.

A rule succeeds if the parameters it’s called with match its parameter list and if the conditions in the body of the rule succeed.

Rules with conditions

You can write conditions in the body of a rule. For example:

has_permission(_user: User, "read", repository: Repository) if
	repository.is_public = true;

This rule succeeds if the repository ’s is_public property is equal to true and the parameter list matches.

In this rule, we used types on our parameters, and specified a literal as the action parameter, meaning the rule will only match if the action is "read".

Combining conditions

You can specify multiple conditions in a rule body using the and operator:

allow(user, action, resource) if
	user.is_blocked = false and
	has_permission(user, action, resource);

This rule succeeds if the user is_blocked field is false and the has_permission rule succeeds.

Extending resource blocks

We defined shorthand rules in our resource blocks in Model your authorization policy. Let’s take a look at extending these rules:

resource Repository {
    permissions = ["read", "push", "delete"];
    roles = ["contributor", "maintainer", "admin"];

    "read" if "contributor";
    "push" if "maintainer";
    "delete" if "admin";

    "maintainer" if "admin";
    "contributor" if "maintainer";
}

Each shorthand rule expands to a rule when the resource block is parsed. For example, "read" if "contributor" expands to:

has_permission(user: User, "read", repository: Repository) if
	has_role(user, "contributor", repository);

We can grant users permissions based on roles by writing our own rules:

has_permission(user: User, "delete", repository: Repository) if
	# User has the "admin" role.
	has_role(user, "admin", repository) and
	user.auth_token.has_sudo_mode() = true;

This rule succeeds if the user has an "admin" role, and the has_sudo_mode() method on the auth_token field of user returns true. That’s right! You can call Ruby methods directly from Polar, and access their return values. This can help you share business logic between the policy and your application, for example whether a user has elevated their privileges sufficiently to perform dangerous actions.

What’s next

• For more detail on writing policies, see the Write Oso Policies guide.
• To go deeper on any of the concepts from this getting started section, see our How to guides.
• To see how to apply authorization to large collections of data that cannot be loaded into memory, read Filter collections of data.

Connect with us on Slack

If you have any questions, or just want to talk something through, jump into Slack. An Oso engineer or one of the thousands of developers in the growing community will be happy to help.