Walruz: Simple yet Powerful Policy Composition Authorization Framework

Rails Integration

Basic and Terminology

Walruz facilitates the separation between the authorization process on the business logic and the actions executed after the validation of the authorizations. To understand how it works, we will follow the following terminology:

Subject: Object that is going to be managed (Profile, Posts).
Actor: Entity that wants to perform an action on a subject (User, Admin).
Policy: A set of rules that tells if the actor can perform the desired action on the subject.

Walruz Architecture

Walruz provides modules and classes that help on the implementation of the concepts given previously, this are:

Walruz::Subject: Module that provides the interface to associate policies to an action in the subject.
Walruz::Actor: Module that provides the interface to perform queries to validate if an action can be done between the actor and the subject.
Walruz::Policy: Class that provides the interface to implement authorization logic.

Subject classes may specify a set of actions that can be performed to them using the check_authorization method

class User
  include Walruz::Subject

  check_authorization :read => UserReadPolicy,
                      :update => UserUpdatePolicy
end

If there is just one policy to every possible action performed to the subject, you may specify the :default action, or just specify the Policy class.

Example:

class User
  include Walruz::Subject

  check_authorization UserPolicy
end

class User
  include Walruz::Subject

  check_authorization :default => UserPolicy
end

You can also specify other flags with the default flag.

class User
  include Walruz::Subject

  check_authorization :read => UserReadPolicy,
                      :update => UserUpdatePolicy,
                      :default => UserPolicy
end

Actors verify if they are able to perform an action on a subject

Actor classes can use several methods to check if the actor instance can perform the given action on a subject. This are:

can?(action, subject): Returns boolean that says if the actor can execute or not the action on the subject.
authorize(action, subject): In case the actor can execute the action on the subject, it returns the parameters hash from the policy, otherwise it will return nil
authorize!(action, subject): In case the actor can execute the action on the subject, it returns the parameters hash from the policy, otherwise it will raise a Walruz::NotAuthorized error.
satisfies?(policy_label, subject): It behaves just like the can? method, but instead of giving an action to be executed to the subject, it receives a policy label (More on policy labels next).

In case the given action is not assigned to any policy, a default Policy will be executed (if given), if no default policy is given then a Walruz::ActionNotFound exception will be raised.

Examples:

current_user.can?(:read, friends_profile)      #=> true
current_user.satisfies?(:actor_is_admin, nil)  #=> false
current_user.satisfies(:actor_is_admin, nil)  #=> nil
current_user.authorize(:read, friends_profile) #=> Hash
current_user.authorize!(:read, other_person_profile) # => raises Walruz::NotAuthorized

Implementing Policies

To implement a policy, it is necessary to inherit from the Walruz::Policy class. This class provides a method called authorized? that return either a Boolean, or an Array of two items, the first one being a Boolean, and the second being a Hash of parameters returned from the Policy.

Every Policy Class also has a label associated to it, by default the label will be the name of the class in underscore case; if you want to have a custom label for a Policy Class, you can invoke the set_policy_label method on the class context and specify the label that you want for it. This label is used on the satisfies? method.

Examples:

class ActorIsAdmin < Walruz::Policy
  set_policy_label :is_admin

  def authorized?(actor, _)
    actor.is_admin?
  end

end

class UserIsFriend < Walruz::Policy

  def authorized?(current_user, friend)
    friendship = Friendship.first(:conditions => { :friend_id => current_user.id, :owner_id => friend.id})
    if !friendship.nil?
      [true, {
        :friendship => friendship
      }]
    else
      false
    end
  end

end

# Examples using this policies with the satisfies method

current_user.satisfies?(:is_admin, nil)

# By default, the policy label is the name of the class in underscore case.
current_user.satisfies?(:user_is_friend, other_user)

Composing basic policies to create complex ones

Sometimes policies can turn really messy, specially when you have a complex business model. The good news is that normally this complex policies are a composition of more simple policies (e.g. ActorCanSeeUserPictures). Instead of creating this new classes that replicates the same logic of basic policies, we could merge them together in the following way:

ActorCanSeeUserPictures = Walruz::Utils.all(UserIsFriend, UserAllowsDisclosureOfPictures)

There is also the utility methods any and not, to create combinations of policies.

If your policy returns a parameters hash, and you are using the all method, the parameters of each policy will be merged together, if you are using the any method, the parameters of the first policy that returns true will be returned.

One other thing that the utility methods does for you is that it leaves its track on the returned policy parameters, when you invoke a composite policy, every policy will leave in the parameters hash the policy_label with a question mark at the end, that way you can know which policies were successful or not.

Example:

class ActorIsAdmin < Walruz::Policy 
  set_policy_label :is_admin

  def authorized?(actor, _)
    actor.is_admin?
  end

end

class ActorIsSubject < Walruz::Policy
  def authorized?(actor, subject); actor == subject; end
end

UserReadPolicy = any(ActorIsSubject, ActorIsAdmin)

class User < AbstractORM
  include Walruz::Subject

  check_authorizations :read => UserReadPolicy
end

class UsersController < Framework::Controller
  def show
    policy_params = current_user.authorize(:read, other_user)
    if policy_params[:actor_is_subject?]
      # do logic of the user interacting with herself
    elsif policy_params[:is_admin?]
      # do logic of the admin user interacting with other user
    else
      # do other logic here...
    end
  end
end

Dependencies between Policies

Sometimes you would like to have a Policy that strictly depends in other policies, on the previous example UserAllowsDisclosureOfPictures could have a dependency that says that only the User allows the disclosure of pictures if and only if there is a friend relationship, so we could re-implement this policy as:

Example:

class UserAllowsDisclosureOfPictures < Walruz::Policy
  depends_on UserIsFriend
  # ...
end

Suppose you need the parameters returned by the previous Policy, you can have them with the params method.

Example:

class UserAllowsDisclosureOfPictures < Walruz::Policy
  depends_on UserIsFriend

  def authorized?(_, _)
    params[:friendship].allows_disclosure_of_images?
  end

end

Policy combinators

Sometimes you would like to execute policies that are not directly related to a subject, but to the association of a subject. Given the example above of the friendship relationship and the disclosure of pictures, sometimes you would like to check if a user can see a picture directly on the picture model.

Suppose we have the following model in our system:

class Picture < AbstractORM
  belongs_to :owner
end

and we would like to check if the current_user can see (read) the picture using:

current_user.can?(:read, picture_instance)

If you may recall, we already implemented the logic that checks that authorization in UserAllowsDisclosureOfPictures, but that policy only works when the subject is of class User; given that you have a subject of class Picture you can not re-use this policy.

You could solve this issue doing the following:

class PictureReadPolicy < Walruz::Policy

  def authorized?(user, image)
    user.satisfies?(UserAllowsDisclosureOfPictures, image.owner)
  end

end

But as you may see, we are just creating new policies to handle old ones, we are not combining the policies effectively. To avoid this caveat, you can use the PolicyClass.for_subject method:

PictureReadPolicy = UserAllowsDisclosureOfPictures.for_subject(:owner)

class Picture < AbstractORM
  include Walruz::Subject
  belongs_to :owner

  check_authorizations :read => PictureReadPolicy
end

The parameter of for_subject is the name of the subject’s method that will return a new subject, this new subject is then passed through the policy. Pretty neat eh?

Returning custom errors

Suppose you want to add an error to the authorization failure that is a more descriptive, you can do so on the authorized? method passing a hash with a :error_message key on the false return. If you use the authorize! method on the actor model, this will become the Walruz::NotAuthorized error message.

Example:

class SomePolicy < Walruz::Policy

  def authorized?(actor, subject)
    # some complex logic here
    return [false, {
      :error_message => 'More descriptive error message'
    }]
  end
end

Conventions

You’ll notice that once you start implementing policies for your system, you’ll be lost soon enough asking yourself which type of subject a Policy receives; to avoid such confusions, we suggest that you apply the following rules of thumb:

The first name of the policy should be the Subject class (e.g. UserIsFriend)
If the policy only applies to the actor, the policy class name should start with the Actor word (e.g. ActorIsAdmin)
You should always have the compositions of policies in just one place in your library folder (e.g. in policies.rb file).
The result of policy compositions should finish with the word Policy (e.g UserDeletePolicy = any(ActorIsSubject, ActorIsAdmin))
Use PolicyClass.for_subject when you are combining the policy class with other policies, if you are not doing this, consider checking authorizations on parents of the subject instead of the subject (e.g. current_user.can?(:see_pictures_of, picture.owner))

If you follow this rules, it will be much easier for you to merge policies together in an efficient way.

More examples

You may check the project in the examples/ directory for more info; on the rails project, take a look on the spec/models/beatle_spec.rb file, it’s really illustrating.

Walruz: Simple yet Powerful Policy Composition Authorization Framework

Rails Integration

Basic and Terminology

Walruz Architecture

Subjects specify which policies are related to which actions

Actors verify if they are able to perform an action on a subject

Implementing Policies

Composing basic policies to create complex ones

Dependencies between Policies

Policy combinators

Returning custom errors

Conventions

More examples

Copyright