View Source Janus (Janus v0.2.1)

Authorization superpowers for applications using Ecto.

Priorities:

  • Single source of truth - The same rules that authorize loaded data should be able to load authorized data.

  • Authentication-agnostic - Janus should not care about how users are modeled or authenticated.

  • Minimal library footprint - Expose a small but flexible API that can be used to create an optimal authorization interface for each application.

  • Escape hatches where necessary - Complex authorization rules and use-cases should be representable when Janus neglects to provide a short cut.

Janus is split into two primary components:

  • Janus.Policy - functions and behaviour for defining policy modules, which describe the allowed actors, actions, and resources in your application. This is where you look if you're writing a policy module.

  • Janus.Authorization - functions and behaviour used by the rest of your application to authorize and load resources. This is where you look if you're using a policy module.

Janus defines a Mix task to generate the basic policy module that will get you started:

$ mix janus.gen.policy

policies

Policies

Policy modules are created by invoking use Janus, which implements both the Janus.Policy and Janus.Authorization behaviours:

defmodule Policy do
  use Janus

  @impl true
  def build_policy(policy, _user) do
    policy
  end
end

When you invoke use Janus, default implementations are injected for required callbacks, except for Janus.Policy.build_policy/2. This callback is your foundation, as it returns the authorization policy for an individual user of your application.

The policy above is not very useful (it doesn't allow anyone to do anything) but that can be changed by using the Janus.Policy API to define actions, resources, and conditions that make up your authorization rules.

def build_policy(policy, %User{role: :moderator} = mod) do
  policy
  |> allow(:read, Post)
  |> allow([:edit, :archive, :unarchive], Post, where: [user: [role: :member]])
  |> allow([:edit, :archive, :unarchive], Post, where: [user_id: mod.id])
  |> deny(:unarchive, Post, where: [archived_by: [role: :admin]])
end

See the Janus.Policy documentation for more on defining policies.

authorization

Authorization

With our policy module defined, it can now be used to load and authorize resources.

iex> Policy.authorize(some_post, :archive, moderator)
{:ok, some_post}

iex> Policy.authorize(post_archived_by_admin, :unarchive, moderator)
{:error, :not_authorized}

iex> Policy.scope(Post, :read, moderator)
%Ecto.Query{}

iex> Policy.scope(Post, :read, moderator) |> Repo.all()
[ ... posts the moderator can read ]

iex> Policy.any_authorized?(Post, :edit, moderator)
true # there are rules allowing moderators to edit posts

iex> Policy.any_authorized?(Post, :delete, moderator)
false # there are no rules that allow moderators to delete posts

These functions make up the Janus.Authorization behaviour, and their definitions were injected by default when we invoked use Janus. This is the "public API" that the rest of your application will use to authorize resources.

See the Janus.Authorization documentation for more.

integration-with-ecto-query

Integration with Ecto.Query

The primary assumption that Janus makes is that your resources are backed by an Ecto.Schema. Using Ecto's schema reflection capabilities, Janus is able to use the same policy to authorize a single resource and to construct a composable Ecto query that is aware of field types and associations.

# This query would result in the 5 latest posts that the current
# user is authorized to see, preloaded with the user # who made
# the post (but only if the current user is  allowed to see that
# user).

Post
|> Policy.scope(:read, current_user,
  preload_authorized: :user
)
|> order_by(desc: :inserted_at)
|> limit(5)

This integration with Ecto queries is main reason Janus exists.

configuration

Configuration

Some defaults can be configured by passing them as options when invoking use Janus. Those are:

  • :repo - Ecto.Repo used to load associations when required by your authorization rules

  • :load_associations - Load associations when required by your authorization rules (requires :repo config option to be set or to be passed explicitly at the call site), defaults to false

For example:

defmodule MyApp.Policy do
    use Janus,
      repo: MyApp.Repo,
      load_associations: true

    # ...
end

These defaults will be referenced in the Janus.Authorization documentation where they are used.

Link to this section Summary

Functions

Sets up a module to implement the Janus.Policy and Janus.Authorization behaviours.

Link to this section Types

@type action() :: any()
@type actor() :: any()
@type schema_module() :: module()

Link to this section Functions

Link to this macro

__using__(opts \\ [])

View Source (macro)

Sets up a module to implement the Janus.Policy and Janus.Authorization behaviours.

Using use Janus does the following:

options

Options

  • :load_associations - Load associations when required by your authorization rules (requires :repo config option to be set or to be passed explicitly at the call site), defaults to false

  • :repo - Ecto.Repo used to load associations when required by your authorization rules

See "Configuration" section for details.

example

Example

defmodule MyApp.Policy do
  use Janus, repo: MyApp.Repo

  @impl true
  def build_policy(policy, _actor) do
    policy
    # |> allow(...)
  end
end