The Strategy is the work-horse of the system.

Strategies are implemented outside this library to meet your needs, the strategy provides a consistent API and behaviour.

Each strategy operates through two phases.

1. request phase
2. callback phase

These phases can be understood with the following psuedocode.

Request Phase

request (for the request phase - default /auth/:provider)
|> relevant_strategy.handle_request!(conn)
|> continue with request plug pipeline

The request phase follows normal plug pipeline behaviour. The request will not continue if the strategy halted the connection.

Callback Phase

request (for a callback phase - default /auth/:provider/callback)
|> relevant_strategy.handle_auth!(conn)
if connection does not have ueberauth failure
  |> set ueberauth auth with relevant_strategy.auth
|> cleanup from the strategy with relevant_strategy.handle_cleanup!
|> continue with plug pipeline

The callback phase is essentially a decorator and does not usually redirect or halt the request. Its result is that one of two cases will end up in your connections assigns when it reaches your controller.

• On Failure - An Ueberauth.Failure struct is available at :ueberauth_failure
• On Success - An Ueberauth.Auth struct is available at :ueberauth_auth

An example

The simplest example is an email/password strategy. This does not intercept the request and just decorates it with the Ueberauth.Auth struct. (it is always successful)

defmodule Ueberauth.Strategies.Identity do
  use Ueberauth.Strategy

  alias Ueberauth.Auth.Credentials
  alias Ueberauth.Auth.Extra

  def uid(conn), do: conn.params["email"]

  def extra(conn), do: struct(Extra, raw_info: conn.params)

  def credentials(conn) do
    %Credentials{
      other: %{
        password: conn.params["password"],
        password_confirmation: conn.params["password_confirmation"]
      }
    }
  end
end

After the strategy has run through the handle_callback!/1 function, since there are no errors added, Ueberauth will add the constructed auth struct to the connection.

The Auth struct is constructed like:

def auth(conn) do
  %Auth{
    provider: strategy_name(conn),
    strategy: strategy(conn),
    uid: uid(conn),
    info: info(conn),
    extra: extra(conn),
    credentials: credentials(conn)
  }
end

Each component of the struct is a separate function and receives the connection object. From this Ueberauth will construct and assign the struct for processing in your own controller.

Redirecting during the request phase

Many strategies may require a redirect (looking at you OAuth). To do this, implement the handle_request!/1 function.

def handle_request!(conn) do
  callback_url = callback_url(conn)
  redirect!(conn, callback_url)
end

Callback phase

The callback phase may not do anything other than instruct the strategy where to get the information to construct the auth struct. In that case define the functions for the components of the struct and fetch the information from the connection struct.

In the case where you do need to take some other step, the handle_callback!/1 function is where its at.

def handle_callback!(conn) do
  conn
  |> call_external_service_and_assign_result_to_private
end

def uid(conn) do
  fetch_from_my_private_area(conn, :username)
end

def handle_cleanup!(conn) do
  remove_my_private_area(conn)
end

This provides a simplistic psuedocode look at what a callback + cleanup phase might look like. By setting the result of your call to the external service in the connections private assigns, you can use that to construct the auth struct in the auth component functions. Of course, as a good citizen you also cleanup the connection before the request continues.

Cleanup phase

The cleanup phase is provided for you to be a good citizen and clean up after your strategy. During the callback phase, you may need to temporarily store information in the private section of the conn struct. Once this is done, the cleanup phase exists to cleanup that temporary storage after the strategy has everything it needs.

Implement the handle_cleanup!/1 function and return the cleaned conn struct.

Adding errors during callback

You have two options when you're in the callback phase. Either you can let the connection go through and Ueberauth will construct the auth hash for you, or you can add errors.

You should add errors before you leave your handle_callback!/1 function.

def handle_callback!(conn) do
  errors = []
  if (something_bad), do: errors = [error("error_key", "Some message") | errors]

  if (length(errors) > 0) do
    set_errors!(errors)
  else
    conn
  end
end

Once you've set errors, Ueberauth will not set the auth struct in the connections assigns at :ueberauth_auth, instead it will set a Ueberauth.Failure struct at :ueberauth_failure with the information provided detailing the failure.