Authenticate your Dancer app with Google
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README.md

Dancer-Plugin-Auth-Google

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This plugin provides a simple way to authenticate your users through Google's OAuth API. It provides you with a helper to build easily a redirect to the authentication URI, defines automatically a callback route handler and saves the authenticated user to your session when done.

    package MyApp;
    use Dancer ':syntax';
    use Dancer::Plugin::Auth::Google;

    auth_google_init;  # <-- don't forget to call this first!

    before sub {
        return unless request->path_info !~ m{^/auth/google/callback};
        redirect auth_google_authenticate_url unless session('google_user');
    };

    get '/' => sub {
        "welcome, " . session('google_user')->{displayName}
    };

    get '/fail' => sub {
        "oh noes!"
    };

Prerequisites

In order for this plugin to work, you need the following:

Session backend

For the authentication process to work, you need a session backend so the plugin can store the authenticated user's information.

Use the session backend of your choice, it doesn't make a difference, see Dancer::Session for details about supported session engines, or search the CPAN for new ones.

Google Application

Anyone with a valid Google account can register an application. Go to http://console.developers.google.com, then select a project or create a new one. After that, in the sidebar on the left, select "Credentials" under the "APIs and auth" option. In the "OAuth" section of that page, select Create New Client ID. A dialog will appear.

screenshot for creating new id

In the "Application type" section of the dialog, make sure you select "Web application". In the "Authorized JavaScript origins" field, make sure you put the domains of both your development server and your production one (e.g.: http://localhost:3000 and http://mywebsite.com). Same thing goes for the "Redirect URIs": those ** MUST ** be the same as you will set in your app and Google won't redirect to any page that is not listed (don't worry, you can edit this later too).

screenshot for authorized uris

Again, make sure the "Redirect URIs" contains both your development url (e.g. http://localhost:3000/auth/google/callback) and production (e.g. http://mycoolwebsite.com/auth/google/callback).

After you're finished, copy the "Client ID" and "Client Secret" data of your newly created app. It should be listed on that same panel (you can check it anytime by going to the "Credentials" option under "APIs & auth":

screenshot for client id

Configuration

After you set up your app, you need to configure this plugin. To do that, copy the "Client ID" and "Client Secret" generated on the previous step into your Dancer's configuration under Plugins / Auth::Google, like so:

    # config.yml
    plugins:
        'Auth::Google':
            client_id:        'your-client-id'
            client_secret:    'your-client-secret'
            scope:            'profile'
            access_type:      'online'
            callback_url:     'http://localhost:3000/auth/google/callback'
            callback_success: '/'
            callback_fail:    '/fail'

Of those, only "client_id", "client_secret" and "callback_url" are mandatory. If you omit the other ones, they will assume their default values, as listed above.

Specifically, it is a good practice to change the "callback_url" depending on whether you're on a development or production environment. Dancer makes this easier for you by letting you split your settings, leaving the basic plugin settings on "config.yml" and specific "callback_url" definitions on "environments/development.yml" and "environments/production.yml":

    # environments/development.yml
    plugins:
        'Auth::Google':
            callback_url:   'http://localhost:3000/auth/google/callback'

And

    # environments/production.yml
    plugins:
        'Auth::Google':
            callback_url:   'http://myproductionserver.com/auth/google/callback'

Setting your permissions' scope

Since this plugin is meant mainly for authentication, the default scope is 'profile'. That should give you general profile data for the user, such as full name, id, profile url, etc. See https://developers.google.com/+/api/oauth#login-scopes for available scopes to chose from. You can set as many as you like, separated by space. A usual combination is 'profile email'. If you want a Google-specific scope (i.e. those with a "." in the name) make sure you add the full URL as specified in the document above. For example, the proper way to ask for a user's social features is not "plus.login", but "https://www.googleapis.com/auth/plus.login".

Installation

cpanm Dancer::Plugin::Auth::Google

Exports

The plugin exports the following symbols to your application's namespace:

auth_google_init

This function should be called before your route handlers in order to initialize the underlying object and set up the proper routes. It will read your configuration and create everything that it needs.

auth_google_authenticate_url

This function returns an authorize URI for redirecting unauthenticated users. You should use this in a before filter like the "synopsis" demo above.

Route Handlers

The plugin defines the following route handler automatically:

  • /auth/google/callback

This route handler is responsible for catching back a user that has just authenticated herself with Google's OAuth. The route handler saves tokens and user information in the session and then redirects the user to the URI specified by callback_success.

If the validation of the token returned by Google failed or was denied, the user will be redirected to the URI specified by callback_fail. Otherwise, this route will point the user to callback_success.

Accessing Other Google APIs

Once the user is authenticated, your session data will contain the access token:

    my $token = session('google_user')->{access_token};

You can use that access token to make calls to a Google API on behalf of the user. See https://developers.google.com/accounts/docs/OAuth2WebServer for more information on this.

Acknowledgements

This plugin was written following the same design as Dancer::Plugin::Auth::Twitter and Dancer::Plugin::Auth::Facebook.

COPYRIGHT AND LICENCE

Copyright (C) 2014, Breno G. de Oliveira

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.