path: root/README.md

# Action Cable -- Integrated websockets for Rails

Action Cable seamlessly integrates websockets with the rest of your Rails application.
It allows for real-time features to be written in Ruby in the same style
and form as the rest of your Rails application, while still being performant
and scalable. It's a full-stack offering that provides both a client-side
JavaScript framework and a server-side Ruby framework. You have access to your full
domain model written with ActiveRecord or whatever.


## Terminology

A single Action Cable server can handle multiple connection instances. It goes one
connection instance per websocket connection. A single user may well have multiple
websockets open to your application if they use multiple browser tabs or devices.
The client of a websocket connection is called the consumer.

Each consumer can in turn subscribe to multiple cable channels. Each channel encapsulates
a logical unit of work, similar to what a controller does in a regular MVC setup. So 
you may have a ChatChannel and a AppearancesChannel. The consumer can be subscribed to either
or to both. At the very least, a consumer should be subscribed to one channel.

When the consumer is subscribed to a channel, they act as a subscriber. The connection between
the subscriber and the channel is, surprise-surprise, called a subscription. A consumer
can act as a subscriber to a given channel via a subscription only once. (But remember that
a physical user may have multiple consumers, one per tab/device open to your connection).

Each channel can then again be streaming zero or more broadcastings. A broadcasting is a
pubsub link where anything transmitted by the broadcaster is sent directly to the channel 
subscribers who are streaming that named broadcasting.

As you can see, this is a fairly deep architectural stack. There's a lot of new terminology
to identify the new pieces, and on top of that, you're dealing with both client and server side
reflections of each unit.


## A full-stack example

The first thing you must do is defined your ApplicationCable::Connection class in Ruby. This
is the place where you do authorization of the incoming connection, and proceed to establish it
if all is well. Here's the simplest example starting with the server-side connection class:

  # app/channels/application_cable/connection.rb
  module ApplicationCable
    class Connection < ActionCable::Connection::Base
      identified_by :current_user
  
      def connect
        self.current_user = find_verified_user
      end
  
      protected
        def find_verified_user
          if current_user = User.find cookies.signed[:user_id]
            current_user
          else
            reject_unauthorized_connection
          end
        end
    end
  end

This relies on the fact that you will already have handled authentication of the user, and
that a successful authentication sets a signed cookie with the user_id. This cookie is then
automatically sent to the connection instance when a new connection is attempted, and you
use that to set the current_user. By identifying the connection by this same current_user,
you're also ensuring that you can later retrieve all open connections by a given user (and
potentially disconnect them all if the user is deleted or deauthorized).

The client-side needs to setup a consumer instance of this connection. That's done like so:

  # app/assets/javascripts/cable.coffee
  @App = {}
  App.cable = Cable.createConsumer "http://cable.example.com"

The http://cable.example.com address must point to your set of Action Cable servers, and it
must share a cookie namespace with the rest of the application (which may live under http://example.com).
This ensures that the signed cookie will be correctly sent.

That's all you need to establish the connection! But of course, this isn't very useful in
itself. This just gives you the plumbing. To make stuff happen, you need content. That content
is defined by declaring channels on the server and allowing the consumer to subscribe to them.


## Channel example 1: User appearances

Here's a simple example of a channel that tracks whether a user is online or not and what page they're on.
(That's useful for creating presence features like showing a green dot next to a user name if they're online).

First you declare the server-side channel:

  # app/channels/appearance_channel.rb
  class AppearanceChannel < ApplicationCable::Channel
    def subscribed
      current_user.appear
    end
  
    def unsubscribed
      current_user.disappear
    end
  
    def appear(data)
      current_user.appear on: data['appearing_on']
    end
  
    def away
      current_user.away
    end
  end

The #subscribed callback is invoked when, as we'll show below, a client-side subscription is initiated. In this case,
we take that opportunity to say "the current user has indeed appeared". That appear/disappear API could be backed by
Redis or a database or whatever else. Here's what the client-side of that looks like:

  # app/assets/javascripts/cable/subscriptions/appearance.coffee
  App.appearance = App.cable.subscriptions.create "AppearanceChannel",
    connected: ->
      # Called once the subscription has been successfully completed
  
    appear: ->
      @perform 'appear', appearing_on: @appearingOn()
  
    away: ->
      @perform 'away'
  
    appearingOn: ->
      $('main').data 'appearing-on'

  $(document).on 'page:change', ->
    App.appearance.appear()

  $(document).on 'click', '[data-behavior~=appear_away]', ->
    App.appearance.away()
    false

Simply calling App.cable.subscriptions.create will setup the subscription, which will call AppearanceChannel#subscribed,
which in turn is linked to original App.consumer -> ApplicationCable::Connection instances.

We then link App.appearance#appear to AppearanceChannel#appear(data). This is possible because the server-side
channel instance will automatically expose the public methods declared on the class (minus the callbacks), so that these
can be reached as remote procedure calls via App.appearance#perform.

Finally, we expose App.appearance to the machinations of the application itself by hooking the #appear call into the 
Turbolinks page:change callback and allowing the user to click a data-behavior link that triggers the #away call.


## Channel example 2: Receiving new web notifications

The appearance example was all about exposing server functionality to client-side invocation over the websocket connection.
But the great thing about websockets is that it's a two-way street. So now let's show an example where the server invokes
action on the client.

This is a web notification channel that allows you to trigger client-side web notifications when you broadcast to the right
streams:

  # app/channels/web_notifications.rb
  class WebNotificationsChannel < ApplicationCable::Channel
     def subscribed
       stream_from "web_notifications_#{current_user.id}"
     end
   end

   # Somewhere in your app this is called, perhaps from a NewCommentJob
   ActionCable.server.broadcast \
     "web_notifications_1", { title: 'New things!', body: 'All shit fit for print' }

   # Client-side which assumes you've already requested the right to send web notifications
   App.cable.subscriptions.create "WebNotificationsChannel",
     received: (data) ->
      web_notification = new Notification data['title'], body: data['body']

The ActionCable.server.broadcast call places a message in the Redis' pubsub queue under the broadcasting name of "web_notifications_1".
The channel has been instructed to stream everything that arrives at "web_notifications_1" directly to the client by invoking the
#received(data) callback. The data is the hash sent as the second parameter to the server-side broadcast call, JSON encoded for the trip
across the wire, and unpacked for the data argument arriving to #received.


## Dependencies

Action Cable is currently tied to Redis through its use of the pubsub feature to route
messages back and forth over the websocket cable connection. This dependency may well
be alleviated in the future, but for the moment that's what it is. So be sure to have
Redis installed and running.


## Deployment

Action Cable is powered by a combination of EventMachine and threads. The
framework plumbing needed for connection handling is handled in the 
EventMachine loop, but the actual channel, user-specified, work is handled
in a normal Ruby thread. This means you can use all your regular Rails models
with no problem, as long as you haven't committed any thread-safety sins.

But this also means that Action Cable needs to run in its own server process.
So you'll have one set of server processes for your normal web work, and another
set of server processes for the Action Cable. The former can be single-threaded,
like Unicorn, but the latter must be multi-threaded, like Puma.


## Alpha disclaimer

Action Cable is currently considered alpha software. The API is almost guaranteed to change between
now and its first production release as part of Rails 5.0. Real applications using the framework
are all well underway, but as of July 8th, 2015, there are no deployments in the wild yet.

So this current release, which resides in rails/actioncable, is primarily intended for
the adventurous kind, who do not mind reading the full source code of the framework. And it
serves as an invitation for all those crafty folks to contribute to and test what we have so far,
in advance of that general production release.

Action Cable will move from rails/actioncable to rails/rails and become a full-fledged default
framework alongside Action Pack, Active Record, and the like once we cross the bridge from alpha
to beta software (which will happen once the API and missing pieces have solidified).

Finally, note that testing is a unfinished, hell unstarted, area of this framework. The framework
has been developed in-app up until this point. We need to find a good way to test both the framework
itself and allow the user to test their connection and channel logic.


## Download and installation

The latest version of Action Pack can be installed with RubyGems:

  % gem install actionpack

Source code can be downloaded as part of the Rails project on GitHub

* https://github.com/rails/rails/tree/master/actionpack


## License

Action Pack is released under the MIT license:

* http://www.opensource.org/licenses/MIT


## Support

Bug reports can be filed for the alpha development project here:

* https://github.com/rails/actioncable/issues