rails--rails/actioncable

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 Active Record or your ORM of choice.

Terminology

A single Action Cable server can handle multiple connection instances. It has one connection instance per WebSocket connection. A single user may 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. For example, you could have a ChatChannel and an AppearancesChannel, and a consumer could be subscribed to either or to both of these channels. 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 any number of times. For example, a consumer could subscribe to multiple chat rooms at the same time. (And 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.

Examples

A full-stack example

The first thing you must do is define your ApplicationCable::Connection class in Ruby. This is the place where you authorize 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_by(id: cookies.signed[:user_id])
          current_user
        else
          reject_unauthorized_connection
        end
      end
  end
end

Here identified_by is a connection identifier that can be used to find the specific connection again or later. Note that anything marked as an identifier will automatically create a delegate by the same name on any channel instances created off the connection.

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).

Next, you should define your ApplicationCable::Channel class in Ruby. This is the place where you put shared logic between your channels.

# app/channels/application_cable/channel.rb
module ApplicationCable
  class Channel < ActionCable::Channel::Base
  end
end

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

// app/assets/javascripts/cable.js
//= require action_cable
//= require_self
//= require_tree ./channels

(function() {
  this.App || (this.App = {});

  App.cable = ActionCable.createConsumer("ws://cable.example.com");
}).call(this);

The ws://cable.example.com address must point to your Action Cable server(s), 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 also what page they are currently on. (This is useful for creating presence features like showing a green dot next to a user's 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.cable.subscriptions.create "AppearanceChannel",
  # Called when the subscription is ready for use on the server
  connected: ->
    @install()
    @appear()

  # Called when the WebSocket connection is closed
  disconnected: ->
    @uninstall()

  # Called when the subscription is rejected by the server
  rejected: ->
    @uninstall()

  appear: ->
    # Calls `AppearanceChannel#appear(data)` on the server
    @perform("appear", appearing_on: $("main").data("appearing-on"))

  away: ->
    # Calls `AppearanceChannel#away` on the server
    @perform("away")


  buttonSelector = "[data-behavior~=appear_away]"

  install: ->
    $(document).on "page:change.appearance", =>
      @appear()

    $(document).on "click.appearance", buttonSelector, =>
      @away()
      false

    $(buttonSelector).show()

  uninstall: ->
    $(document).off(".appearance")
    $(buttonSelector).hide()

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

Next, we link the client-side appear method 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 a subscription's perform method.

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 an 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_channel.rb
class WebNotificationsChannel < ApplicationCable::Channel
  def subscribed
    stream_from "web_notifications_#{current_user.id}"
  end
end

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

# Somewhere in your app this is called, perhaps from a NewCommentJob
ActionCable.server.broadcast \
  "web_notifications_#{current_user.id}", { title: 'New things!', body: 'All the news that is fit to print' }

The ActionCable.server.broadcast call places a message in the Action Cable pubsub queue under a separate broadcasting name for each user. For a user with an ID of 1, the broadcasting name would be 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.

Passing Parameters to Channel

You can pass parameters from the client side to the server side when creating a subscription. For example:

# app/channels/chat_channel.rb
class ChatChannel < ApplicationCable::Channel
  def subscribed
    stream_from "chat_#{params[:room]}"
  end
end

If you pass an object as the first argument to subscriptions.create, that object will become the params hash in your cable channel. The keyword channel is required.

# Client-side, which assumes you've already requested the right to send web notifications
App.cable.subscriptions.create { channel: "ChatChannel", room: "Best Room" },
  received: (data) ->
    @appendLine(data)

  appendLine: (data) ->
    html = @createLine(data)
    $("[data-chat-room='Best Room']").append(html)

  createLine: (data) ->
    """
    <article class="chat-line">
      <span class="speaker">#{data["sent_by"]}</span>
      <span class="body">#{data["body"]}</span>
    </article>
    """

# Somewhere in your app this is called, perhaps from a NewCommentJob
ActionCable.server.broadcast \
  "chat_#{room}", { sent_by: 'Paul', body: 'This is a cool chat app.' }

Rebroadcasting message

A common use case is to rebroadcast a message sent by one client to any other connected clients.

# app/channels/chat_channel.rb
class ChatChannel < ApplicationCable::Channel
  def subscribed
    stream_from "chat_#{params[:room]}"
  end

  def receive(data)
    ActionCable.server.broadcast "chat_#{params[:room]}", data
  end
end

# Client-side, which assumes you've already requested the right to send web notifications
App.chatChannel = App.cable.subscriptions.create { channel: "ChatChannel", room: "Best Room" },
  received: (data) ->
    # data => { sent_by: "Paul", body: "This is a cool chat app." }

App.chatChannel.send({ sent_by: "Paul", body: "This is a cool chat app." })

The rebroadcast will be received by all connected clients, including the client that sent the message. Note that params are the same as they were when you subscribed to the channel.

More complete examples

See the rails/actioncable-examples repository for a full example of how to setup Action Cable in a Rails app, and how to add channels.

Download and installation

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

$ gem install actioncable

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

• https://github.com/rails/rails/tree/master/actioncable

License

Action Cable is released under the MIT license:

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

Support

API documentation is at:

• http://api.rubyonrails.org

Bug reports can be filed for the Ruby on Rails project here:

• https://github.com/rails/rails/issues

Feature requests should be discussed on the rails-core mailing list here:

• https://groups.google.com/forum/?fromgroups#!forum/rubyonrails-core