Action Cable Overview

1 Introduction

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.

2 What is Pub/Sub

Pub/Sub, or Publish-Subscribe, refers to a message queue paradigm whereby senders of information (publishers), send data to an abstract class of recipients (subscribers), without specifying individual recipients. Action Cable uses this approach to communicate between the server and many clients.

3 Server-Side Components

3.1 Connections

Connections form the foundation of the client-server relationship. For every WebSocket accepted by the server, a connection object is instantiated. This object becomes the parent of all the channel subscriptions that are created from there on. The connection itself does not deal with any specific application logic beyond authentication and authorization. The client of a WebSocket connection is called the connection consumer. An individual user will create one consumer-connection pair per browser tab, window, or device they have open.

Connections are instances of ApplicationCable::Connection. In this class, you authorize the incoming connection, and proceed to establish it if the user can be identified.

3.1.1 Connection Setup

# 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 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 example relies on the fact that you will already have handled authentication of the user somewhere else in your application, and that a successful authentication sets a signed cookie with the user ID.

The 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).

3.2 Channels

A channel encapsulates a logical unit of work, similar to what a controller does in a regular MVC setup. By default, Rails creates a parent ApplicationCable::Channel class for encapsulating shared logic between your channels.

3.2.1 Parent Channel Setup

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

Then you would create your own channel classes. For example, you could have a ChatChannel and an AppearanceChannel:

# app/channels/chat_channel.rb
class ChatChannel < ApplicationCable::Channel
end

# app/channels/appearance_channel.rb
class AppearanceChannel < ApplicationCable::Channel
end

A consumer could then be subscribed to either or both of these channels.

3.2.2 Subscriptions

Consumers subscribe to channels, acting as subscribers. Their connection is called a subscription. Produced messages are then routed to these channel subscriptions based on an identifier sent by the cable consumer.

# app/channels/chat_channel.rb
class ChatChannel < ApplicationCable::Channel
  # Called when the consumer has successfully
  # become a subscriber of this channel.
  def subscribed
  end
end

4 Client-Side Components

4.1 Connections

Consumers require an instance of the connection on their side. This can be established using the following JavaScript, which is generated by default by Rails:

4.1.1 Connect Consumer

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

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

  App.cable = ActionCable.createConsumer();
}).call(this);

This will ready a consumer that'll connect against /cable on your server by default. The connection won't be established until you've also specified at least one subscription you're interested in having.

4.1.2 Subscriber

A consumer becomes a subscriber by creating a subscription to a given channel:

# app/assets/javascripts/cable/subscriptions/chat.coffee
App.cable.subscriptions.create { channel: "ChatChannel", room: "Best Room" }

# app/assets/javascripts/cable/subscriptions/appearance.coffee
App.cable.subscriptions.create { channel: "AppearanceChannel" }

While this creates the subscription, the functionality needed to respond to received data will be described later on.

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:

App.cable.subscriptions.create { channel: "ChatChannel", room: "1st Room" }
App.cable.subscriptions.create { channel: "ChatChannel", room: "2nd Room" }

5 Client-Server Interactions

5.1 Streams

Streams provide the mechanism by which channels route published content (broadcasts) to their subscribers.

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

If you have a stream that is related to a model, then the broadcasting used can be generated from the model and channel. The following example would subscribe to a broadcasting like comments:Z2lkOi8vVGVzdEFwcC9Qb3N0LzE

class CommentsChannel < ApplicationCable::Channel
  def subscribed
    post = Post.find(params[:id])
    stream_for post
  end
end

You can then broadcast to this channel like this:

CommentsChannel.broadcast_to(@post, @comment)

5.2 Broadcasting

A broadcasting is a pub/sub link where anything transmitted by a publisher is routed directly to the channel subscribers who are streaming that named broadcasting. Each channel can be streaming zero or more broadcastings.

Broadcastings are purely an online queue and time dependent. If a consumer is not streaming (subscribed to a given channel), they'll not get the broadcast should they connect later.

Broadcasts are called elsewhere in your Rails application:

WebNotificationsChannel.broadcast_to(
  current_user,
  title: 'New things!',
  body: 'All the news fit to print'
)

The WebNotificationsChannel.broadcast_to call places a message in the current subscription adapter (Redis by default)'s 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 callback.

5.3 Subscriptions

When a consumer is subscribed to a channel, they act as a subscriber. This connection is called a subscription. Incoming messages are then routed to these channel subscriptions based on an identifier sent by the cable consumer.

# app/assets/javascripts/cable/subscriptions/chat.coffee
# 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>
    """

5.4 Passing Parameters to Channels

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

An object passed as the first argument to subscriptions.create becomes the params hash in the cable channel. The keyword channel is required:

# app/assets/javascripts/cable/subscriptions/chat.coffee
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.
ChatChannel.broadcast_to(
  "chat_#{room}",
  sent_by: 'Paul',
  body: 'This is a cool chat app.'
)

5.5 Rebroadcasting a 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

# app/assets/javascripts/cable/subscriptions/chat.coffee
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.

6 Full-Stack Examples

The following setup steps are common to both examples:

Setup your connection.
Setup your parent channel.
Connect your consumer.

6.1 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. (This is useful for creating presence features like showing a green dot next to a user name if they're online).

Create the server-side appearance 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

When a subscription is initiated the subscribed callback gets fired and we take that opportunity to say "the current user has indeed appeared". That appear/disappear API could be backed by Redis, a database, or whatever else.

Create the client-side appearance channel subscription:

# 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()

6.1.1 Client-Server Interaction

Client connects to the Server via App.cable = ActionCable.createConsumer("ws://cable.example.com"). (cable.js). The Server identifies this connection by current_user.
Client subscribes to the appearance channel via App.cable.subscriptions.create(channel: "AppearanceChannel"). (appearance.coffee)
Server recognizes a new subscription has been initiated for the appearance channel and runs its subscribed callback, calling the appear method on current_user. (appearance_channel.rb)
Client recognizes that a subscription has been established and calls connected (appearance.coffee) which in turn calls @install and @appear. @appear calls AppearanceChannel#appear(data) on the server, and supplies a data hash of { appearing_on: $("main").data("appearing-on") }. This is possible because the server-side channel instance automatically exposes all 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.
Server receives the request for the appear action on the appearance channel for the connection identified by current_user (appearance_channel.rb). Server retrieves the data with the :appearing_on key from the data hash and sets it as the value for the :on key being passed to current_user.appear.

6.2 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:

Create the server-side web notifications channel:

# app/channels/web_notifications_channel.rb
class WebNotificationsChannel < ApplicationCable::Channel
  def subscribed
    stream_for current_user
  end
end

Create the client-side web notifications channel subscription:

# app/assets/javascripts/cable/subscriptions/web_notifications.coffee
# 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"]

Broadcast content to a web notification channel instance from elsewhere in your application:

# Somewhere in your app this is called, perhaps from a NewCommentJob
WebNotificationsChannel.broadcast_to(
  current_user,
  title: 'New things!',
  body: 'All the news fit to print'
)

The WebNotificationsChannel.broadcast_to call places a message in the current subscription adapter's 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 callback. The data passed as argument 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.

6.3 More Complete Examples

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

7 Configuration

Action Cable has two required configurations: a subscription adapter and allowed request origins.

7.1 Subscription Adapter

By default, Action Cable looks for a configuration file in config/cable.yml. The file must specify an adapter and a URL for each Rails environment. See the Dependencies section for additional information on adapters.

development:
  adapter: async

test:
  adapter: async

production:
  adapter: redis
  url: redis://10.10.3.153:6381

7.2 Allowed Request Origins

Action Cable will only accept requests from specified origins, which are passed to the server config as an array. The origins can be instances of strings or regular expressions, against which a check for match will be performed.

config.action_cable.allowed_request_origins = ['http://rubyonrails.com', %r{http://ruby.*}]

To disable and allow requests from any origin:

config.action_cable.disable_request_forgery_protection = true

By default, Action Cable allows all requests from localhost:3000 when running in the development environment.

7.3 Consumer Configuration

To configure the URL, add a call to action_cable_meta_tag in your HTML layout HEAD. This uses a URL or path typically set via config.action_cable.url in the environment configuration files.

7.4 Other Configurations

The other common option to configure is the log tags applied to the per-connection logger. Here's close to what we're using in Basecamp:

config.action_cable.log_tags = [
  -> request { request.env['bc.account_id'] || "no-account" },
  :action_cable,
  -> request { request.uuid }
]

For a full list of all configuration options, see the ActionCable::Server::Configuration class.

Also note that your server must provide at least the same number of database connections as you have workers. The default worker pool size is set to 100, so that means you have to make at least that available. You can change that in config/database.yml through the pool attribute.

8 Running Standalone Cable Servers

8.1 In App

Action Cable can run alongside your Rails application. For example, to listen for WebSocket requests on /websocket, specify that path to config.action_cable.mount_path:

# config/application.rb
class Application < Rails::Application
  config.action_cable.mount_path = '/websocket'
end

You can use App.cable = ActionCable.createConsumer() to connect to the cable server if action_cable_meta_tag is invoked in the layout. A custom path is specified as first argument to createConsumer (e.g. App.cable = ActionCable.createConsumer("/websocket")).

For every instance of your server you create and for every worker your server spawns, you will also have a new instance of Action Cable, but the use of Redis keeps messages synced across connections.

8.2 Standalone

The cable servers can be separated from your normal application server. It's still a Rack application, but it is its own Rack application. The recommended basic setup is as follows:

# cable/config.ru
require_relative 'config/environment'
Rails.application.eager_load!

run ActionCable.server

Then you start the server using a binstub in bin/cable ala:

#!/bin/bash
bundle exec puma -p 28080 cable/config.ru

The above will start a cable server on port 28080.

8.3 Notes

The WebSocket server doesn't have access to the session, but it has access to the cookies. This can be used when you need to handle authentication. You can see one way of doing that with Devise in this article.

9 Dependencies

Action Cable provides a subscription adapter interface to process its pubsub internals. By default, asynchronous, inline, PostgreSQL, evented Redis, and non-evented Redis adapters are included. The default adapter in new Rails applications is the asynchronous (async) adapter.

The Ruby side of things is built on top of websocket-driver, nio4r, and concurrent-ruby.

10 Deployment

Action Cable is powered by a combination of WebSockets and threads. Both the framework plumbing and user-specified channel work are handled internally by utilizing Ruby's native thread support. This means you can use all your regular Rails models with no problem, as long as you haven't committed any thread-safety sins.

The Action Cable server implements the Rack socket hijacking API, thereby allowing the use of a multithreaded pattern for managing connections internally, irrespective of whether the application server is multi-threaded or not.

Accordingly, Action Cable works with popular servers like Unicorn, Puma, and Passenger.

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