Action Cable Overview

1. What is Action Cable?

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 entire domain model written with Active Record or your ORM of choice.

2. Terminology

Action Cable uses WebSockets instead of the HTTP request-response protocol. Both Action Cable and WebSockets introduce some less familiar terminology:

2.1. Connections

Connections form the foundation of the client-server relationship. 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.

2.2. Consumers

The client of a WebSocket connection is called the consumer. In Action Cable, the consumer is created by the client-side JavaScript framework.

2.3. Channels

Each consumer can, in turn, subscribe to multiple channels. Each channel encapsulates a logical unit of work, similar to what a controller does in a typical MVC setup. For example, you could have a ChatChannel and an AppearancesChannel, and a consumer could be subscribed to either or both of these channels. At the very least, a consumer should be subscribed to one channel.

2.4. Subscribers

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).

2.5. 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.

2.6. Broadcastings

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

3. Server-Side Components

3.1. Connections

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 thereon. 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, which extends ActionCable::Connection::Base. In ApplicationCable::Connection, 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

    private
      def find_verified_user
        if verified_user = User.find_by(id: cookies.encrypted[:user_id])
          verified_user
        else
          reject_unauthorized_connection
        end
      end
  end
end

Here identified_by designates 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 an encrypted 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 unauthorized).

If your authentication approach includes using a session, you use cookie store for the session, your session cookie is named _session and the user ID key is user_id you can use this approach:

verified_user = User.find_by(id: cookies.encrypted["_session"]["user_id"])

3.1.2. Exception Handling

By default, unhandled exceptions are caught and logged to Rails' logger. If you would like to globally intercept these exceptions and report them to an external bug tracking service, for example, you can do so with rescue_from:

# app/channels/application_cable/connection.rb
module ApplicationCable
  class Connection < ActionCable::Connection::Base
    rescue_from StandardError, with: :report_error

    private
      def report_error(e)
        SomeExternalBugtrackingService.notify(e)
      end
  end
end

3.1.3. Connection Callbacks

ActionCable::Connection::Callbacks provides callback hooks that are invoked when sending commands to the client, such as when subscribing, unsubscribing, or performing an action:

• before_command
• after_command
• around_command

3.2. Channels

A channel encapsulates a logical unit of work, similar to what a controller does in a typical MVC setup. By default, Rails creates a parent ApplicationCable::Channel class (which extends ActionCable::Channel::Base) for encapsulating shared logic between your channels, when you use the channel generator for the first time.

3.2.1. Parent Channel Setup

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

Your own channel classes could then look like these examples:

# 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 channel consumer.

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

3.2.3. Exception Handling

As with ApplicationCable::Connection, you can also use rescue_from on a specific channel to handle raised exceptions:

# app/channels/chat_channel.rb
class ChatChannel < ApplicationCable::Channel
  rescue_from "MyError", with: :deliver_error_message

  private
    def deliver_error_message(e)
      # broadcast_to(...)
    end
end

3.2.4. Channel Callbacks

ActionCable::Channel::Callbacks provides callback hooks that are invoked during the life cycle of a channel:

• before_subscribe
• after_subscribe (aliased as on_subscribe)
• before_unsubscribe
• after_unsubscribe (aliased as on_unsubscribe)

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/javascript/channels/consumer.js
// Action Cable provides the framework to deal with WebSockets in Rails.
// You can generate new channels where WebSocket features live using the `bin/rails generate channel` command.

import { createConsumer } from "@rails/actioncable"

export default createConsumer()

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.

The consumer can optionally take an argument that specifies the URL to connect to. This can be a string or a function that returns a string that will be called when the WebSocket is opened.

// Specify a different URL to connect to
createConsumer('wss://example.com/cable')
// Or when using websockets over HTTP
createConsumer('https://ws.example.com/cable')

// Use a function to dynamically generate the URL
createConsumer(getWebSocketURL)

function getWebSocketURL() {
  const token = localStorage.get('auth-token')
  return `wss://example.com/cable?token=${token}`
}

4.1.2. Subscriber

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

// app/javascript/channels/chat_channel.js
import consumer from "./consumer"

consumer.subscriptions.create({ channel: "ChatChannel", room: "Best Room" })

// app/javascript/channels/appearance_channel.js
import consumer from "./consumer"

consumer.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/javascript/channels/chat_channel.js
import consumer from "./consumer"

consumer.subscriptions.create({ channel: "ChatChannel", room: "1st Room" })
consumer.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. For example, the following code uses stream_from to subscribe to the broadcasting named chat_Best Room when the value of the :room parameter is "Best Room":

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

Then, elsewhere in your Rails application, you can broadcast to such a room by calling broadcast:

ActionCable.server.broadcast("chat_Best Room", { body: "This Room is Best Room." })

If you have a stream that is related to a model, then the broadcasting name can be generated from the channel and model. For example, the following code uses stream_for to subscribe to a broadcasting like posts:Z2lkOi8vVGVzdEFwcC9Qb3N0LzE, where Z2lkOi8vVGVzdEFwcC9Qb3N0LzE is the GlobalID of the Post model.

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

You can then broadcast to this channel by calling broadcast_to:

PostsChannel.broadcast_to(@post, @comment)

5.2. Broadcastings

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.

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/javascript/channels/chat_channel.js
import consumer from "./consumer"

consumer.subscriptions.create({ channel: "ChatChannel", room: "Best Room" }, {
  received(data) {
    this.appendLine(data)
  },

  appendLine(data) {
    const html = this.createLine(data)
    const element = document.querySelector("[data-chat-room='Best Room']")
    element.insertAdjacentHTML("beforeend", html)
  },

  createLine(data) {
    return `
      <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/javascript/channels/chat_channel.js
import consumer from "./consumer"

consumer.subscriptions.create({ channel: "ChatChannel", room: "Best Room" }, {
  received(data) {
    this.appendLine(data)
  },

  appendLine(data) {
    const html = this.createLine(data)
    const element = document.querySelector("[data-chat-room='Best Room']")
    element.insertAdjacentHTML("beforeend", html)
  },

  createLine(data) {
    return `
      <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."
  }
)

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/javascript/channels/chat_channel.js
import consumer from "./consumer"

const chatChannel = consumer.subscriptions.create({ channel: "ChatChannel", room: "Best Room" }, {
  received(data) {
    // data => { sent_by: "Paul", body: "This is a cool chat 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:

1. Set up your connection.
2. Set up your parent channel.
3. 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 username 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/javascript/channels/appearance_channel.js
import consumer from "./consumer"

consumer.subscriptions.create("AppearanceChannel", {
  // Called once when the subscription is created.
  initialized() {
    this.update = this.update.bind(this)
  },

  // Called when the subscription is ready for use on the server.
  connected() {
    this.install()
    this.update()
  },

  // Called when the WebSocket connection is closed.
  disconnected() {
    this.uninstall()
  },

  // Called when the subscription is rejected by the server.
  rejected() {
    this.uninstall()
  },

  update() {
    this.documentIsActive ? this.appear() : this.away()
  },

  appear() {
    // Calls `AppearanceChannel#appear(data)` on the server.
    this.perform("appear", { appearing_on: this.appearingOn })
  },

  away() {
    // Calls `AppearanceChannel#away` on the server.
    this.perform("away")
  },

  install() {
    window.addEventListener("focus", this.update)
    window.addEventListener("blur", this.update)
    document.addEventListener("turbo:load", this.update)
    document.addEventListener("visibilitychange", this.update)
  },

  uninstall() {
    window.removeEventListener("focus", this.update)
    window.removeEventListener("blur", this.update)
    document.removeEventListener("turbo:load", this.update)
    document.removeEventListener("visibilitychange", this.update)
  },

  get documentIsActive() {
    return document.visibilityState === "visible" && document.hasFocus()
  },

  get appearingOn() {
    const element = document.querySelector("[data-appearing-on]")
    return element ? element.getAttribute("data-appearing-on") : null
  }
})

6.1.1. Client-Server Interaction

1. Client connects to the Server via createConsumer(). (consumer.js). The Server identifies this connection by current_user.

2. Client subscribes to the appearance channel via consumer.subscriptions.create({ channel: "AppearanceChannel" }). (appearance_channel.js)

3. 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)

4. Client recognizes that a subscription has been established and calls connected (appearance_channel.js), which in turn calls install and appear. appear calls AppearanceChannel#appear(data) on the server, and supplies a data hash of { appearing_on: this.appearingOn }. 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.

5. 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 relevant 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/javascript/channels/web_notifications_channel.js
// Client-side which assumes you've already requested
// the right to send web notifications.
import consumer from "./consumer"

consumer.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 an 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 as received.

6.3. More Complete Examples

See the rails/actioncable-examples repository for a full example of how to set up 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 for each Rails environment. See the Dependencies section for additional information on adapters.

development:
  adapter: async

test:
  adapter: test

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

7.1.1. Adapter Configuration

Below is a list of the subscription adapters available for end-users.

7.1.1.1. Async Adapter

The async adapter is intended for development/testing and should not be used in production.

7.1.1.2. Redis Adapter

The Redis adapter requires users to provide a URL pointing to the Redis server. Additionally, a channel_prefix may be provided to avoid channel name collisions when using the same Redis server for multiple applications. See the Redis Pub/Sub documentation for more details.

The Redis adapter also supports SSL/TLS connections. The required SSL/TLS parameters can be passed in ssl_params key in the configuration YAML file.

production:
  adapter: redis
  url: rediss://10.10.3.153:tls_port
  channel_prefix: appname_production
  ssl_params:
    ca_file: "/path/to/ca.crt"

The options given to ssl_params are passed directly to the OpenSSL::SSL::SSLContext#set_params method and can be any valid attribute of the SSL context. Please refer to the OpenSSL::SSL::SSLContext documentation for other available attributes.

If you are using self-signed certificates for redis adapter behind a firewall and opt to skip certificate check, then the ssl verify_mode should be set as OpenSSL::SSL::VERIFY_NONE.

7.1.1.3. PostgreSQL Adapter

The PostgreSQL adapter uses Active Record's connection pool, and thus the application's config/database.yml database configuration, for its connection. This may change in the future. #27214

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 the match will be performed.

config.action_cable.allowed_request_origins = ["https://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. Worker Pool Configuration

The worker pool is used to run connection callbacks and channel actions in isolation from the server's main thread. Action Cable allows the application to configure the number of simultaneously processed threads in the worker pool.

config.action_cable.worker_pool_size = 4

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 4, so that means you have to make at least 4 database connections available. You can change that in config/database.yml through the pool attribute.

7.5. Client-side Logging

Client-side logging is disabled by default. You can enable this by setting the ActionCable.logger.enabled to true.

import * as ActionCable from '@rails/actioncable'

ActionCable.logger.enabled = true

7.6. Other Configurations

The other common option to configure is the log tags applied to the per-connection logger. Here's an example that uses the user account id if available, else "no-account" while tagging:

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

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

8. Running Standalone Cable Servers

Action Cable can either run as part of your Rails application, or as a standalone server. In development, running as part of your Rails app is generally fine, but in production you should run it as a standalone.

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 ActionCable.createConsumer() to connect to the cable server if action_cable_meta_tag is invoked in the layout. Otherwise, a path is specified as first argument to createConsumer (e.g. 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 Redis or PostgreSQL adapter 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 to start the server:

$ bundle exec puma -p 28080 cable/config.ru

This starts a cable server on port 28080. To tell Rails to use this server, update your config:

# config/environments/development.rb
Rails.application.configure do
  config.action_cable.mount_path = nil
  config.action_cable.url = "ws://localhost:28080" # use wss:// in production
end

Finally, ensure you have configured the consumer correctly.

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, and 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 existing 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 multi-threaded 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.

11. Testing

You can find detailed instructions on how to test your Action Cable functionality in the testing guide.