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:
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.
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 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 ason_subscribe
)before_unsubscribe
after_unsubscribe
(aliased ason_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:
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
Client connects to the Server via
createConsumer()
. (consumer.js
). The Server identifies this connection bycurrent_user
.Client subscribes to the appearance channel via
consumer.subscriptions.create({ channel: "AppearanceChannel" })
. (appearance_channel.js
)Server recognizes a new subscription has been initiated for the appearance channel and runs its
subscribed
callback, calling theappear
method oncurrent_user
. (appearance_channel.rb
)Client recognizes that a subscription has been established and calls
connected
(appearance_channel.js
), which in turn callsinstall
andappear
.appear
callsAppearanceChannel#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'sperform
method.Server receives the request for the
appear
action on the appearance channel for the connection identified bycurrent_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 tocurrent_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
.
It is not recommended to use VERIFY_NONE
in production unless you absolutely understand the security implications. In order to set this option for the Redis adapter, the config should be ssl_params: { verify_mode: <%= 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
PostgreSQL has a 8000 bytes limit on NOTIFY
(the command used under the hood for sending notifications) which might be a constraint when dealing with large payloads.
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.
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