Action Cable Overview
In this guide you will learn how Action Cable works and how to use WebSockets to incorporate real-time features into your Rails application.
After reading this guide, you will know:
- How to setup Action Cable
- How to setup channels
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) ChatChannel.broadcast_to("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:
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 bycurrent_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 theappear
method oncurrent_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
callsAppearanceChannel#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'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 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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