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

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.

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.

What is Action Cable
--------------------

Action Cable is a server which can handle multiple connection instances, with one
client-server connection instance established per WebSocket connection.

## Server-Side Components

### Connections

Connections form the foundation of the client-server relationship. For every WebSocket
the cable server is accepting, a Connection object will be instantiated on the server side.
This instance 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 a consumer. An individual
user will create one consumer-connection pair per browser tab, window, or device they have open.

Connections are instantiated via the `ApplicationCable::Connection` class in Ruby.
In this class, you authorize the incoming connection, and proceed to establish it
if the user can be identified.

#### Connection Setup

```ruby
# 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).

### 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. 

#### Parent Channel Setup

```ruby
# 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**:

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

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

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

#### 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.

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

## Client-Side Components

### Connections

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

#### Connect Consumer

```coffeescript
# app/assets/javascripts/cable.coffee
#= require action_cable

@App = {}
App.cable = ActionCable.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.

#### Subscriber

When a consumer is subscribed to a channel, they act as a subscriber. 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.
(remember that a physical user may have multiple consumers, one per tab/device open to your connection).

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

```coffeescript
# 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.

## Client-Server Interactions

### Streams

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

```ruby
# app/channels/application_cable/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`

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

You can then broadcast to this channel using: `CommentsChannel.broadcast_to(@post, @comment)`

### 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.

Broadcasts are called elsewhere in your Rails application:
```ruby
  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(data)`
callback.

### 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.

```coffeescript
# 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>
    """
```

### Passing Parameters to Channel

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

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

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

```coffeescript
# 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>
    """
```

```ruby
# 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.'
```


### Rebroadcasting message

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

```ruby
# 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
```

```coffeescript
# 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.

## Full-stack examples

The following setup steps are common to both examples:

  1. [Setup your connection](#connection-setup)
  2. [Setup your parent channel](#parent-channel-setup)
  3. [Connect your consumer](#connect-consumer)

### 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:

```ruby
# 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 `#subscribed` callback is invoked by the consumer, a client-side subscription
is initiated. In this case, we take that opportunity to say "the current user has
indeed appeared". That appear/disappear API could be backed by Redis, a database,
or whatever else.

#### Create the client-side Appearance Channel subscription:

```coffeescript
# 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()
```

##### Client-Server Interaction
1. **Client** establishes a connection with the **Server** via `App.cable = ActionCable.createConsumer("ws://cable.example.com")`. [*` cable.coffee`*] The **Server** identified this connection instance by `current_user`.
2. **Client** initiates a subscription to the `Appearance Channel` for their connection via `App.cable.subscriptions.create "AppearanceChannel"`. [*`appearance.coffee`*]
3. **Server** recognizes a new subscription has been initiated for `AppearanceChannel` channel performs the `subscribed` callback, which calls the `appear` method on the `current_user`. [*`appearance_channel.rb`*]
4. **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 will automatically expose the public methods declared on the class (minus the callbacks), so that these can be reached as remote procedure calls via a subscription's `perform` method.
5. **Server** receives the request for the `appear` action on the `AppearanceChannel` channel for the connection identified by `current_user`. [*`appearance_channel.rb`*] The server retrieves the data with the `appearing_on` key from the data hash, and sets it as the the value for the `on:` key being passed to `current_user.appear`.

### 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:

```ruby
# 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:
```coffeescript
# 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

```ruby
# 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 (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(data)`
callback. The data is the hash sent as the second parameter to the server-side
broadcast call, JSON encoded for the trip across the wire, and unpacked for
the data argument arriving to `#received`.

### More complete examples

See the [rails/actioncable-examples](http://github.com/rails/actioncable-examples)
repository for a full example of how to setup Action Cable in a Rails app and adding channels.

## Configuration

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

### Subscription Adapter

By default, `ActionCable::Server::Base` will look for a configuration file
in `Rails.root.join('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.

```yaml
production: &production
  adapter: redis
  url: redis://10.10.3.153:6381
development: &development
  adapter: async
test: *development
```

This format allows you to specify one configuration per Rails environment.
You can also change the location of the Action Cable config file in
a Rails initializer with something like:

```ruby
Rails.application.paths.add "config/redis/cable", with: "somewhere/else/cable.yml"
```

### 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.

```ruby
Rails.application.config.action_cable.allowed_request_origins = ['http://rubyonrails.com', /http:\/\/ruby.*/]
```

To disable and allow requests from any origin:

```ruby
Rails.application.config.action_cable.disable_request_forgery_protection = true
```

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


### 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.

### 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:

```ruby
Rails.application.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 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.

## Running standalone cable servers

### In App

Action Cable can run alongside your Rails application. For example, to
listen for WebSocket requests on `/websocket`, mount the server at that path:

```ruby
# config/routes.rb
Example::Application.routes.draw do
  mount ActionCable.server => '/cable'
end
```

You can use `App.cable = ActionCable.createConsumer()` to connect to the
cable server if `action_cable_meta_tag` is included 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 ActionCable,
but the use of Redis keeps messages synced across connections.

### 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:

```ruby
# cable/config.ru
require ::File.expand_path('../../config/environment', __FILE__)
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.

### 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](http://www.rubytutorial.io/actioncable-devise-authentication).

## 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](https://github.com/faye/websocket-driver-ruby),
[nio4r](https://github.com/celluloid/nio4r), and [concurrent-ruby](https://github.com/ruby-concurrency/concurrent-ruby).

## 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 all the popular application servers -- Unicorn, Puma and Passenger.