Create action_cable_overview.md resolves #23176 [ci skip]

Rails Guide for Action Cable

Added content from README

Pull additional information from Action Cable README and restructured Client/Server elements

Update to list numbering

Switched from list numbers to headings in the examples
Change AppearancesChannel to AppearanceChannel

Word missing

fixed missing word.

Removed Mistaken nesting of Channel class

Removed nesting of ChatChannel and AppearanceChannel from within ApplicationChannel Module. (Copy&Paste error)

Incorporated first round of comments

1. Capitalize "action cable"
2. Separated "Consumers require... via JavaScript" into two sentences
3. Minor typographical correction (remember...)
4. Extra backtick
5. Revised text which implied Redis was the exclusive storage adapter to reflect it's position as default.
6. Revised reference to denote correct config file location: config/cable.yml
7. Added adapter: redis to environment configuration blocks
8. Capitalized "R" for "Rack"
9. Revised syntax for routing to reflect the hash syntax. (to: NOT :to =>)
10. Removed reference to Action Cable being separate from Rails.
11. Began revision for adapter API [I believe this requires a reformatting of the 'Redis' portion of the configuration section to simply "Storage Adapters"]
12. Celluloid -> Concurrent-ruby

Moved errant grave mark [ci skip]

Reordered "In App" above "Standalone" [ci skip]

Reordered to reflect "In App" as preferable to "Standalone"

Action Cable Deployment [ci skip]

Removed paragraph that alludes to Action Cable not being able to run in the same process as Rails.

Removed EM reference and updated deployment [ci skip]

Removed explicit multi-threaded server dependency reference [ci skip]

Revised Configuration and fixed typos [ci skip]

Switched Lifecycle Graphic for Detailed Description [ci skip]

Switched from Lifecycle graphic to detailed description to facilitate revision and maintainability in the future.

Capitalized Heading (typo) [ci skip]

Implemented merged commits from README [ci skip]

Pulled over all (or at least I believe all) merged commits from README. (Dec 14, 2015 - Feb 11, 2016)

Editorial - Capitalize WebSockets [ci skip]

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+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.
+
+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 foundaton of the client-server relationship. For every WebSocket
+the cable server is accepting, a Connection object will be instantiated. 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 the consumer. 
+A single consumer may have multiple WebSockets open to your application if they
+use multiple browser tabs or devices. 
+
+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 again or 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 relies on the fact that you will already have handled authentication of the user,
+and that a successful authentication sets a signed cookie with the `user_id`. 
+This 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. 
+You should first define a parent `ApplicationCable::Channel` class in Ruby, which
+encapsulates 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 child 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
+  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:
+
+#### Connect Consumer
+```coffeescript
+# app/assets/javascripts/cable.coffee
+#= require action_cable
+
+@App = {}
+App.cable = ActionCable.createConsumer("ws://cable.example.com")
+```
+The `ws://cable.example.com` address must point to your set of Action Cable servers, and it
+must share a cookie namespace with the rest of the application (which may live under http://example.com).
+This ensures that the signed cookie will be correctly sent.
+
+#### 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 subscribtion to a given channel:
+```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" }
+
+# app/assets/javascripts/cable/subscriptions/appearance.coffee
+# Assumes you've already requested the right to send web notifications
+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
+```
+
+### 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
+ActionCable.server.broadcast \
+  "web_notifications_#{current_user.id}", { title: 'New things!', body: 'All the news that is fit to print' }
+```
+
+The `ActionCable.server.broadcast` 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
+# Client-side, which 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>
+    """
+```
+
+```ruby
+# 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.' }
+```
+
+
+### 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)
+    ActionCable.server.broadcast "chat_#{params[:room]}", data
+  end
+end
+```
+
+```coffeescript
+# app/assets/javascripts/cable/subscriptions/chat.coffee
+# Client-side which assumes you've already requested the right to send web notifications
+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 invokesan 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_from "web_notifications_#{current_user.id}"
+  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
+ActionCable.server.broadcast \
+  "web_notifications_#{current_user.id}", { title: 'New things!', body: 'All the news that is fit to print' }
+```
+
+The `ActionCable.server.broadcast` 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 three required configurations: a subscription adapter,
+allowed request origins, and the cable server URL.
+
+### 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 # Optional as default is redis
+  url: redis://10.10.3.153:6381
+development: &development
+  url: redis://localhost:6379
+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
+
+Once you have decided how to run your cable server (see below), you must provide
+the server url (or path) to your client-side setup. There are two ways you can do this.
+
+The first is to simply pass it in when creating your consumer. For a standalone server,
+this would be something like: `App.cable = ActionCable.createConsumer("ws://example.com:28080")`,
+and for an in-app server, something like: `App.cable = ActionCable.createConsumer("/cable")`.
+
+The second option is to pass the server url through the `action_cable_meta_tag` in your layout.
+This uses a url or path typically set via `config.action_cable.url` 
+in the environment configuration files, or defaults to "/cable".
+
+This method is especially useful if your WebSocket url might change
+between environments. If you host your production server via https,
+you will need to use the wss scheme for your ActionCable server, but 
+development might remain http and use the ws scheme. You might use localhost
+in development and your domain in production.
+
+In any case, to vary the WebSocket url between environments, add the following
+configuration to each environment:
+
+```ruby
+config.action_cable.url = "ws://example.com:28080"
+```
+
+Then add the following line to your layout before your JavaScript tag:
+
+```erb
+<%= action_cable_meta_tag %>
+```
+
+And finally, create your consumer like so:
+
+```coffeescript
+App.cable = ActionCable.createConsumer()
+```
+
+### 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 }
+]
+```
+
+Your WebSocket URL might change between environments. If you host your
+production server via https, you will need to use the wss scheme
+for your ActionCable server, but development might remain http and
+use the ws scheme. You might use localhost in development and your
+domain in production. In any case, to vary the WebSocket URL between
+environments, add the following configuration to each environment:
+
+```ruby
+config.action_cable.url = "ws://example.com:28080"
+```
+
+Then add the following line to your layout before your JavaScript tag:
+
+```erb
+<%= action_cable_meta_tag %>
+```
+
+And finally, create your consumer like so:
+
+```coffeescript
+App.cable = Cable.createConsumer()
+```
+
+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 the cable server
+
+### 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 server(s) 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
+
+Beware that currently the cable server will _not_ auto-reload any
+changes in the framework. As we've discussed, long-running cable
+connections mean long-running objects. We don't yet have a way of
+reloading the classes of those objects in a safe manner. So when 
+you change your channels, or the model your channels use, you must
+restart the cable server.
+
+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.