Flush out the README overview of Action Cable

README

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-# ActionCable
-
-Action Cable is a framework for realtime communication over websockets.

README.md

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+# Action Cable -- Integrated websockets for Rails
+
+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 ActiveRecord or whatever.
+
+
+## Terminology
+
+A single Action Cable server can handle multiple connection instances. It goes one
+connection instance per websocket connection. A single user may well have multiple
+websockets open to your application if they use multiple browser tabs or devices.
+The client of a websocket connection is called the consumer.
+
+Each consumer can in turn subscribe to multiple cable channels. Each channel encapsulates
+a logical unit of work, similar to what a controller does in a regular MVC setup. So 
+you may have a ChatChannel and a AppearancesChannel. The consumer can be subscribed to either
+or to both. At the very least, a consumer should be subscribed to one channel.
+
+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 via a subscription only once. (But remember that
+a physical user may have multiple consumers, one per tab/device open to your connection).
+
+Each channel can then again be streaming zero or more broadcastings. A broadcasting is a
+pubsub link where anything transmitted by the broadcaster is sent directly to the channel 
+subscribers who are streaming that named broadcasting.
+
+As you can see, this is a fairly deep architectural stack. There's a lot of new terminology
+to identify the new pieces, and on top of that, you're dealing with both client and server side
+reflections of each unit.
+
+
+## A full-stack example
+
+The first thing you must do is defined your ApplicationCable::Connection class in Ruby. This
+is the place where you do authorization of the incoming connection, and proceed to establish it
+if all is well. Here's the simplest example starting with the server-side connection class:
+
+  # 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 cookies.signed[:user_id]
+            current_user
+          else
+            reject_unauthorized_connection
+          end
+        end
+    end
+  end
+
+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).
+
+The client-side needs to setup a consumer instance of this connection. That's done like so:
+
+  # app/assets/javascripts/cable.coffee
+  @App = {}
+  App.cable = Cable.createConsumer "http://cable.example.com"
+
+The http://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.
+
+That's all you need to establish the connection! But of course, this isn't very useful in
+itself. This just gives you the plumbing. To make stuff happen, you need content. That content
+is defined by declaring channels on the server and allowing the consumer to subscribe to them.
+
+
+## Channel 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.
+(That's useful for creating presence features like showing a green dot next to a user name if they're online).
+
+First you declare the server-side 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
+
+The #subscribed callback is invoked when, as we'll show below, 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 or a database or whatever else. Here's what the client-side of that looks like:
+
+  # app/assets/javascripts/cable/subscriptions/appearance.coffee
+  App.appearance = App.cable.subscriptions.create "AppearanceChannel",
+    connected: ->
+      # Called once the subscription has been successfully completed
+  
+    appear: ->
+      @perform 'appear', appearing_on: @appearingOn()
+  
+    away: ->
+      @perform 'away'
+  
+    appearingOn: ->
+      $('main').data 'appearing-on'
+
+  $(document).on 'page:change', ->
+    App.appearance.appear()
+
+  $(document).on 'click', '[data-behavior~=appear_away]', ->
+    App.appearance.away()
+    false
+
+Simply calling App.cable.subscriptions.create will setup the subscription, which will call AppearanceChannel#subscribed,
+which in turn is linked to original App.consumer -> ApplicationCable::Connection instances.
+
+We then link App.appearance#appear to AppearanceChannel#appear(data). 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 App.appearance#perform.
+
+Finally, we expose App.appearance to the machinations of the application itself by hooking the #appear call into the 
+Turbolinks page:change callback and allowing the user to click a data-behavior link that triggers the #away call.
+
+
+## Channel 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
+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:
+
+  # app/channels/web_notifications.rb
+  class WebNotificationsChannel < ApplicationCable::Channel
+     def subscribed
+       stream_from "web_notifications_#{current_user.id}"
+     end
+   end
+
+   # Somewhere in your app this is called, perhaps from a NewCommentJob
+   ActionCable.server.broadcast \
+     "web_notifications_1", { title: 'New things!', body: 'All shit fit for print' }
+
+   # Client-side which assumes you've already requested the right to send web notifications
+   App.cable.subscriptions.create "WebNotificationsChannel",
+     received: (data) ->
+      web_notification = new Notification data['title'], body: data['body']
+
+The ActionCable.server.broadcast call places a message in the Redis' pubsub queue under the broadcasting name of "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.
+
+
+## Dependencies
+
+Action Cable is currently tied to Redis through its use of the pubsub feature to route
+messages back and forth over the websocket cable connection. This dependency may well
+be alleviated in the future, but for the moment that's what it is. So be sure to have
+Redis installed and running.
+
+
+## Deployment
+
+Action Cable is powered by a combination of EventMachine and threads. The
+framework plumbing needed for connection handling is handled in the 
+EventMachine loop, but the actual channel, user-specified, work is handled
+in a normal Ruby thread. This means you can use all your regular Rails models
+with no problem, as long as you haven't committed any thread-safety sins.
+
+But this also means that Action Cable needs to run in its own server process.
+So you'll have one set of server processes for your normal web work, and another
+set of server processes for the Action Cable. The former can be single-threaded,
+like Unicorn, but the latter must be multi-threaded, like Puma.
+
+
+## Alpha disclaimer
+
+Action Cable is currently considered alpha software. The API is almost guaranteed to change between
+now and its first production release as part of Rails 5.0. Real applications using the framework
+are all well underway, but as of July 8th, 2015, there are no deployments in the wild yet.
+
+So this current release, which resides in rails/actioncable, is primarily intended for
+the adventurous kind, who do not mind reading the full source code of the framework. And it
+serves as an invitation for all those crafty folks to contribute to and test what we have so far,
+in advance of that general production release.
+
+Action Cable will move from rails/actioncable to rails/rails and become a full-fledged default
+framework alongside Action Pack, Active Record, and the like once we cross the bridge from alpha
+to beta software (which will happen once the API and missing pieces have solidified).
+
+
+## Download and installation
+
+The latest version of Action Pack can be installed with RubyGems:
+
+  % gem install actionpack
+
+Source code can be downloaded as part of the Rails project on GitHub
+
+* https://github.com/rails/rails/tree/master/actionpack
+
+
+## License
+
+Action Pack is released under the MIT license:
+
+* http://www.opensource.org/licenses/MIT
+
+
+## Support
+
+Bug reports can be filed for the alpha development project here:
+
+* https://github.com/rails/actioncable/issues