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: * What Action Cable is and its integration on backend and frontend * How to setup Action Cable * How to setup channels * Deployment and Architecture setup for running Action Cable -------------------------------------------------------------------------------- Introduction ------------ Action Cable seamlessly integrates [WebSockets](https://en.wikipedia.org/wiki/WebSocket) 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](https://en.wikipedia.org/wiki/Publish%E2%80%93subscribe_pattern), 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. ## Server-Side Components ### 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. #### 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 unauthorized). ### 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/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. #### 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. ```ruby # 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 ``` ## 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 by Rails: #### Connect Consumer ```js // 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. #### Subscriber 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. 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: ```coffeescript App.cable.subscriptions.create { channel: "ChatChannel", room: "1st Room" } App.cable.subscriptions.create { channel: "ChatChannel", room: "2nd Room" } ``` ## Client-Server Interactions ### Streams *Streams* provide the mechanism by which channels route published content (broadcasts) to their subscribers. ```ruby # 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` ```ruby 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: ```ruby CommentsChannel.broadcast_to(@post, @comment) ``` ### 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: ```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` 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) -> """
#{data["sent_by"]} #{data["body"]}
""" ``` ### Passing Parameters to Channels 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 ``` An object passed as the first argument to `subscriptions.create` becomes the params hash in the 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) -> """
#{data["sent_by"]} #{data["body"]}
""" ``` ```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 a 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 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 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: ```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** connects to the **Server** via `App.cable = ActionCable.createConsumer("ws://cable.example.com")`. (`cable.js`). The **Server** identifies this connection by `current_user`. 2. **Client** subscribes to the appearance channel via `App.cable.subscriptions.create(channel: "AppearanceChannel")`. (`appearance.coffee`) 3. **Server** recognizes a new subscription has been initiated for the appearance channel and runs its `subscribed` callback, calling the `appear` method on `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 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's `perform` method. 5. **Server** receives the request for the `appear` action on the appearance channel for the connection identified by `current_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 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'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`. ### More Complete Examples See the [rails/actioncable-examples](https://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, 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](#dependencies) section for additional information on adapters. ```yaml development: adapter: async test: adapter: async production: adapter: redis url: redis://10.10.3.153:6381 ``` ### 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 config.action_cable.allowed_request_origins = ['http://rubyonrails.com', %r{http://ruby.*}] ``` To disable and allow requests from any origin: ```ruby 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 an example that uses the user account id if available, else "no-account" while tagging: ```ruby 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. 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 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`, specify that path to `config.action_cable.mount_path`: ```ruby # 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. ### 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_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. ### 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 popular servers like Unicorn, Puma, and Passenger.