Cable

It’s like ActionCable (100% compatible with JS Client), but you know, for Crystal.

Installation

1. Add the dependency to your shard.yml:

dependencies:
  cable:
    github: cable-cr/cable
    branch: master # or use the latest version
  # Specify which backend you want to use
  cable-redis:
    github: cable-cr/cable-redis
    branch: main

Cable supports multiple backends. The most common one is Redis, but there’s a few to choose from with more being added:

Since there are multiple different versions of Redis for Crystal, you can choose which one you want to use.

• jgaskins/redis
• stefanwille/crystal-redis

Or if you don’t want to use Redis, you can try one of these alternatives

• NATS

1. Run shards install

Usage

Application code

require "cable"
# Or whichever backend you chose
require "cable-redis"

Lucky example

To help better illustrate how the entire setup looks, we’ll use Lucky, but this will work in any Crystal web framework.

Load the shard

# src/shards.cr
require "cable"
require "cable-redis"

Mount the middleware

Add the Cable::Handler before Lucky::RouteHandler

# src/app_server.cr
class AppServer < Lucky::BaseAppServer
  def middleware
    [
      Cable::Handler(ApplicationCable::Connection).new, # place before the middleware below
      Honeybadger::Handler.new,
      Lucky::ErrorHandler.new(action: Errors::Show),
      Lucky::RouteHandler.new,
    ]
   end
end

Configure cable settings

After that, you can configure your Cable server. The defaults are:

# config/cable.cr
Cable.configure do |settings|
  settings.route = "/cable"    # the URL your JS Client will connect
  settings.token = "token"     # The query string parameter used to get the token
  settings.url = ENV.fetch("CABLE_BACKEND_URL", "redis://localhost:6379")
  settings.backend_class = Cable::RedisBackend
  settings.backend_ping_interval = 15.seconds
  settings.restart_error_allowance = 20
  settings.on_error = ->(error : Exception, message : String) do
    # or whichever error reportings you're using
    Bugsnag.report(error) do |event|
      event.app.app_type = "lucky"
      event.meta_data = {
        "error_class" => JSON::Any.new(error.class.name),
        "message"     => JSON::Any.new(message),
      }
    end
  end
end

Configure logging level

You may want to tune how to report logging.

# config/log.cr
log_levels = {
  "debug" => Log::Severity::Debug,
  "info"  => Log::Severity::Info,
  "error" => Log::Severity::Error,
}
# use the `CABLE_DEBUG_LEVEL` env var to choose any of the 3 log levels above
Cable::Logger.level = log_levels[ENV.fetch("CABLE_DEBUG_LEVEL", "info")]

Alternatively, use a global log level which matches you application log code also.

See Crystal API docs for more details..

# config/log.cr
# use the `LOG_LEVEL` env var
Cable::Logger.setup_from_env(default_level: :warn)

NOTE: The volume of logs produced are high… If log costs are a concern, use warn level to only receive critical logs

Setup the main application connection and channel classes

Then you need to implement a few classes.

The connection class is how you are going to handle connections. It’s referenced in the src/app_server.cr file when creating the handler.

# src/channels/application_cable/connection.cr
module ApplicationCable
  class Connection < Cable::Connection
    # You need to specify how you identify the class, using something like:
    # Remembering that it must be a String
    # Tip: Use your `User#id` converted to String
    identified_by :identifier
    # If you'd like to keep a `User` instance together with the Connection, so
    # there's no need to fetch from the database all the time, you can use the
    # `owned_by` instruction
    owned_by current_user : User
    def connect
      UserToken.decode_user_id(token.to_s).try do |user_id|
        self.identifier = user_id.to_s
        self.current_user =  UserQuery.find(user_id)
      end
    end
  end
end

Then you need you a base channel to make it easy to inherit your app’s Cable logic.

# src/channels/application_cable/channel.cr
module ApplicationCable
  class Channel < Cable::Channel
    # some potential shared logic or helpers
  end
end

Create your app channels

Kitchen sink example

Then create your cables, as much as your want!! Let’s set up a ChatChannel as an example:

# src/channels/chat_channel.cr
class ChatChannel < ApplicationCable::Channel
  def subscribed
    # We don't support stream_for, needs to generate your own unique string
    stream_from "chat_#{params["room"]}"
  end
  def receive(data)
    broadcast_message = {} of String => String
    broadcast_message["message"] = data["message"].to_s
    broadcast_message["current_user_id"] = connection.identifier
    ChatChannel.broadcast_to("chat_#{params["room"]}", broadcast_message)
  end
  def perform(action, action_params)
    user = UserQuery.new.find(connection.identifier)
    # Perform actions on a user object. For example, you could manage
    # its status by adding some .away and .status methods on it like below
    # user.away if action == "away"
    # user.status(action_params["status"]) if action == "status"
    ChatChannel.broadcast_to("chat_#{params["room"]}", {
      "user"      => user.email,
      "performed" => action.to_s,
    })
  end
  def unsubscribed
    #  Perform any action after the client closes the connection.
    user = UserQuery.new.find(connection.identifier)
    # You could, for example, call any method on your user
    # user.logout
  end
end

Rejection example

Reject channel subscription if the request is invalid:

# src/channels/chat_channel.cr
class ChatChannel < ApplicationCable::Channel
  def subscribed
    reject if user_not_allowed_to_join_chat_room?
    stream_from "chat_#{params["room"]}"
  end
end

Callbacks example

Use callbacks to perform actions or transmit messages once the connection/channel has been subscribed.

# src/channels/chat_channel.cr
class ChatChannel < ApplicationCable::Channel
  # you can name these callbacks anything you want...
  # `after_subscribed` can accept 1 or more callbacks to be run in order
  after_subscribed :broadcast_welcome_pack_to_single_subscribed_user,
                   :announce_user_joining_to_everyone_else_in_the_channel,
                   :process_some_stuff
  def subscribed
    stream_from "chat_#{params["room"]}"
  end
  # If you ONLY need to send the current_user a message
  # and none of the other subscribers
  #
  # use -> transmit(message), which accepts Hash(String, String) or String
  def broadcast_welcome_pack_to_single_subscribed_user
    transmit({ "welcome_pack" => "some cool stuff for this single user" })
  end
  # On the other hand,
  # if you want to broadcast a message
  # to all subscribers connected to this channel
  #
  # use -> broadcast(message), which accepts Hash(String, String) or String
  def announce_user_joining_to_everyone_else_in_the_channel
    broadcast("username xyz just joined")
  end
  # you don't need to use the transmit functionality
  def process_some_stuff
    send_welcome_email_to_user
    update_their_profile
  end
end

Error handling

You can setup a hook to report errors to any 3rd party service you choose.

# config/cable.cr
Cable.configure do |settings|
  settings.on_error = ->(exception : Exception, message : String) do
    # new 3rd part service handler
    ExceptionService.notify(exception, message: message)
    # default logic
    Cable::Logger.error(exception: exception) { message }
  end
end

Default Handler

Habitat.create do
  setting on_error : Proc(Exception, String, Nil) = ->(exception : Exception, message : String) do
    Cable::Logger.error(exception: exception) { message }
  end
end

NOTE: The message field will contain details regarding which class/method raised the error

Client-Side

Check below on the JavaScript section how to communicate with the Cable backend.

JavaScript

It works with ActionCable JS Client out-of-the-box!! Yeah, that’s really cool no? If you need to adapt, make a hack, or something like that?!

No, you don’t need it! Just read the few lines below and start playing with Cable in 5 minutes!

ActionCable JS Example

examples/action-cable-js-client.md

Vanilla JS Examples

If you want to use this shard with iOS clients or vanilla JS using react etc., there is an example in the examples folder.

Note - If you are using a vanilla - non-action-cable JS client, you may want to disable the action cable response headers as they cause issues for clients who don’t know how to handle them. Set a Habitat disable_sec_websocket_protocol_header like so to disable those headers;

# config/cable.cr
Cable.configure do |settings|
  settings.disable_sec_websocket_protocol_header = true
end

Debugging

You can create a JSON endpoint to ping the server and check how things are going.

# src/actions/debug/index.cr
class Debug::Index < ApiAction
  include RequireAuthToken
  get "/debug" do
    json(Cable.server.debug_json) # Cable.server.debug_json is provided by this shard
  end
end

Alternatively, you can ping Redis directly using the redis-cli as follows;

PUBLISH _internal debug

This will dump a debug status into the logs.

Contributing

1. Fork it (https://github.com/cable-cr/cable/fork)
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new Pull Request