A WebSocket backend for a Rack-based (using Puma) Ruby app that distributes messages received via Redis

websocket_backend.rb

# This file's job is to accept WebSocket connections
# and pass along Redis messages to them

require 'faye/websocket'

class WebSocketBackend
  def initialize(app)
    @app     = app
    @clients = []
    @running = false
    @thread = nil
  end

  def call(env)
    if Faye::WebSocket.websocket?(env)
      ws = Faye::WebSocket.new(env, nil, { ping: 15 })

      ws.on :open do |event|
        @clients << ws

        # Only start the streaming process after the first WS connection
        unless @running
          @running = true
          @thread = Thread.new do
            Redis.new.subscribe('event_channel') do |on|
              on.message do |channel, msg|
                @clients.each { |ws| ws.send(msg) }
              end
            end
          end
        end
      end

      ws.on :close do |event|
        @clients.delete(ws)
        ws = nil

        if @clients.empty?
          @running = false
          @thread.kill
        end
      end

      ws.rack_response
    else
      @app.call(env)
    end
  end
end