Redis + ActionController::Live threads not dying

Question

Background: We\'ve built a chat feature in to one of our existing Rails applications. We\'re using the new ActionController::Live module and ru

Answer 1

I'm currently making an app that revolves around ActionController:Live, EventSource and Puma and for those that are encountering problems closing streams and such, instead of rescuing an IOError, in Rails 4.2 you need to rescue ClientDisconnected. Example:

def stream
  #Begin is not required
  twitter_client = Twitter::Streaming::Client.new(config_params) do |obj|
    # Do something
  end
rescue ClientDisconnected
  # Do something when disconnected
ensure
  # Do something else to ensure the stream is closed
end

I found this handy tip from this forum post (all the way at the bottom): http://railscasts.com/episodes/401-actioncontroller-live?view=comments

Answer 2

Here you are solution with timeout that will exit blocking Redis.(p)subscribe call and kill unused connection tread.

class Stream::FixedController < StreamController
  def events
    # Rails reserve a db connection from connection pool for
    # each request, lets put it back into connection pool.
    ActiveRecord::Base.clear_active_connections!

    # Last time of any (except heartbeat) activity on stream
    # it mean last time of any message was send from server to client
    # or time of setting new connection
    @last_active = Time.zone.now

    # Redis (p)subscribe is blocking request so we need do some trick
    # to prevent it freeze request forever.
    redis.psubscribe("messages:*", 'heartbeat') do |on|
      on.pmessage do |pattern, event, data|
        # capture heartbeat from Redis pub/sub
        if event == 'heartbeat'
          # calculate idle time (in secounds) for this stream connection
          idle_time = (Time.zone.now - @last_active).to_i

          # Now we need to relase connection with Redis.(p)subscribe
          # chanel to allow go of any Exception (like connection closed)
          if idle_time > 4.minutes
            # unsubscribe from Redis because of idle time was to long
            # that's all - fix in (almost)one line :)
            redis.punsubscribe
          end
        else
          # save time of this (last) activity
          @last_active = Time.zone.now
        end
        # write to stream - even heartbeat - it's sometimes chance to
        # capture dissconection error before idle_time
        response.stream.write("event: #{event}\ndata: #{data}\n\n")
      end
    end
    # blicking end (no chance to get below this line without unsubscribe)
  rescue IOError
    Logs::Stream.info "Stream closed"
  rescue ClientDisconnected
    Logs::Stream.info "ClientDisconnected"
  rescue ActionController::Live::ClientDisconnected
    Logs::Stream.info "Live::ClientDisconnected"
  ensure
    Logs::Stream.info "Stream ensure close"
    redis.quit
    response.stream.close
  end
end

You have to use reds.(p)unsubscribe to end this blocking call. No exception can break this.

My simple app with information about this fix: https://github.com/piotr-kedziak/redis-subscribe-stream-puma-fix

Answer 3

Building on @James Boutcher, I used the following in clustered Puma with 2 workers, so that I have only 1 thread created for the heartbeat in config/initializers/redis.rb:

config/puma.rb

on_worker_boot do |index|
  puts "worker nb #{index.to_s} booting"
  create_heartbeat if index.to_i==0
end

def create_heartbeat
  puts "creating heartbeat"
  $redis||=Redis.new
  heartbeat = Thread.new do
    ActiveRecord::Base.connection_pool.release_connection
    begin
      while true
        hash={event: "heartbeat",data: "heartbeat"}
        $redis.publish("heartbeat",hash.to_json)
        sleep 20.seconds
      end
    ensure
      #no db connection anyway
    end
  end
end

Answer 4

Here's a potentially simpler solution which does not use a heartbeat. After much research and experimentation, here's the code I'm using with sinatra + sinatra sse gem (which should be easily adapted to Rails 4):

class EventServer < Sinatra::Base
 include Sinatra::SSE
 set :connections, []
 .
 .
 .
 get '/channel/:channel' do
 .
 .
 .
  sse_stream do |out|
    settings.connections << out
    out.callback {
      puts 'Client disconnected from sse';
      settings.connections.delete(out);
    }
  redis.subscribe(channel) do |on|
      on.subscribe do |channel, subscriptions|
        puts "Subscribed to redis ##{channel}\n"
      end
      on.message do |channel, message|
        puts "Message from redis ##{channel}: #{message}\n"
        message = JSON.parse(message)
        .
        .
        .
        if settings.connections.include?(out)
          out.push(message)
        else
          puts 'closing orphaned redis connection'
          redis.unsubscribe
        end
      end
    end
  end
end

The redis connection blocks on.message and only accepts (p)subscribe/(p)unsubscribe commands. Once you unsubscribe, the redis connection is no longer blocked and can be released by the web server object which was instantiated by the initial sse request. It automatically clears when you receive a message on redis and sse connection to the browser no longer exists in the collection array.

Answer 5

Instead of sending a heartbeat to all the clients, it might be easier to just set a watchdog for each connection. [Thanks to @NeilJewers]

class Stream::FixedController < StreamController
  def events
    # Rails reserve a db connection from connection pool for
    # each request, lets put it back into connection pool.
    ActiveRecord::Base.clear_active_connections!

    redis = Redis.new

    watchdog = Doberman::WatchDog.new(:timeout => 20.seconds)
    watchdog.start

    # Redis (p)subscribe is blocking request so we need do some trick
    # to prevent it freeze request forever.
    redis.psubscribe("messages:*") do |on|
      on.pmessage do |pattern, event, data|
        begin
          # write to stream - even heartbeat - it's sometimes chance to
          response.stream.write("event: #{event}\ndata: #{data}\n\n")
          watchdog.ping

        rescue Doberman::WatchDog::Timeout => e
          raise ClientDisconnected if response.stream.closed?
          watchdog.ping
        end
      end
    end

  rescue IOError
  rescue ClientDisconnected

  ensure
    response.stream.close
    redis.quit
    watchdog.stop
  end
end

Answer 6

A solution I just did (borrowing a lot from @teeg) which seems to work okay (haven't failure tested it, tho)

config/initializers/redis.rb

$redis = Redis.new(:host => "xxxx.com", :port => 6379)

heartbeat_thread = Thread.new do
  while true
    $redis.publish("heartbeat","thump")
    sleep 30.seconds
  end
end

at_exit do
  # not sure this is needed, but just in case
  heartbeat_thread.kill
  $redis.quit
end

And then in my controller:

def events
    response.headers["Content-Type"] = "text/event-stream"
    redis = Redis.new(:host => "xxxxxxx.com", :port => 6379)
    logger.info "New stream starting, connecting to redis"
    redis.subscribe(['parse.new','heartbeat']) do |on|
      on.message do |event, data|
        if event == 'parse.new'
          response.stream.write("event: parse\ndata: #{data}\n\n")
        elsif event == 'heartbeat'
          response.stream.write("event: heartbeat\ndata: heartbeat\n\n")
        end
      end
    end
  rescue IOError
    logger.info "Stream closed"
  ensure
    logger.info "Stopping stream thread"
    redis.quit
    response.stream.close
  end