Is it possible to create a thread in Rails subscribe to Redis message channel?

Is it possible to create a thread in Rails subscribe to Redis message channel?

[胡明]

Hi there

I have one question about related to thread in Unicorn; I have asked the
quesiton on stackoverflow.com but no one answers:

http://stackoverflow.com/questions/29180275/is-it-possible-to-create-a-thread-in-rails-subscribe-to-redis-message-channel

I also pasted the question in this email, hope that someone could help me
with this:

==============================================

I am trying to create a thread in Rails to subscribe a message channel of
Redis. Is there a way to do this? I am using unicorn.

I have tried to do this in the unicorn configuration like this:

after_fork do |server, worker|

  Thread.new do
    begin
      $redis.subscribe(:one, :two) do |on|
        on.subscribe do |channel, subscriptions|
          puts "Subscribed to ##{channel} (#{subscriptions} subscriptions)"
        end
        on.message do |channel, message|
          puts "##{channel}: #{message}"
          $redis.unsubscribe if message == "exit"
        end
        on.unsubscribe do |channel, subscriptions|
          puts "Unsubscribed from ##{channel} (#{subscriptions} subscriptions)"
        end
      end
    rescue Redis::BaseConnectionError => error
      puts "#{error}, retrying in 1s"
      sleep 1
      retry
    end
  endend

But it will make the unicorn server unable to handle any web request. I
thought that if I am using a different thread to subscribe to Redis, it
won't block the main thread; am I missing something here?

-- 
Regards,
Hu, Ming

Re: Is it possible to create a thread in Rails subscribe to Redis message channel?

[Eric Wong]

胡明 <humings@gmail.com> wrote:
> I am trying to create a thread in Rails to subscribe a message channel of
> Redis. Is there a way to do this? I am using unicorn.

Theoretically, yes, similar things are done with other services.

<snip>

> But it will make the unicorn server unable to handle any web request. I
> thought that if I am using a different thread to subscribe to Redis, it
> won't block the main thread; am I missing something here?

I'm not familiar with Redis, but I know it's a server based on stream
sockets similar to anything else HTTP/IMAP/MySQL/Postgres/memcached-based.

But based on your observation, your client library is probably blocking
the entire Ruby VM by not releasing the GVL when it the thread waits on
Redis.

Which Redis client library are you using?

If the client library is written in C, it should have rb_thread_* calls
when it needs to wait on the server for anything (e.g.
rb_thread_call_without_gvl, rb_thread_fd_select, rb_thread_fd_select,
etc...).  If you don't see those in the source code, get it fixed :)