Perlbal for chat polling

Brad Fitzpatrick brad at
Mon Mar 27 05:01:48 UTC 2006

Very cool!

I'll get your new hook into the codebase.  Btw, the Perlbal codebase is
now in subversion:

And yes, that comment is wrong:  writability isn't disabled automatically.
You need to do it.

- Brad

On Mon, 20 Mar 2006, Chris Hondl wrote:

> We've been using Perlbal at IMVU since January of this year.  IMVU is a
> community service built around a 3D avatar chat client.  We've been made
> some extensions to Perlbal that others might find useful.
> Our chat protocol is XML over HTTP with the client polling for messages.  We
> store the message queues for all the chats in memcached (the data is
> inherently ephemeral and the protocol can recover if messages get dropped
> due to a restart).  About a week ago we switched to an implementation where
> Perlbal handles the polling requests internally using a plugin.  Our plugin
> recognizes inbound polling requests, makes the calls to memcached to
> determine if there are any messages for the client, and if there are
> messages sends the response.   If there are no messages, the plugin puts the
> request into a buffer and retries again one second later.
> We've been very happy with Perlbal.  The new code is much more efficient
> because there is no HTTP traffic to the client unless there are messages
> that need to go out, because Perlbal can keep many more connections alive
> than Apache, and because Perlbal can share state across connections.  Since
> Perlbal was written in Perl (as opposed to C) and has a plugin architecture
> I was able to write and ship this extension in under a week.
> I made one change to the Perlbal 1.41 distribution to add a hook after the
> POST data has been received from the client.  I've appended the patch to the
> end of the email if anyone is interested.
> There is a comment in the source for Danga socket that seemed inaccurate.
> The Danga socket source says:
>     # watch for writability.  not needed with ->write().  write()
>     # will automatically turn on watch_write when you wrote too much
>     # and turn it off when done
>     $my_sock->watch_write($bool);
> The behavior I observed was that Danga::Socket never actually called
> watch_write(0) to turn off polling for writes and that CPU usage went to
> 100%.  My work around was to copy the code used elsewhere in Perlbal to turn
> off write notifications by passing a coderef to a subroutine as the last
> thing to write.
> I also was tripped up by the management code in that assumes all
> instances of Danga::Socket are also instances of Perlbal::Socket.   I
> created an asynchronous memcached client that initially inherited from
> Danga::Socket, but was forced to inherit from Perlbal::Socket to avoid
> failures in the management code.
> In addition to chat, the approach we used here might be interesting on any
> site using AJAX techniques to keep data on web pages up to date.  Although
> our plugin is pretty specific to our situation, we would be glad to share
> with anyone interested.  I've got a few more things I want to do to the
> asynchronous memcached client, but I expect to post that to the memcached
> mailing list in a week or so.
> Chris
> --
> Index: C:/imvu/operations/software/Perlbal- 1.41/lib/Perlbal/
> ===================================================================
> --- C:/imvu/operations/software/Perlbal-1.41/lib/Perlbal/
> (revision 8072)
> +++ C:/imvu/operations/software/Perlbal- 1.41/lib/Perlbal/
> (revision 8150)
> @@ -684,6 +684,10 @@
>      # over the buffer-to-memory size, see if we should start spooling to
> disk.
>      return if $self->{service}->{buffer_uploads} &&
> $self->decide_to_buffer_to_disk;
> +    # give plugins a chance to act on the request before we request a
> backend
> +    my $svc = $self->{service};
> +    return if $svc->run_hook('proxy_read_request', $self);
> +
>      # if we fall through to here, we need to ensure that a backend is on
> the
>      # way, because no specialized handling took over above
>      print "  finally requesting a backend\n" if Perlbal::DEBUG >= 3;

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