From: James Gray <james@...>
Date: 2009-05-11T07:31:40+09:00
Subject: Re: Communicate with a daemon

On May 10, 2009, at 4:22 PM, Paul A. wrote:

> After daemonize a Ruby process (thanks Ruby version 1.9), it is  
> possible
> to communicate with on the same machine by using socket on  
> localhost. Do
> you know some other methods instead of socket?

Sure.  In I daemon I've been working on, I just use SQLite as a queue  
for messages.  I use the Amalgalite extension, which is terrific and  
works well on Ruby 1.9.

When you're going to write to the queue, make sure to grab an  
"exclusive" lock (via the "transaction" SQL command).  When reading,  
grab an "immediate" lock first.  Set a busy handler through Amalgalite  
that retries for a reasonable time (I use 60 seconds) to grab locks.   
Add some error handling code for if the lock wait does eventually  
timeout, but that shouldn't happen under normal operations.

About the only challenges I ran into were deleting data from the  
queue.  Deleting counts as a write, so grab an "exclusive" lock.  That  
was easy of course.  The downside is that the SQLite database needs to  
have the "vacuum" command run on it every so often to reclaim wasted  
disk space.  You can't be holding a lock when you do that though.   
Thus I locked on an external source about once a day, and vacuumed the  
database.  If it's just a simple queue you need (with no long term  
storage), you could also just nuke the file and rebuild it periodically.

Anyway, I hope that gives you some fresh ideas.

James Edward Gray II