From: Guillaume Marcais <guslist@...>
Date: 2006-03-19T05:21:11+09:00
Subject: Re: How to write a Ruby service(?)?

Le 18 mars 06, � 08:19, Randy Kramer a �crit :

> I need to write what might be called a Ruby service or server(?)--a 
> Ruby
> program that:
>    * runs only one instance even if "invoked" multiple times
>    * can be sent additional "commands" via CLI commands, e.g., if the 
> name of
> the program is, for example, "program" I can issue multiple CLI 
> commands
> like:
>
> program <parameters>
>
> If program is not running, it will start and perform appropriate 
> actions based
> on the parameters.
>
> If program is running, the existing instance of the program will 
> receive those
> parameters and perform appropriate actions.
>
> At the moment (and maybe permanently ;-) I'm drawing a blank as to 
> what a
> program like that is called (service, server, ...?), and where to dig 
> to
> learn how to implement such a thing (preferably in Ruby).
>
> Hints?
>
> Initially I plan to implement this under Linux--some day it may get 
> ported to
> Windows, etc.

I have some code that do kind of what you are describing. But I have to 
admit that lately, when I run a daemon/service on my Linux box, I am 
incline to manage it with something like runit 
(http://smarden.sunsite.dk/runit/). I find it easier than to manage 
than the SysV services.

On to some code. The server says:

module TelnetShellServer
   def self.daemonize
     return true if fork             # Parent exits, child continues.
     Process.setsid                  # Become session leader.
     exit if fork                    # Zap session leader. See [1].
     Dir.chdir "/"                   # Release old working directory.
     File.umask 0000                 # Ensure sensible umask. Adjust as 
needed.
     STDIN.reopen("/dev/null", "r")  # Free file descriptors and
     stdout_r, stdout_w = IO.pipe
     stderr_r, stderr_w = IO.pipe
     Thread.new { loop { $logger.info(stdout_r.gets.chomp) } }
     Thread.new { loop { $logger.error(stderr_r.gets.chomp) } }
     STDOUT.reopen(stdout_w)
     STDERR.reopen(stderr_w)
     nil
   end

   class Broker
     [... actual useful service code ...]
   end
end

[ ... read options and configuration file ...]

unless foreground
   TelnetShellServer.daemonize and exit 0
end

$SAFE = 1   # disable eval() and friends
BROKER = Broker.new(login_files, auto_kill)
$logger.info("Starting DRb")
DRb.start_service(uri.to_s, BROKER)
$logger.info("Broker listening at #{uri}")
trap("TERM") { $logger.debug("TERM"); BROKER.stop }
trap("QUIT") { $logger.debug("QUIT"); BROKER.stop }
trap("INT") { $logger.debug("INT"); BROKER.stop }
trap("HUP") { $logger.debug("HUP"); BROKER.reread_logins }

DRb.thread.join


The client says:

[ ... read options and configuration file ...]
uri = URI.parse(uri)
DRb.start_service
broker = DRbObject.new_with_uri(uri.to_s)
tries = 2
host, cmd = ARGV.shift, ARGV.join(' ')
begin
   res = broker.connection_do(host, cmd)
   puts(res) if res
rescue DRb::DRbConnError
   if auto_vivify && ((tries -= 1) > 0)
     system("telnet_shell_serverd")  # server not present, start it!
     sleep(2)
     retry
   end
   raise
end

HTH,
Guillaume.


>
> Thanks!
> Randy Kramer
>
> Incidentally, if the program crashes, the next invocation of the 
> program will
> restart it and recover as much of the previous "context" as possible 
> (which
> is, of course, only what has been recorded to disk).
>
>