Hello Monks,
I wrote a small SNMP based monitor that hooks onto an OID and responds to various SNMP commands.
(I use my Organizations OID, but for Internet purposes 9999 works just fine.)

I would like to modify this so I have a root monitor "driver" which loads the monitors from individual files placed in a ./plugins/ directory. In other words how do I design this in pluggable type architecture? Can you help me out with some reading material or basic examples?

Also, I am always looking for ways to improve my code so, if you have any constructive criticism or ideas I would like to hear them.

For bonus points and extremely low on the priority scale: how to I get Perl to tell snmpwalk the OID names?

As always, thanks for your help.

There is a colorized version here: http://pastebin.com/QKH4H9ta

#!/usr/bin/perl
#   
#   for this to work place this file in: /usr/share/snmp/exim_queue_mo
+nitor.pl
#   insert the following line in /usr/share/snmp/snmpd.conf
#   perl do "/usr/share/snmp/exim_queue_monitor.pl
#
use strict;
use warnings;

# Global Vars:
our $agent;

# Private vars
my $root_oid = '.1.3.6.1.4.1.9999';
my $running;
# set to 1 to get debugging information
my $debugging = 0;
# since we're embedded, set this to 0
my $subagent = 0;

 
BEGIN {
    print STDERR "starting exim_queue_monitor.pl\n";
}

use NetSNMP::OID     (':all');
use NetSNMP::agent     (':all');
use NetSNMP::ASN     (':all');
 
print STDERR "exim_queue_monitor.pl loaded ok\n";
 
# where we are going to hook onto
my $reg_oid = new NetSNMP::OID($root_oid);
print STDERR "registering at ",$reg_oid,"\n" if $debugging;

# we register ourselves with the master agent we're embedded in.  
$agent->register('exim_queue_monitor', $reg_oid, \&exim_queue_monitor,
+ );
 
# processes system command and updates request obj.
sub process_exim_mail_queue {
    my $request = shift;
    my $queue_len = `exim -bpc`;
    chomp $queue_len;
    print STDERR "   -> queue length = $queue_len\n" if $debugging;
    $request->setValue(ASN_INTEGER, $queue_len);
}

sub process_postfix_mail_queue {
    my $request = shift;
    my $queue_len = `find /var/spool/postfix/deferred/ /var/spool/post
+fix/active/ /var/spool/postfix/maildrop/ | wc -l`;
    chomp $queue_len;
    print STDERR "   -> queue length = $queue_len\n" if $debugging;
    $request->setValue(ASN_INTEGER, $queue_len);
}

# define a handler for incoming SNMP requests for our part of the OID 
+tree.  
sub exim_queue_monitor {
    my ($handler, $registration_info, $request_info, $requests) = @_;
    my $request;
    
    #debug messages    
    do {
        print STDERR "refs: ",join(", ", ref($handler), ref($registrat
+ion_info), 
                   ref($request_info), ref($requests)),"\n";
        print STDERR "processing a request of type " . $request_info->
+getMode() . "\n";
    } 
    if $debugging;

    for($request = $requests; $request; $request = $request->next()) {
        my $oid = $request->getOID();
        print STDERR "  processing request of $oid\n" if $debugging;

        # if SNMP monitor request mode is GET
        if ($request_info->getMode() == MODE_GET) {
            # queue request for exim
            if ($oid == new NetSNMP::OID($root_oid . ".1.2.1")) {
               process_exim_mail_queue $request;
            }
            # queue request for postfix
            elsif ($oid == new NetSNMP::OID($root_oid . ".1.2.2")) {
               process_postfix_mail_queue $request;
            }
        } 

        # if SNMP monitor request mode is GETNEXT 
        elsif ($request_info->getMode() == MODE_GETNEXT) {
            # queue request for exim
            if ($oid == new NetSNMP::OID($root_oid . ".1.2.1")) {
               process_exim_mail_queue $request;
            }
            # queue request for postfix
            elsif ($oid == new NetSNMP::OID($root_oid . ".1.2.2")) {
               process_postfix_mail_queue $request;
            }
        }
    }# end for loop

    print STDERR "  finished processing\n" if $debugging;
}
[download]

EDIT:Updated to more recent version, fixed minor naming.