Message     Process
abc         A
def         D
ghi         D
jkl         C

##</code><code>##

SELECT DISTINCT(Process) FROM Queue;

##</code><code>##

$SIG{USR1} = \&CatchSignal;

...

sub CatchSignal {
	my $signal = shift;

	if ($signal eq "USR1") {
		$SIG_CHECK_DATABASE = 1;
	}
}

...

if ($SIG_CHECK_DATABASE) {
	# Process all messages from the database queue
	$SIG_CHECK_DATABASE = 0;
}