set('devel_dsn','dbi:mysql:host=192.168.1.115:username=foo:password=bar');

(
#
# Execute shell commands
#

	{
		match		=> qr{^shell (.*)}io,
		columns		=> sub { return $_[1] },
		data		=> sub { split (m{[\r\n]}o, `$_[1]`) }
	},

#
# Print the current environment as a response to an 'env' command
#
	{
		match		=> 'env',
		columns		=> ['key','value'],
		data		=> sub { \%ENV }
	},

#
# Send queries prefixed with devel to another server. Since a regular expression is used, the outgoing query
# will be automatially taken from the string matched by the first set of brackets in the regexp.
#
	{
		match		=> qr{^devel (.*)}o,
		dsn		=> sub { get('devel_dsn') }
	},

#
# Redirect all queries issued after "devel" to another server. Restore with "restore"
#

	{
		match		=> 'devel',
		ok		=> sub {
					set('old_dsn', get('dsn'));
					set('dsn', get('devel_dsn'));
				}
	}, {
		match		=> 'restore',
		ok		=> sub {
					set('dsn', get('old_dsn'));
				}
	},

#
# Wrap every query starting with 'STATS' in SHOW STATUS
# In 'before', we store the counters before the query, then on 'after', we calculate the difference and store
# it again so that it can be displayed on 'SHOW STATS'
#

	{
		match		=> qr{^stats (.*)}io,
		before		=> sub { set('stats_before', get('dbh')->selectall_hashref("SHOW STATUS","Variable_name")) },
		after		=> sub {
					my $before = get('stats_before');
					my $after = get('dbh')->selectall_hashref("SHOW STATUS","Variable_name");
					my $diff;
					foreach my $key (keys %{$after}) {
						$diff->{$key} = $after->{$key}->{Value} - $before->{$key}->{Value};
						delete $diff->{$key} if $diff->{$key} == 0;	# Skip unchanged values
					}
					set('stats_diff', $diff);
				}
	},
	{
		match		=> 'show stats',
                columns		=> ['Variable_name','Value'],
                data            => sub { return (get('stats_diff') || []) }
	},

#
# Final rule - Forward all other queries as they are to the default DBH
#

	{
		match		=> qr{(.*)}o
	}
);