my $s = YAML::Dump($params);
    print "pack length " . length($s) . "\n";
    my $packed_data = pack("N/aN/a", $s, $in);

    $conn->publish($channel, "rpc.$function",
                   $packed_data,
               { exchange => $exchange_name },
                   {
                       content_type => $type,
                       correlation_id => $uuid_str,
                       reply_to => $queue_name,
                   });