send xml data via socket with message (VLI) Ruby - ruby-on-rails

Question

Ruby novice. First time post so excuse any communication protocol inadequacies :)
This site has been a great help and a "HUGE!!!" shoutout of thanks to all.
I need to connect my rails app to an electricity providers api so I can vend electricity to my web customers. I'm needing some help to simply get an initial request sent to the API
IP: 41.204.194.188
Port: 8945
First block: What is a message variable length indicator (VLI)?
"2 bytes precede every message sent to/from BizSwitch. The 2 bytes are referred to as a variable length indicator. Bytes 1-2 indicate the number of bytes in the message (excluding the first 2 bytes). The 2 bytes represent a 16bit unsigned integer in network byte order. Note that if a compressed message is being sent, the message will have to first be compressed, in order to determine its length, before being sent."
Ignore compression.
link to api doc: https://dl.dropboxusercontent.com/u/3815995/Ipay-prepaidElecTransactionSpec.pdf
Simple Vend Request example:
<ipayMsg client="ipay" term="1" seqNum="0" time="2002-05-16 10:55:30 +0200">
<elecMsg ver="2.37">
<vendReq>
<ref>136105500001</ref>
<amt cur="ZAR">11400</amt>
<numTokens>1</numTokens>
<meter>A12C3456789</meter>
<payType>cash</payType>
</vendReq >
</elecMsg>
</ipayMsg>
Simple Vend Response example
<ipayMsg client="ipay" term="1" seqNum="0" time="2002-05-16 10:55:35 +0200">
<elecMsg ver="2.37"
<vendRes> <ref>136105500001</ref>
<res code="elec000">OK</res>
<util addr="Megawatt Park, Contact Centre tel [phone]" taxRef="4740101508" distId="6004708001509">Eskom Online</util>
<stdToken units="346.34" rctNum="12345678" amt="10000" tax="1400">12345678901234567890</stdToken>
<rtlrMsg>060000 Warning: This meter is not configured for FBE.</rtlrMsg>
<customerMsg>Meter not registered for Free Basic Electricity. Please apply at your local office.</customerMsg>
</vendRes>
</elecMsg>
</ipayMsg>
I've got this far and I seem connected but how do I actually send and receive responses? I've tried googling for help but yet to find how to send the XML packet and then receive the response.
#!/usr/bin/env ruby
require 'socket'
begin
socket = TCPSocket.new('41.204.194.188', 8945)
rescue => e
puts "error: #{e}"
else
puts "connected"
end
socket.close
Would appreciate any assistance or a nudge in the right direction.
Kind regards,
Jamie

Answer 1

Great I figured it out. Main issue regarding communication to the socket was sending a message variable length indicator. This stackoverflow question put me on the write path "Ruby - How to represent message length as 2 binary bytes"
Step 1: determine the length of my xml message length = message.size
The first field in the header must be the message length which is defined as a 2 binary byte message length in network byte order.
Step 2: message_variable_length_indicator = [length].pack("n")
Step 3: Connect to socket streamSock = TCPSocket::new('41.204.194.188', 8945)
Step 4: streamSock.write(message_variable_length_indicator)
Step 5: streamSock.write(message)
Step 6: get a response str = streamSock.recvfrom(1000)
Now to deal with timeout but at least I'm connecting :)

Answer 2

I have done this in PHP may be you will get idea from below code its working fine for my request.
function sendSocketRequest($XmlString, $Socket_Request) {
if (!($sock = socket_create(AF_INET, SOCK_STREAM, 0))) {
$errorcode = socket_last_error();
$errormsg = socket_strerror($errorcode);
die("Couldn't create socket: [$errorcode] $errormsg \n");
}
if (!socket_connect($sock, $Socket_Request['HostName'], $Socket_Request['Port'])) {
$errorcode = socket_last_error();
$errormsg = socket_strerror($errorcode);
die("Could not connect: [$errorcode] $errormsg \n");
}
$status = socket_write($sock, pack_int32be(strlen($XmlString)), 4);
$status = socket_write($sock, $XmlString, strlen($XmlString));
$response = socket_read($sock, $this->_socketReadLength);
socket_close($sock);
return substr($response, 2);
}
function pack_int32be($i) {
if ($i < -2147483648 || $i > 2147483647) {
die("Out of bounds");
}
return pack('C4', ($i >> 24) & 0xFF, ($i >> 16) & 0xFF, ($i >> 8) & 0xFF, ($i >> 0) & 0xFF
);
}
$socketResponse = sendSocketRequest($yourXMLString, array('HostName'=>'<HostName>','Port'=>'<Port>');