How to create a custom object using node.js

Question

I have a java-based server that allows client applications to connect via other programming languages (java, unity, obj-c). I would like to know how to add javascript to this list using node.js and socket.io. The server listens on a set port and accepts simple json data plus an int for length in bytes, it response in the same format. The format of the "packet" is like so:

first four bytes are the length
00 00 00 1c

remaining bytes are the data
7b 22 69 64 22 3a 31 2c 22 6e 61 6d 65 22 3a 22 73 6f 6d 65 77 69 64 67 65 74 22 7d

The data is sent over TCP and is encoded in little endian. The object in originating from Java is modeled like so:

public class Widget {
    private int id;
    private String name;
    public int getId() { return id; }
    public String getName() { return name; }
}

The JSON would be:

{"id":1,"name":"somewidget"}

Describe packet data. What does mean "0..3" and "data: 4..(data length + 4)" is it an comment in brackets? Could you give an example in other formats? — Paul Rumkin
– Paul Rumkin, Commented Aug 11, 2011 at 22:21
Most likely they are the byte indexes of the given fields. That is at least what I assumed in my answer. — Daniel Baulig
– Daniel Baulig, Commented Aug 11, 2011 at 22:40

Daniel Baulig · Accepted Answer · 2011-08-11 23:26:01Z

2

You will need a TCP socket. Connect it to the service and listen for the data event. When the data event is fired look at the buffers length. If it is <= 4 byte, you propably should discard it*. Else read the first 4 bytes using readUInt32() specifying 'little' as the endianess. Then use this number for the length of the remainding buffer. If the buffer is shorter than the given length, only "read" the remaining length, else "read" the given length. Use the toString method for this. You will get a string that can be parsed using the JSON.parse method, which will return you the JavaScript object matching the JSON.

You can build your packets basicly the same way by instanciating a buffer and writing all the data to it.

Also see the Buffers and net documentation for details.

* I do not know when node fires it's data events but your data might be received fragmented (that is splitted up into multiple data events). It could happen and due to the streaming nature of the tcp protocol it most likely will happen if the JSON string is long enough to not fit into a single frame. In that case, you propably should not simply discard the buffer, but try to reassemble the fragmented data.

edited Aug 11, 2011 at 23:26

answered Aug 11, 2011 at 22:36

Daniel Baulig

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4 Comments

Paul Gregoire Over a year ago

Yes, we definitely want to use TCP here. It seems that you get-it but I am looking for a sample code block.

Daniel Baulig Over a year ago

Everything you need is there in text, translating it to code should be trivial at this level of detail. However, please note my comment on the accepted answer. The code will fail eventually. To understand why, read my remarks flagged with a star *.

Paul Gregoire Over a year ago

Are you saying that "soc.on('data', function(buffer)" is not the correct means for accessing the incoming data?

Daniel Baulig Over a year ago

I am not. What I am saying is that because how the TCP protocol works it may happen that your data is not received in one chunk but in two, three or more data events. Also, it may happen that you receive 2 or more data chunks in a single data event. You should prepare for those cases and handle them.

Paul Rumkin · Accepted Answer · 2011-08-11 23:57:11Z

1

So if you want just send request to server and get response you could do this:

var net = require('net');

var soc = net.Socket();
soc.connect(8088);

soc.on('connect', function(){
  var data, request, header;

  data = {request : true};
  data = JSON.stringify(data);

  request = new Buffer(Buffer.byteLength(data));
  request.write(data);

  header = new Buffer(4);
  header.write(request.length.toString());


  // send request  
  soc.end(header.toString('binary') + request.toString('binary'));
});

soc.on('data', function(buffer){
  // Crop length bytes
  var data = JSON.parse(buffer.slice(4).toString('utf-8'));
  soc.destroy();

  console.log(data);
});

edited Aug 11, 2011 at 23:57

answered Aug 11, 2011 at 23:25

Paul Rumkin

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5 Comments

Daniel Baulig Over a year ago

We are talking of a binary header here, not a string based header. Besides, I don't think you may alter the specified protocol by adding linefeeds and carriage return characters to it.

Paul Gregoire Over a year ago

This looks close to what I was looking for minus the header stuff and crlf. Our "header" is only 4 bytes that give the length of the data bytes.

Paul Rumkin Over a year ago

Fixed! So that's binary version of what you want.

Paul Rumkin Over a year ago

I could publish test server for this client if you need it

Daniel Baulig Over a year ago

You should absolutely incorporate what I said on fragmentation and defragmentation or your code will blow up sooner or later.

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