node.js - Nodejs: Set highWaterMark of socket object

Question

is it possible to set the highWaterMark of a socket object after it was created:

var http = require('http');

var server = http.createServer();

server.on('upgrade', function(req, socket, head) {
    socket.on('data', function(chunk) {
        var frame = new WebSocketFrame(chunk);

        // skip invalid frames
        if (!frame.isValid()) return;

        // if the length in the head is unequal to the chunk 
        // node has maybe split it
        if (chunk.length != WebSocketFrame.getLength()) {
            socket.once('data', listenOnMissingChunks);
        });
    });
});

function listenOnMissingChunks(chunk, frame) {
    frame.addChunkToPayload(chunk);

    if (WebSocketFrame.getLength()) {
        // if still corrupted listen once more
    } else {
        // else proceed
    }
}

The above code example does not work. But how do I do it instead?

Further explaination: When I receive big WebSocket frames they get split into multiple data events. This makes it hard to parse the frames because I do not know if this is a splitted or corrupted frame.

Answer 1

I think you misunderstand the nature of a TCP socket. Despite the fact that TCP sends its data over IP packets, TCP is not a packet protocol. A TCP socket is simply a stream of data. Thus, it is incorrect to view the data event as a logical message. In other words, one socket.write on one end does not equate to a single data event on the other.

There are many reasons that a single write to a socket does not map 1:1 to a single data event:

• The sender's network stack may combine multiple small writes into a single IP packet. (The Nagle algorithm)
• An IP packet may be fragmented (split into multiple packets) along its journey if its size exceeds any one hop's MTU.
• The receiver's network stack may combine multiple packets into a single data event (as seen by your application).

Because of this, a single data event might contain multiple messages, a single message, or only part of a message.

In order to correctly handle messages sent over a stream, you must buffer incoming data until you have a complete message.

var net = require('net');


var max = 1024 * 1024 // 1 MB, the maximum amount of data that we will buffer (prevent a bad server from crashing us by filling up RAM)
    , allocate = 4096; // how much memory to allocate at once, 4 kB (there's no point in wasting 1 MB of RAM to buffer a few bytes)
    , buffer=new Buffer(allocate) // create a new buffer that allocates 4 kB to start
    , nread=0 // how many bytes we've buffered so far
    , nproc=0 // how many bytes in the buffer we've processed (to avoid looping over the entire buffer every time data is received)
    , client = net.connect({host:'example.com', port: 8124}); // connect to the server

client.on('data', function(chunk) {
    if (nread + chunk.length > buffer.length) { // if the buffer is too small to hold the data
        var need = Math.min(chunk.length, allocate); // allocate at least 4kB
        if (nread + need > max) throw new Error('Buffer overflow'); // uh-oh, we're all full - TODO you'll want to handle this more gracefully

        var newbuf = new Buffer(buffer.length + need); // because Buffers can't be resized, we must allocate a new one
        buffer.copy(newbuf); // and copy the old one's data to the new one
        buffer = newbuf; // the old, small buffer will be garbage collected
    }

    chunk.copy(buffer, nread); // copy the received chunk of data into the buffer
    nread += chunk.length; // add this chunk's length to the total number of bytes buffered

    pump(); // look at the buffer to see if we've received enough data to act
});

client.on('end', function() {
    // handle disconnect
});


client.on('error', function(err) {
    // handle errors
});


function find(byte) { // look for a specific byte in the buffer
    for (var i = nproc; i < nread; i++) { // look through the buffer, starting from where we left off last time
        if (buffer.readUInt8(i, true) == byte) { // we've found one
            return i;
        }
    }
}
function slice(bytes) { // discard bytes from the beginning of a buffer
    buffer = buffer.slice(bytes); // slice off the bytes
    nread -= bytes; // note that we've removed bytes
    nproc = 0; // and reset the processed bytes counter
}

function pump() {
    var pos; // position of a NULL character

    while ((pos = find(0x00)) >= 0) { // keep going while there's a NULL (0x00) somewhere in the buffer
        if (pos == 0) { // if there's more than one NULL in a row, the buffer will now start with a NULL
            slice(1); // discard it
            continue; // so that the next iteration will start with data
        }
        process(buffer.slice(0,pos)); // hand off the message
        slice(pos+1); // and slice the processed data off the buffer
    }
}

function process(msg) { // here's where we do something with a message
    if (msg.length > 0) { // ignore empty messages
        // here's where you have to decide what to do with the data you've received
        // experiment with the protocol
    }
}