How can i thread image processing in NodeJS? - multithreading

Question

I am writing a image processing web service on NodeJS and in basic i need to figure out how to make the actual image processing threaded. When i test it through Apache AB NodeJS is only using one core and stalling, i am surely doing something wrong here. How can i redesign this simple app to make use of the multiple cores on my server CPU.
I scaled away all input filtering and simplified the image processing function to give you a idea of the app structure instead of long code bits.
In app.js
app.get('/generate/:q', function(req, res){
var textString = req.params.q;
res.setHeader("Content-Type", "image/png");
res.end(generator.s());
});
In generate.js
var Canvas = require('canvas')
, Font = Canvas.Font
, fs = require('fs')
, path = require("path")
, plate = new Canvas.Image;
//To keep plate in RAM between requests.
fs.readFile(__dirname + '/plates/dhw132.png', function(err, squid){
if (err) throw err;
plate.src = squid;
});
exports.s = function () {
canvas = new Canvas(731,1024);
ctx = canvas.getContext('2d');
ctx.drawImage(plate, 0, 0, plate.width, plate.height);
return canvas.toBuffer();
}
How can i rewrite this to make the generator.s() threaded?

Answer 1

Node is single thread of course, but need for multi-threading is a very valid use-case. There's two ways I'm aware of.
Why don't you use clusters. You'll get a configurable number of process/threads, by default the number of cpus on your machine. Cluster essentially load-balances your application across processes, and transparently handles those processes sharing of the single listening http port.
http://nodejs.org/api/cluster.html
There's a wrapper for it as well here: https://github.com/dpweb/cluster-node
A different option, you could fork the process directly, here's an example where an uploaded file gets converted to mp3 using lame.. For your case, you would encapsulate all the image processing in a separate app, so the clusters option may be cleaner that doing that.
app.post('/process', function(req, res){
var f = req.files.filen;
fs.rename(f.path, f.name, function(err) {
if (err){
fs.unlink(f.name, ef);
throw err;
return;
}
fs.unlink(f.path, function() {
var ext = "." + req.body.extn;
require('child_process').exec("lame "+f.name, function(err, out, er) {
var nfn = f.name + '.mp3';
res.setHeader('Content-Type', 'application/octet-stream');
res.setHeader('Content-disposition', 'attachment; filename='+nfn);
res.setHeader("Content-Transfer-Encoding: binary");
res.setHeader('Accept-Ranges: bytes');
var size = fs.statSync(nfn).size;
console.log(size, f.name, nfn)
res.setHeader('Content-Length', size);
fs.createReadStream(nfn).pipe(res);
fs.unlink(nfn, ef); fs.unlink(f.name, ef);
})
})
})
})