使用NodeJS和SocketIO通过Websockets提供图像

Question

I am trying to develop a very simple image server with NodeJS & SocketIO. A project I am working on requires me to load several hundred images on page-load (customer requirement). Currently, a HTTP request is made for each image via use of the HTML "img" tag. With the reduced latency and overall efficiency of websockets compared to HTTP or Ajax, I was hoping to improve performance by sending images over websockets instead.

Unfortunately, reading images from the server's file-system with NodeJS and sending them over websockets with SocketIO has been significantly slower than the traditional HTTP requests served over Apache. Below is my server code:

var express = require('express'),
    app = express(),
    http = require('http'),
    fs = require("fs"),
    mime = require('mime'),
    server = http.createServer(app),
    io = require('socket.io').listen(server);

server.listen(151);

io.sockets.on('connection',function(socket){
    socket.emit('connected');

    socket.on('getImageData',function(file,callback){
        var path = 'c:/restricted_dir/'+file;

        fs.readFile(path,function(err,data){
            if (!err){
                var prefix = "data:" + mime.lookup(path) + ";base64,";
                var base64Image = prefix+data.toString('base64');
                socket.emit('imageData',data,callback);
            }
        });
    });
});

I have also tried buffering with "createReadStream", but I saw no significant speed improvements with this. I should also note that it is desirable to receive the image data as a Base64-encoded dataURI so I can simply throw that into the "src" attribute of the "img" tag. I understand Base64 means roughly a 30% increase in the data's size, but even when using binary image data, it still takes about 10 times longer than HTTP.

EDIT: I suppose the real question here is, "are websockets really the best way to serve static files?" After further thought and additional reading, I strongly suspect the issue here is related to parallel processing. Since NodeJS operates on a single thread, maybe it is not the best solution for serving all these static image files? Does anyone have any thoughts on this?

Answer 1

Browsers usually open multiple connections to the same server to perform requests in parallel, and can also perform multiple requests per single connection , whereas you only have one websocket connection.

Also, the combo fs.readFile()/Base64-encode/socket.emit() introduces a significant overhead, where a regular httpd can use system calls like sendfile() and don't even have to touch the file contents before they are being sent to the client.

The single-threaded nature of Node isn't an issue here, because Node can do I/O (which is what you're doing, minus the Base64-encoding) really well.

So I would say that websockets aren't very suitable for static file serving :)