Why you should never use Socket.IO

µWS is one of the most lightweight, efficient & scalable WebSocket server implementations available. It features an easy-to-use, fully async object-oriented interface and scales to millions of connections using only a fraction of memory compared to the competition. While performance and scalability are two of our top priorities, we consider security, stability and standards compliance paramount. License is zlib/libpng (very permissive & suits commercial applications).

  • Autobahn testsall pass.
  • Significantly outperforms WebSocket++ , libwebsockets , Beast , Crow , Kaazing Gateway , ws and Socket.IO in every tested dimension (see benchmark table below).
  • Linux, OS X & Windows support.
  • Valgrind / AddressSanitizer clean.
  • Built-in load balancing and multi-core scalability.
  • SSL/TLS support & integrates with foreign HTTPS servers.
  • Permessage-deflate built-in.
  • Node.js binding exposed as the well-known ws interface ( uws is at least 10x faster).

Benchmarks table -validate

Implementation User space memory scaling Connection performance Short message throughput Huge message throughput
Beast 1.0.0 b17 µWS is 7x as lightweight :+1: µWS is 4x as performant µWS is 22x as performant µWS is 3x as performant
libwebsockets 2.0 µWS is 11x as lightweight µWS is equal in performance :+1: µWS is 6x as performant µWS is 4x as performant
Crow [Sep 21] µWS is 13x as lightweight µWS is 2x as performant µWS is 12x as performant unable to measure
ws v1.1.0 + binary addons µWS is 47x as lightweight µWS is 18x as performant µWS is 33x as performant µWS is 2x as performant
Kaazing Gateway Community 5.0.0 µWS is 62x as lightweight µWS is 15x as performant µWS is 18x as performant unable to measure
Socket.IO 1.5.1 µWS is 62x as lightweight µWS is 42x as performant :-1: µWS is 61x as performant :-1: data missing
WebSocket++ v0.7.0 µWS is 63x as lightweight :-1: µWS is 4x as performant µWS is 3x as performant :+1: µWS is 2x as performant :+1:

Benchmarks are run with default settings in all libraries, except for ws which is run with the native performance addons. These results were achieved with the native C++ server, not the Node.js addon. Expect worse performance and scalability when using Node.js (don't worry, the Node.js addon will run circles around ws ).

Built with µWS



µWebSockets is a high performance C++ library with optional bindings to Node.js. It is greatly recommended investing in a proper C++ implementation if performance and memory scalability is considered critical for the solution in whole. The C++ interface has been designed for simplicity and only requires you to write a few lines of code to get a working server:

#include <uWS.h>

int main()
    uWS::Hub h;

    h.onMessage([](uWS::WebSocket<uWS::SERVER> ws, char *message, size_t length, uWS::OpCode opCode) {
        ws.send(message, length, opCode);



We built µWS with the existing Node.js infrastructure in mind. That's why we target the widespread ws interface, allowing us to seamlessly integrate with already existing projects. You simply swap require('ws') with require('uws') :

var WebSocketServer = require('uws').Server;
var wss = new WebSocketServer({ port: 8080 });

wss.on('connection', function (ws) {
    ws.on('message', function (message) {
        console.log('received: ' + message);


Deviations from ws

There are some important incompatibilities with ws though, we aim to be ~90% compatible but will never implement behavior that is deemed too inefficient:

  • Binary data is passed zero-copy as an ArrayBuffer . This means you need to copy it to keep it past the callback. It also means you need to convert it with Buffer.from(message) if you expect a Node.js Buffer .
  • webSocket._socket is not a net.Socket , it is just a getter function with very basic functionalities.
  • webSocket._socket.remote... might fail, you need to cache it at connection.
  • webSocket acts like an EventEmitter with one listener per event maximum.
  • webSocket.upgradeReq is only valid during execution of the connection handler. If you want to keep properties of the upgradeReq for the entire lifetime of the webSocket you better attach that specific property to the webSocket at connection.

Consider these projects

µWS is already the default engine in recent versions of deepstream.io and SocketCluster .

I would stay away from these projects

You can enable uws in Socket.IO using something like this:

var io = require('socket.io')(80);
io.engine.ws = new (require('uws').Server)({
    noServer: true,
    perMessageDeflate: false

You can set 'uws' as transformer in Primus :

var primus = new Primus(server, { transformer: 'uws' });


Node.js developers

  • Node.js 4.x, 5.x & 6.x supported (Windows version requires Node.js 6.4.0+)
  • Linux, Mac OS X & Windows supported
  • gcc >= 4.8.0 and make (or compatible) are required to build from source. This translates to Visual Studio >= 2015 on Windows and Clang >= 3.3 on macOS.

On installation, the module will be attempted to be build from source. If that fails, it will attempt to fall back to prebuilt modules which are provided for most platforms. If that fails too, uws will throw on require .

C++ developers


First of all you need to install the required dependencies. On Unix systems this is typically done via package managers, like homebrew in the case of OS X or dnf in the case of Fedora Linux. On Windows you need to search the web for pre-compiled binaries or simply compile the dependencies yourself.

  • libuv 1.3+
  • OpenSSL 1.0.x
  • zlib 1.x
  • CMake 3.x


Obviously you will need to clone this repo to get the sources. We use CMake as build system.

  • git clone https://github.com/uWebSockets/uWebSockets.git && cd uWebSockets
  • cmake .

Now, on Unix systems it should work by simply running make . Run [sudo] make install as you wish.

Windows, in all its glory

If you are running Windows you should now have a bunch of Visual Studio project files and one solution file. Open the solution file, now you need to make sure the header include paths and library paths are all set according to where you installed the dependencies. You might also need to change the names of the libraries being linked against, all according to the names of the installed library files. You know the drill.