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Primus, the creator god of transformers but now also known as universal wrapper for real-time frameworks. There are a lot of real-time frameworks available for Node.js and they all have different opinions on how real-time should be done. Primus provides a common low level interface to communicate in real-time using various of real-time frameworks.


  1. Effortless switching between real-time frameworks and message parsers.
  2. Clean and stream compatible interface for client and server.
  3. Fixes bugs in frameworks and real-time communication where needed.
  4. Build with love and passion for real-time.
  5. Reconnect that actually works.


Primus is released in npm and can be installed using:

npm install primus --save

Table of Contents

Getting Started

Primus doesn't ship with real-time frameworks as dependencies, it assumes that you as user adds them your self as a dependency. This is done to keep the module as light weight as possible. This works because require in will walk through your directories searching for node_module folders that have these matching dependencies.

Primus needs to be "attached" to a HTTP compatible server. These includes the build in http and https servers but also the spdy module as it has the same API as node servers. Creating a new Primus instance is relatively straight forward:

'use strict';

var Primus = require('primus')
  , http = require('http');

var server = http.createServer(/* request handler */)
  , primus = new Primus(server, {/* options */});

In addition to support different frameworks we've also made it possible to use custom encoding and decoding libraries. We're using JSON by default but you could also use msgpack or JSONH for example (but these parsers need to be supported by Primus, so check out the parser folder for examples). To set parser you can supply a parser configuration option:

var primus = new Primus(server, { parser: 'JSON' });

All parsers have an async interface for error handling.

As most libraries come with their own client-side framework for making the connection we've also created a small wrapper for this. The library can be retrieved using:


Which returns the client-side library. It's not minified as that is out of the scope of this project. You can store this on a CDN or on your static server. Do what ever you want with it, but I would advice you to regenerate that file every time you redeploy so it always contains a client side library that is compatible with your back-end. To save the file you can use: +'/primus.js');

This will store the compiled library in your current directory. If you want to save it asyncronously, you can supply the method with an callback method: +'/primus.js', function save(err) {


But to make it easier for you during development we've automatically added an extra route to the supplied HTTP server, this will serve the library for you so you don't have to save it. Please note, that this route isn't optimized for serving static assets and should only be used during development. In your HTML page add:

<script src="/primus/primus.js"></script>

If you've configured a different pathname in the options deploy on a different domain then your primus server you would ofcourse need to update the src attribute to the correct location. It's always available at:

<protocol>://<server location>/<pathname>/primus.js

Once you're all set up you can start listening for connections. These connections are announced through the connection event.

primus.on('connection', function (spark) {
  // spark is the new connection.

Disconnects are announced using a disconnection event:

primus.on('disconnected', funciton (spark) {
  // the spark that disconnected

The spark the actual real-time socket/connection. Sparks have a really low level interface and only expose a couple properties that are cross engine supported. The interface is modeled towards a Node.js stream compatible interface.


The spark.headers property contains contains the headers of either the request that started a handshake with the server or the headers of the actual real-time connection. This depends on the module you are using.


The spark.address property contains the ip and port of the connection. If you're running your server behind a reverse proxy it will automatically use the x-forwarded-for headers. This way you will always have the address of the connecting client and not the ip address of your proxy.


The spark.query contains the query string you used to connect to server. It's parsed to a object. Please note that this is not available for all supported transformers, but it's proven to be to useful to not implement it because one silly tranformer refuses to support it. Yes.. I'm looking at you, browserchannel.

This is the connection id we use to identify the connection. This should not be seen as a "session id" and can change between disconnects and reconnects.


You can use the spark.write method to send data over the socket. The data is automatically encoded for you using the parser that you've set while creating the Primus instance. This method always returns true so back pressure isn't handled.

spark.write({ foo: 'bar' });


The spark.end() closes the connection.

spark.emits(event, parser)

This method is mostly used internally. It returns a function that emits assigned event every time it's called. It only emits the first received argument or the result of the optional parser call. The parser function receives all arguments and can parse it down to a single value or just extracts the useful information from the data. Please note that the data that is received here isn't decoded yet.

spark.emits('event', function parser(structure) {


The data event is emitted when a message is received from the client. It's automatically decoded by the specified decoder.

spark.on('data', function message(data) {
  // the message we've received.


The end event is emitted when the client has disconnected.

primus.on('connection', function (spark) {
  console.log('connection has the following headers', spark.headers);
  console.log('connection was made from', spark.address);
  console.log('connection id',;

  spark.on('data', function (data) {
    console.log('recieved data from the client', data);

    if ('foo' !== data.secrethandshake) spark.end();
    spark.write({ foo: 'bar' });

  spark.write('Hello world');

Connecting from the Browser.

Primus comes with it's client framework which can be compiled using primus.library() as mentioned above. To create a connection you can simply create a new Primus instance:

var primus = new Primus(url, { options });

// But it can be easier, with some syntax sugar.
var primus = Primus.connect(url, { options });


Once you've created your primus instance you're ready to go. When you want to write data to your server you can just call the .write method:


It automatically encodes your messages using the parser that you've specified on the server. So sending objects back and forth between the server is nothing different then just writing:

primus.write({ foo: 'bar' });

When you are sending messages to the server, you don't have to wait for the open event to happen, the client will automatically buffer all the data you've send and automatically write it to the server once it's connected. The client supports a couple of different events.


The data event is the most important event of the whole library. It's emitted when we receive data from the server. The data that is received is already decoded by the specified parser.

primus.on('data', function message(data) {
  console.log('Received a new message from the server', data);


The open event is emitted when we've successfully created a connection with the server. It will also be emitted when we've successfully reconnected when the connection goes down unintentionally.

primus.on('open', function open() {
  console.log('Connection is alive and kicking');


The error event is emitted when something breaks that is out of our control. Unlike Node.js, we do not throw an error if no error event listener is specified. The cause of an error could be that we've failed to encode or decode a message or we failed to create a connection.

primus.on('error', function error(err) {
  console.error('Something horrible has happend', err, err.message);


The reconnect event is emitted when we're attempting to reconnect to the server. This all happens transparently and it's just a way for you to know when these reconnects are actually happening.

primus.on('reconnecting', function () {


The end event is emitted when we've closed the connection. When this event is emitted you should consider your connection to be fully dead with no way of reconnecting. But it's also emitted when the server closes the connection.

primus.on('end', function () {
  console.log('connection closed');


When you want to close the connection you can call the primus.end() method. After this the connection should be considered dead and a new connection needs to be made using Primus.connect(url) or primus = new Primus(url) if you want to talk with the server again.



When the connection goes down unexpectedly a automatic reconnect process is started. It's using a randomized exponential backoff algorithm to prevent clients to DDOS your server when you reboot as they will all be re-connecting at different times. The reconnection can be configured using the options argument in Primus and you should add these options to the backoff property:

primus = Primus.connect(url, {
  backoff: {
    maxDelay: Infinity // Number: The max delay for a reconnect retry.
  , minDelay: 500 // Number: The minimum delay before we reconnect.
  , retries: 10 // Number: How many times should we attempt to reconnect.
  , factor: 2 // Number The backoff factor.

Please do note when we reconnect, you will receive a new connection event on the server. As the previous connection was completely dead and should there for be considered a new connection.

If you are interested in learning more about the backoff algorithm you might want to read

var primus = Primus.connect(url);

primus.on('data', function (message) {
  console.log('recieved a message', message);

  primus.write({ echo: message });

primus.write('hello world');

Supported Real-time Frameworks

The following transformers/transports are supported in Primus:

Engine.IO is the low level transport functionality of 1.0. It supports multiple transports for creating a real-time connection. It uses transport upgrading instead of downgrading which makes it more resilient to blocking proxies and firewalls. To enable you need to install the module:

npm install --save

And tell Primus that you want to us as transformer:

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

If you want to use the client interface inside of Node.js you also need to install the

npm install --save

And then you can access it from your server instance:

var Socket = primus.Socket;
  , socket = new Socket('url');


If you are targeting a high end audience or maybe just something for internal uses you can use a pure WebSocket server. This uses the ws WebSocket module which is known to be one if not the fastest WebSocket server available in Node.js and supports all protocol specifications. To use pure WebSockets you need to install the ws module:

npm install ws --save

And tell Primus that you want to use WebSockets as transformer:

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

The WebSockets transformer comes with build in client support and can be accessed using:

var Socket = primus.Socket;
  , socket = new Socket('url');


Browserchannel was the original technology that GMail used for their real-time communication. It's designed for same domain communication and does not use WebSockets. To use browserchannel you need to install the browserchannel module:

npm install browserchannel --save

And tell Primus that you want to use browserchannel as transformer:

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

The browserchannel transformer comes with build in client support and can be accessed using:

var Socket = primus.Socket;
  , socket = new Socket('url');


SockJS is a real-time server that focuses on cross-domain connections and does this by using multiple transports. To use SockJS you need to install the sockjs module:

npm install sockjs --save

And tell Primus that you want to use sockjs as transformer:

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

If yo want to use the client interface inside of Node.js you also need to install the sockjs-client-node module:

npm install --save

And then you can access it from your server instance:

var Socket = primus.Socket;
  , socket = new Socket('url');


The Socket.IO transport was written against Socket.IO 0.9.x. It was one of the first real-time servers written on Node.js and is one of the most used modules in Node.js. It uses multiple transports to connect the server. To use Socket.IO you need to install the module:

npm install --save

And tell Primus that you want to use as transformer:

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

If you want to use the client interface inside of Node.js you also need to install the

npm install --save

And then you can access it from your server instance:

var Socket = primus.Socket;
  , socket = new Socket('url');

As you can see from the examples above, it doesn't matter how you write the name of the transformer, we just toLowerCase() everything.

Transformer Inconsistencies

  • Browserchannel does not give you access to the remotePort of the incoming connection. So when you access spark.address the port property will be set to 1337 by default.
  • Browserchannel and SockJS do not support connections with query strings. You can still supply a query string in the new Primus('http://localhost:80?q=s') but it will not be accessible in the spark.query property.
  • Browserchannel is the only transformer that does not support cross domain connections.
  • SockJS and Browserchannel are originally written in CoffeeScript which can make it harder to debug when their internals are failing.
  • Engine.IO and SockJS do not ship their client-side library with their server side component. We're bundling a snapshot of these libraries inside of Primus. We will always be targeting the latest version of these transformers when we bundle the library.
  • There are small bugs in Engine.IO that are causing our tests to fail. I've submitted patches for these bugs, but they have been reject for silly reasons. The bug causes closed connections to say open. If you're experiencing this you can apply this patch.


All 0.x.x releases should be considered unstable and not ready for production. The version number is layed out as: major.minor.patch and tries to follow semver as closely as possible but this is how we use our version numbering:


A major and possible breaking change has been made in the primus core. These changes are not backwards compatible with older versions.


New features are added or a big change has happend with one of the real-time libraries that we've supporting.


A bug has been fixed, without any major internal and breaking changes.

Community Plugins

These are plugins created by our amazing community. Do you have a module that you want to have listed here? Make sure it has test suite and runs on Travis CI. After that open a pull request where you added your module to this README and see it be merged automatically.

A module that adds rooms capabilities to Primus. It's based on the rooms implementation of Socket.IO.
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