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message-oriented socket library for node.js heavily inspired by zeromq
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Axon is a message-oriented socket library for node.js heavily inspired by zeromq.


$ npm install axon


  • message oriented
  • automated reconnection
  • light-weight wire protocol
  • supports arbitrary binary message (msgpack, json, BLOBS, etc)
  • supports JSON messages out of the box
  • fast (~800 mb/s ~500,000 messages/s)


  • close when server or connection is closed
  • error (err) when an-handled socket error occurs
  • ignored error (err) when an axon-handled socket error occurs, but is ignored
  • socket error (err) emitted regardless of handling, for logging purposes
  • reconnect attempt when a reconnection attempt is made
  • connect when connected to the peer, or a peer connection is accepted
  • disconnect when an accepted peer disconnects
  • bind when the server is bound
  • drop (msg) when a message is dropped due to the HWM
  • flush (msgs) queued when messages are flushed on connection


  • push / pull
  • pub / sub
  • req / rep
  • pub-emitter / sub-emitter

Push / Pull

PushSockets distribute messages round-robin:

var axon = require('axon')
  , sock = axon.socket('push');

console.log('push server started');

}, 150);

Receiver of PushSocket messages:

var axon = require('axon')
  , sock = axon.socket('pull');


sock.on('message', function(msg){

Both PushSockets and PullSockets may .bind() or .connect(). In the following configuration the push socket is bound and pull "workers" connect to it to receive work:

push bind

This configuration shows the inverse, where workers connect to a "sink" to push results:

pull bind

Pub / Sub

PubSockets send messages to all subscribers without queueing. This is an important difference when compared to a PushSocket, where the delivery of messages will be queued during disconnects and sent again upon the next connection.

var axon = require('axon')
  , sock = axon.socket('pub');

console.log('pub server started');

}, 500);

SubSocket simply receives any messages from a PubSocket:

var axon = require('axon')
  , sock = axon.socket('sub');


sock.on('message', function(msg){

SubSockets may optionally .subscribe() to one or more "topics" (the first multipart value), using string patterns or regular expressions:

var axon = require('axon')
  , sock = axon.socket('sub');


sock.on('message', function(topic, msg){


Req / Rep

ReqSocket is similar to a PushSocket in that it round-robins messages to connected RepSockets, however it differs in that this communication is bi-directional, every req.send() must provide a callback which is invoked when the RepSocket replies.

var axon = require('axon')
  , sock = axon.socket('req');


sock.send(img, function(res){


RepSockets receive a reply callback that is used to respond to the request, you may have several of these nodes.

var axon = require('axon')
  , sock = axon.socket('rep');


sock.on('message', function(img, reply){
  // resize the image

Like other sockets you may provide multiple arguments or an array of arguments, followed by the callbacks. For example here we provide a task name of "resize" to facilitate multiple tasks over a single socket:

var axon = require('axon')
  , sock = axon.socket('req');


sock.send('resize', img, function(res){


Respond to the "resize" task:

var axon = require('axon')
  , sock = axon.socket('rep');


sock.on('message', function(task, img, reply){
  switch (task.toString()) {
    case 'resize':
      // resize the image

PubEmitter / SubEmitter

PubEmitter and SubEmitter are higher-level Pub / Sub sockets, using the "json" codec to behave much like node's EventEmitter. When a SubEmitter's .on() method is invoked, the event name is .subscribe()d for you. Each wildcard (*) or regexp capture group is passed to the callback along with regular message arguments.


var axon = require('axon')
  , sock = axon.socket('pub-emitter');


  sock.emit('login', { name: 'tobi' });
}, 500);


var axon = require('axon')
  , sock = axon.socket('sub-emitter');


sock.on('user:login', function(user){
  console.log('%s signed in',;

sock.on('user:*', function(action, user){
  console.log('%s %s',, action);

sock.on('*', function(event){

Socket Options

Every socket has associated options that can be configured via get/set.

  • identity - the "name" of the socket that uniqued identifies it.
  • retry timeout - connection retry timeout in milliseconds [100]
  • retry max timeout - the cap for retry timeout length in milliseconds [5000]
  • hwm - the high water mark threshold for queues [Infinity]

Binding / Connecting

In addition to passing a portno, binding to INADDR_ANY by default, you may also specify the hostname via .bind(port, host), another alternative is to specify the url much like zmq via tcp://<hostname>:<portno>, thus the following are equivalent:

sock.bind(3000, '')

sock.connect(3000, '')


The wire protocol is simple and very much zeromq-like, where <length> is a BE 24 bit unsigned integer representing a maximum length of roughly ~16mb. The <meta> data byte is currently only used to store the codec, for example "json" is simply 1, in turn JSON messages received on the client end will then be automatically decoded for you by selecting this same codec.

 octet:     0      1      2      3      <length>
        | meta | <length>           | data ...

Thus 5 bytes is the smallest message axon supports at the moment. Later if necessary we can use the meta to indicate a small message and ditch octet 2 and 3 allowing for 3 byte messages.


To define a codec simply invoke the axon.codec.define() method, for example here is the JSON codec:

var axon = require('axon');

axon.codec.define('json', {
  encode: JSON.stringify,
  decode: JSON.parse

Note: codecs must be defined on both the sending and receiving ends, otherwise axon cannot properly decode the messages. You may of course ignore this feature all together and simply pass encoded data to .send().


Preliminary benchmarks on my Macbook Pro based on 10 messages per tick as a realistic production application would likely have even less than this. "better" numbers may be acheived with batching and a larger messages/tick count however this is not realistic.

64 byte messages:

      min: 47,169 ops/s
     mean: 465,127 ops/s
   median: 500,000 ops/s
    total: 2,325,636 ops in 5s
  through: 28.39 mb/s

1k messages:

      min: 48,076 ops/s
     mean: 120,253 ops/s
   median: 121,951 ops/s
    total: 601,386 ops in 5.001s
  through: 117.43 mb/s

8k messages:

      min: 36,496 ops/s
     mean: 53,194 ops/s
   median: 50,505 ops/s
    total: 266,506 ops in 5.01s
  through: 405.84 mb/s

32k messages:

      min: 12,077 ops/s
     mean: 14,792 ops/s
   median: 16,233 ops/s
    total: 74,186 ops in 5.015s
  through: 462.28 mb/s

What's it good for?

Axon are not meant to combat zeromq nor provide feature parity, but provide a nice solution when you don't need the insane nanosecond latency or language interoperability that zeromq provides as axon do not rely on any third-party compiled libraries.

Running tests

$ npm install
$ make test





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