pm2-axon

What is pm2-axon?

pm2-axon is a high-performance inter-process messaging library for Node.js, designed to facilitate communication between different processes or services. It provides various messaging patterns such as push/pull, pub/sub, and req/rep, making it suitable for building scalable and distributed systems.

What are pm2-axon's main functionalities?

Push/Pull

The push/pull pattern is used for distributing tasks among multiple workers. The push socket sends messages to connected pull sockets in a round-robin fashion.

const axon = require('pm2-axon');
const push = axon.socket('push');
const pull = axon.socket('pull');
push.bind(3000);
pull.connect(3000);
pull.on('message', function(msg){
  console.log('received: %s', msg);
});
push.send('hello');

Pub/Sub

The pub/sub pattern is used for broadcasting messages to multiple subscribers. The pub socket sends messages to all connected sub sockets.

const axon = require('pm2-axon');
const pub = axon.socket('pub');
const sub = axon.socket('sub');
pub.bind(4000);
sub.connect(4000);
sub.on('message', function(msg){
  console.log('received: %s', msg);
});
pub.send('hello world');

Req/Rep

The req/rep pattern is used for request/response communication. The req socket sends a request and waits for a response from the rep socket.

const axon = require('pm2-axon');
const req = axon.socket('req');
const rep = axon.socket('rep');
req.bind(5000);
rep.connect(5000);
rep.on('message', function(msg, reply){
  console.log('received: %s', msg);
  reply('pong');
});
req.send('ping', function(res){
  console.log('reply: %s', res);
});

Other packages similar to pm2-axon

zeromq

ZeroMQ is a high-performance asynchronous messaging library aimed at use in scalable distributed or concurrent applications. It provides similar messaging patterns like push/pull, pub/sub, and req/rep. Compared to pm2-axon, ZeroMQ is more mature and widely used in various languages, not just Node.js.

amqp

AMQP (Advanced Message Queuing Protocol) is a protocol for message-oriented middleware. The 'amqp' npm package allows Node.js applications to communicate with AMQP brokers like RabbitMQ. It supports various messaging patterns and is more feature-rich compared to pm2-axon, but it requires a message broker to function.

redis

Redis is an in-memory data structure store that can be used as a message broker. The 'redis' npm package allows Node.js applications to use Redis for pub/sub messaging. While Redis is not specifically designed for inter-process communication, it provides robust pub/sub capabilities and can be used for similar purposes as pm2-axon.

README

Axon

Axon is a message-oriented socket library for node.js heavily inspired by zeromq. For a light-weight UDP alternative you may be interested in punt.

Installation

$ npm install axon

Features

• message oriented
• automated reconnection
• light-weight wire protocol
• mixed-type arguments (strings, objects, buffers, etc)
• unix domain socket support
• fast (~800 mb/s ~500,000 messages/s)

Events

• close when server or connection is closed
• error (err) when an un-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

Patterns

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

Mixed argument types

Backed by node-amp-message you may pass strings, objects, and buffers as arguments.

push.send('image', { w: 100, h: 200 }, imageBuffer);
pull.on('message', function(type, size, img){});

Push / Pull

PushSockets distribute messages round-robin:

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

sock.bind(3000);
console.log('push server started');

setInterval(function(){
  sock.send('hello');
}, 150);

Receiver of PushSocket messages:

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

sock.connect(3000);

sock.on('message', function(msg){
  console.log(msg.toString());
});

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:

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

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

sock.bind(3000);
console.log('pub server started');

setInterval(function(){
  sock.send('hello');
}, 500);

SubSocket simply receives any messages from a PubSocket:

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

sock.connect(3000);

sock.on('message', function(msg){
  console.log(msg.toString());
});

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

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

sock.connect(3000);
sock.subscribe('user:login');
sock.subscribe('upload:*:progress');

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

sock.bind(3000);

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

sock.connect(3000);

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

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

sock.bind(3000);

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

});

Respond to the "resize" task:

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

sock.connect(3000);

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

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.

app.js:

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

sock.connect(3000);

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

logger.js:

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

sock.bind(3000);

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

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

sock.on('*', function(event){
  console.log(arguments);
});

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.bind(3000, '0.0.0.0')
sock.bind('tcp://0.0.0.0:3000')

sock.connect(3000)
sock.connect(3000, '0.0.0.0')
sock.connect('tcp://0.0.0.0:3000')

You may also use unix domain sockets:

sock.bind('unix:///some/path')
sock.connect('unix:///some/path')

Protocol

Axon 2.x uses the extremely simple AMP protocol to send messages on the wire. Codecs are no longer required as they were in Axon 1.x.

Performance

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

Authors

• visionmedia
• gjohnson

Links

• Screencast
• Axon RPC

License

MIT