REQ/REP & DEALER/ROUTER for two-way asynchronous worker processing - zeromq

Question

I'm learning ZeroMQ and just went through the tutorial and a few examples. I'm using Node.js as my main environment ( with Python eventually used to replace my workers ).
Trying to sort out how I can create a fully asynchronous messaging system that will allow my API to push tasks ( via a REQ socket ) to a router, have a dealer pass the message to a worker, process the message and send its results back up to my client ( which is an Express route ).
I believe the pattern for this would work something like this ( haven't tested or properly implemented code yet, so please take it as a conceptual outline ):
router.js
const zmq = require('zmq');;
const frontend = zmq.socket('router');
const backend = zmq.socket('dealer');
frontend.on('message', function() {
var args = Array.apply(null, arguments);
backend.send(args);
});
backend.on('message', function() {
var args = Array.apply(null, arguments);
frontend.send(args);
});
frontend.bindSync('tcp://*:5559');
backend.bindSync('tcp://*:5560');
client.js
var zmq = require('zmq'),
var express = require('express');
var app = express();
app.post('send', function(req, res) {
var client = zmq.socket('req');
// listen for responses from the server
client.on('message', function(data) {
console.log(data);
client.close();
});
// connect to the server port
client.connect('tcp://0.0.0.0:5454');
client.send('Request from ' + process.id);
});
app.listen('80');
worker.js
var zmq = require('zmq');
var server = zmq.socket('rep');
server.on('message', function(d){
server.send('Response from ' + process.id);
});
// bind to port 5454
server.bind('tcp://0.0.0.0:5454', function(err){
if (err){
console.error("something bad happened");
console.error( err.msg );
console.error( err.stack );
process.exit(0);
}
});
What I'm not fully understanding is if the ROUTER/DEALER will handle sending the response worker to the correct client. Also in this case the Dealer handles the Fair Queueing as I want my work distributed amongst the workers evenly.
My client could be distributed amongst many different boxes ( load balancer API server ), my router will be on its own server and the workers would be distributed amongst multiple boxes as well.

Answer 1

Forget REQ/REP in any production-grade app, can fall in mutual deadlock
You might find this subject in many other posts on high-risk mutual FSM-FSM deadlocking in REQ/REP Formal Scalable Communication Pattern.
Be sure, XREQ/XREP == DEALER/ROUTER ( already since 2011 )
source code removes all hidden magics behind this, XREQ == DEALER and XREP == ROUTER
+++b/include/zmq.h
...
-#define ZMQ_XREQ 5
-#define ZMQ_XREP 6
+#define ZMQ_DEALER 5
+#define ZMQ_ROUTER 6
...
+#define ZMQ_XREQ ZMQ_DEALER /* Old alias, remove in 3.x */
+#define ZMQ_XREP ZMQ_ROUTER /* Old alias, remove in 3.x */

Answer 2

For anyone reading this in the future, in my further research I stumbled on the Majordomo Protocol/pattern. It's precisely what I'm trying to implement. Documentation on the implementation, benefits and disadvantages can be read here: https://rfc.zeromq.org/spec:18/MDP/. Here's the broker implementation: https://github.com/zeromq/majordomo

Answer 3

Seems like I was using DEALER/ROUTER when I should have been using XREQ and XREP.
broker.js
var zmq = require('zmq');
var frontPort = 'tcp://127.0.0.1:5559';
var backPort = 'tcp://127.0.0.1:5560';
var frontSocket = zmq.socket('xrep');
var backSocket = zmq.socket('xreq');
frontSocket.identity = 'xrep_' + process.pid;
backSocket.identity = 'xreq_' + process.pid;
frontSocket.bind(frontPort, function (err) {
console.log('bound', frontPort);
});
frontSocket.on('message', function() {
//pass to back
console.log('router: sending to server', arguments[0].toString(), arguments[2].toString());
backSocket.send(Array.prototype.slice.call(arguments));
});
backSocket.bind(backPort, function (err) {
console.log('bound', backPort);
});
backSocket.on('message', function() {
//pass to front
console.log('dealer: sending to client', arguments[0].toString(), arguments[2].toString());
frontSocket.send(Array.prototype.slice.call(arguments));
});
console.log('Broker started...');
worker.js
var zmq = require('zmq');
var socket = zmq.socket('rep');
socket.identity = 'worker_' + process.pid;
socket.on('message', function(data) {
console.log(socket.identity + ': received ' + data.toString());
socket.send(data * 2);
});
socket.connect('tcp://127.0.0.1:5560', function(err) {
if (err) throw err;
console.log('server connected!');
});
console.log('Worker started...');
client.js
var zmq = require('zmq');
var socket = zmq.socket('req');
socket.identity = 'client_' + process.pid;
socket.on('message', function(data) {
console.log(socket.identity + ': answer data ' + data);
});
socket.connect('tcp://127.0.0.1:5559');
setInterval(function() {
var value = Math.floor(Math.random()*100);
console.log(socket.identity + ': asking ' + value);
socket.send(value);
}, 100);
console.log('Client started...');
I'm still not sure if it's safe opening a connection on EVERY API inbound request.