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Sending notification to clients in Node.JS

In a Node JS app, I want my server to be able to send a notification to clients, in some specific cases. Searching the net on the subject, I find Service Workers which seems to be what I need; but I am not totally sure.
Here is why. When I look at various tutorials and videos, it seems like the notifications always somewhat originates from some client. But this is not what I want. It should be the server deciding when and what to send as a notification, not some client. So comes my questions:
Am I misunderstanding the way to use Service Workers?
Is this the right path to have my server send a notification to clients?
If the answer to the last question is NO. Then what is the proper way?

Like Dimitar wrote earlier, you could use socket.io to send messages from the server to the client and vice versa. This can be done as simple as that:
//Server sent message:
io.on('connection', (socket) => {
socket.emit('notificationToClient', 'Message'); //message sent from server to client
});
});
//Client receives the message:
const socket = io.connect('http://localhost:3000');
socket.on('notificationToClient', (data) => { //received message
console.log(data);
});
The websocket API is a little less confusing than socket.io but IMO, I'd go with socket.io since it already deals with a lot of the heavy lifting like re-connecting, custom namespaces, rooms etc. There are several fantastic udemy courses on socket.io.
See the socket.io documentation here: https://socket.io/docs/
Update 5/9/2020:
Michel - Since you asked about how to implement Socket.io in your code I'll give you an example of how I implemented this technology in my most recent project. I was working on a web app that connects to ELK Alarm systems and is able to arm/disarm these systems as well as receive sensor input (chime signals) at any remote location. The backend is Node.js and the UI is React.
//client side code
import React, { Component, Suspense } from 'react';
import panels from "./panels.js"
const socketConnection = io("http://localhost:5000"); //Socket.io
function App() {
function handleClick(panel) {
socketConnection.emit("panelData", panel); //Socket.io - sends the connection data for the ELK alarm panel to the express server
socketConnection.on('data', (data) => { //Socket.io - receives data from the server.
//does something with the data
})
}
}
//server side code
const express = require('express');
const elkClient = require('elk-client');
const app = express();
const server = app.listen('5000', () => {
console.log('------Server Running on 5000---------');
});
let io = new sio(server);
io.on("connection", (socket) => { //boilerplate code - establishes the "handshake" with the client.
socket.on("panelData", async (msg) => { //receives the ELK connection parameters from the client. See client side code
const client = new ElkClient({
connection: {
site: msg.name,
host: msg.host,
}
})
await client.connect();
client.on("message", (elkMessage) => { // This is NOT Socket.io - server is listening for emitted data from the alarm system - beam breaks, alarms, arm/disarm etc. [See event emitter][2]
if(elkMessage.messageType == 'A' && elkMessage.subMessageType == 'S') {
const armingStatusReport = {elkMessage};
socket.emit("data", armingStatusReport); //Socket.io - emits the received data to the client.
}
})
I tried to simplify the code above to hit the point. The server is using the connection parameters that it receives from the client to connect to a remote alarm system. The server then waits and listens for incoming data with client.on()(event emitter - not socket.io). As the data is received, I'm using Socket.io to send the received data back to the client with socket.emit(); Due to the nature of how alarm systems work, data is sent event driven so the fetch api wouldn't really fit the bill but socket.io (or websockets) does.
Let me know if I can help with anything else. With this recent project, I have spent the last few months EXTENSIVELY dealing with socket.io, including namespaces, rooms etc. I also had to create a real-time monitoring app for performance management for my server utilizing socket.io since I implemented clustering. Feel free to contact me anytime!

The answer is WebSockets.
Check out Socket.IO. It will give you the possibility of receiving live notifications on the client and you'll have complete server control on them.

What about using twilio library to manage your notification job .twilio library

How to receive data in real time from TCP socket using PHP or JS? [duplicate]

I have a vb.net application that opens a socket and listens on it.
I need to communicate via this socket to that application using a javascript running on a browser. That is i need to send some data on this socket so that the app which is listening on this socket can take that data, do some stuff using some remote calls and get some more data and put it back on the socket that my javascript needs to read and print it in the browser.
Ive tried, socket.io, websockify but none have proved to be useful.
Hence the question, is what i am trying even possible? Is there a way that a javascript running in a browser can connect to a tcp socket and send some data and listen on it for some more data response on the socket and print it to the browser.
If this is possible can some one point me in the right direction as to which would help me establish the goal.

As for your problem, currently you will have to depend on XHR or websockets for this.
Currently no popular browser has implemented any such raw sockets api for javascript that lets you create and access raw sockets, but a draft for the implementation of raw sockets api in JavaScript is under-way. Have a look at these links:
http://www.w3.org/TR/raw-sockets/
https://developer.mozilla.org/en-US/docs/Web/API/TCPSocket
Chrome now has support for raw TCP and UDP sockets in its ‘experimental’ APIs. These features are only available for chrome apps and, although documented, are hidden for the moment. Having said that, some developers are already creating interesting projects using it, such as this IRC client.
To access this API, you’ll need to enable the experimental flag in your extension’s manifest. Using sockets is pretty straightforward, for example:
chrome.experimental.socket.create('tcp', '127.0.0.1', 8080, function(socketInfo) {
chrome.experimental.socket.connect(socketInfo.socketId, function (result) {
chrome.experimental.socket.write(socketInfo.socketId, "Hello, world!");
});
});

This will be possible via the navigator interface as shown below:
navigator.tcpPermission.requestPermission({remoteAddress:"127.0.0.1", remotePort:6789}).then(
() => {
// Permission was granted
// Create a new TCP client socket and connect to remote host
var mySocket = new TCPSocket("127.0.0.1", 6789);
// Send data to server
mySocket.writeable.write("Hello World").then(
() => {
// Data sent sucessfully, wait for response
console.log("Data has been sent to server");
mySocket.readable.getReader().read().then(
({ value, done }) => {
if (!done) {
// Response received, log it:
console.log("Data received from server:" + value);
}
// Close the TCP connection
mySocket.close();
}
);
},
e => console.error("Sending error: ", e)
);
}
);
More details are outlined in the w3.org tcp-udp-sockets documentation.
http://raw-sockets.sysapps.org/#interface-tcpsocket
https://www.w3.org/TR/tcp-udp-sockets/
Another alternative is to use Chrome Sockets
Creating connections
chrome.sockets.tcp.create({}, function(createInfo) {
chrome.sockets.tcp.connect(createInfo.socketId,
IP, PORT, onConnectedCallback);
});
Sending data
chrome.sockets.tcp.send(socketId, arrayBuffer, onSentCallback);
Receiving data
chrome.sockets.tcp.onReceive.addListener(function(info) {
if (info.socketId != socketId)
return;
// info.data is an arrayBuffer.
});
You can use also attempt to use HTML5 Web Sockets (Although this is not direct TCP communication):
var connection = new WebSocket('ws://IPAddress:Port');
connection.onopen = function () {
connection.send('Ping'); // Send the message 'Ping' to the server
};
http://www.html5rocks.com/en/tutorials/websockets/basics/
Your server must also be listening with a WebSocket server such as pywebsocket, alternatively you can write your own as outlined at Mozilla

ws2s project is aimed at bring socket to browser-side js. It is a websocket server which transform websocket to socket.
ws2s schematic diagram
code sample:
var socket = new WS2S("wss://ws2s.feling.io/").newSocket()
socket.onReady = () => {
socket.connect("feling.io", 80)
socket.send("GET / HTTP/1.1\r\nHost: feling.io\r\nConnection: close\r\n\r\n")
}
socket.onRecv = (data) => {
console.log('onRecv', data)
}

See jsocket. Haven't used it myself. Been more than 3 years since last update (as of 26/6/2014).
* Uses flash :(
From the documentation:
<script type='text/javascript'>
// Host we are connecting to
var host = 'localhost';
// Port we are connecting on
var port = 3000;
var socket = new jSocket();
// When the socket is added the to document
socket.onReady = function(){
socket.connect(host, port);
}
// Connection attempt finished
socket.onConnect = function(success, msg){
if(success){
// Send something to the socket
socket.write('Hello world');
}else{
alert('Connection to the server could not be estabilished: ' + msg);
}
}
socket.onData = function(data){
alert('Received from socket: '+data);
}
// Setup our socket in the div with the id="socket"
socket.setup('mySocket');
</script>

In order to achieve what you want, you would have to write two applications (in either Java or Python, for example):
Bridge app that sits on the client's machine and can deal with both TCP/IP sockets and WebSockets. It will interact with the TCP/IP socket in question.
Server-side app (such as a JSP/Servlet WAR) that can talk WebSockets. It includes at least one HTML page (including server-side processing code if need be) to be accessed by a browser.
It should work like this
The Bridge will open a WS connection to the web app (because a server can't connect to a client).
The Web app will ask the client to identify itself
The bridge client sends some ID information to the server, which stores it in order to identify the bridge.
The browser-viewable page connects to the WS server using JS.
Repeat step 3, but for the JS-based page
The JS-based page sends a command to the server, including to which bridge it must go.
The server forwards the command to the bridge.
The bridge opens a TCP/IP socket and interacts with it (sends a message, gets a response).
The Bridge sends a response to the server through the WS
The WS forwards the response to the browser-viewable page
The JS processes the response and reacts accordingly
Repeat until either client disconnects/unloads
Note 1: The above steps are a vast simplification and do not include information about error handling and keepAlive requests, in the event that either client disconnects prematurely or the server needs to inform clients that it is shutting down/restarting.
Note 2: Depending on your needs, it might be possible to merge these components into one if the TCP/IP socket server in question (to which the bridge talks) is on the same machine as the server app.

The solution you are really looking for is web sockets. However, the chromium project has developed some new technologies that are direct TCP connections TCP chromium

How to detect client disconnect from redis in nodejs?

I am using the library node_redis as the client for a micro-service message client that I am writing. Clients get messages from their application in their outbox that they need to send to other services. Everything is working great but I am trying to build some resilience on the part of the application that uses the redis client to communicate with the redis-server.
My idea is that the redis client-server connection status should be highly available to the clients. I mean that if a connection goes down, I would like to know within the second instead of the default timeout of 300 seconds. Currently I am using the free redislabs tier hosted on AWS, but I should be moving this to run in its own container on my kubernetes cluster.
I need to know the state of client connections in the network because I would like to not send messages when the network conditions are not right and not rely on the error handling to handle this sort of event. Knowing how often and when these high latency events occur will also help me diagnose and improve my network and my microservices.
Note: I wanted to set the connect_timeout value in the client options but this is listed as deprecated.

Something like that?
var redis = require('redis');
var client = redis.createClient();
var reconnectAfter = 15000;
client.on( 'error', function () {
console.log( (new Date()) + " Redis: disconnect");
setTimeout( connect, reconnectAfter);
});
connect = function(){
client = redis.createClient();
}

How to update constantly data from server in JS?

There are data that are constantly updated on the server and written to the file (every 100 ms), i need to transfer that to the client side, for the time being I did this through the setInterval function (which should check them at the same interval), checking this JSON file, and im not sure if it's a correct solution of the problem.
Server side code:
function SerialPortStart(COM, Input){
console.log( COM, Input);
var serialPort = new SerialPort(COM, {
parser: SerialPort.parsers.readline('\n'),
baudrate: +Input || 9600
});
serialPort.on('data', function (data) {
thermodata = data.toString();
console.log(data);
fs.writeFileSync(__dirname +'/views/cockpit.json',JSON.stringify(thermodata));
});
};
Client side code:
function SerialPortStart(){
var xhr = new XMLHttpRequest();
setInterval(function() {
xhr.open('GET', 'cockpit.json', true);
xhr.send();
xhr.onreadystatechange = function() {
if(this.status == 200) {
var data = JSON.parse(xhr.responseText);
console.log(data);
obj = data.split(';');
if(cockpitNum[3] != null){
$('header .mainBlock1 .cockpit .elements .blockHaw .cirle').css({
transform: 'rotate('+(obj[cockpitNum[3]]) +'deg)'
}); }
}, 50);

You are now no longer in the realm of AJAX. A normal request can take around 500ms to complete, thats to slow and apart from that, it consumes way to much resource.
Time to learn Websockets. A websocket is (sortof) a small connection (just like http), but it doesnt close (unlike http which closes at the end of the call). This way you can broadcast data to all connected to the websocket and it's super fast.
Node.js is an example which allows you to set this up via Socket.io. You can think of your situation as a kind of one-direction-chat :)

I would suggest that you use the socket.io library and open a socket.io connection from your web page to your server. This is a continuous connection between client and server and allows data to be sent either direction from client to server or server to client with very low overhead. It is ideal for lots of transactions between client and server and is ideal when the server wants to regularly send data to the client. It is way more efficient than Ajax polling from the client.
You would implement a node.js server that listens for incoming socket.io connections, does whatever authentication is needed to establish who is listening on the other end and then your server can regularly send that client data whenever there is new data to send it.
You can read about socket.io here. There are libraries there for both client and server. The client-side library uses the webSocket transport (which all modern browsers support), but socket.io adds a number of very useful features on top of socket.io (such as connection monitoring, auto-reconnection, auto-serialization to JSON, etc...).
See this post for a list of features socket.io adds on top of webSockets:
Socket.io features beyond plain webSocket

Connecting to TCP Socket from browser using javascript

I have a vb.net application that opens a socket and listens on it.
I need to communicate via this socket to that application using a javascript running on a browser. That is i need to send some data on this socket so that the app which is listening on this socket can take that data, do some stuff using some remote calls and get some more data and put it back on the socket that my javascript needs to read and print it in the browser.
Ive tried, socket.io, websockify but none have proved to be useful.
Hence the question, is what i am trying even possible? Is there a way that a javascript running in a browser can connect to a tcp socket and send some data and listen on it for some more data response on the socket and print it to the browser.
If this is possible can some one point me in the right direction as to which would help me establish the goal.

As for your problem, currently you will have to depend on XHR or websockets for this.
Currently no popular browser has implemented any such raw sockets api for javascript that lets you create and access raw sockets, but a draft for the implementation of raw sockets api in JavaScript is under-way. Have a look at these links:
http://www.w3.org/TR/raw-sockets/
https://developer.mozilla.org/en-US/docs/Web/API/TCPSocket
Chrome now has support for raw TCP and UDP sockets in its ‘experimental’ APIs. These features are only available for chrome apps and, although documented, are hidden for the moment. Having said that, some developers are already creating interesting projects using it, such as this IRC client.
To access this API, you’ll need to enable the experimental flag in your extension’s manifest. Using sockets is pretty straightforward, for example:
chrome.experimental.socket.create('tcp', '127.0.0.1', 8080, function(socketInfo) {
chrome.experimental.socket.connect(socketInfo.socketId, function (result) {
chrome.experimental.socket.write(socketInfo.socketId, "Hello, world!");
});
});

This will be possible via the navigator interface as shown below:
navigator.tcpPermission.requestPermission({remoteAddress:"127.0.0.1", remotePort:6789}).then(
() => {
// Permission was granted
// Create a new TCP client socket and connect to remote host
var mySocket = new TCPSocket("127.0.0.1", 6789);
// Send data to server
mySocket.writeable.write("Hello World").then(
() => {
// Data sent sucessfully, wait for response
console.log("Data has been sent to server");
mySocket.readable.getReader().read().then(
({ value, done }) => {
if (!done) {
// Response received, log it:
console.log("Data received from server:" + value);
}
// Close the TCP connection
mySocket.close();
}
);
},
e => console.error("Sending error: ", e)
);
}
);
More details are outlined in the w3.org tcp-udp-sockets documentation.
http://raw-sockets.sysapps.org/#interface-tcpsocket
https://www.w3.org/TR/tcp-udp-sockets/
Another alternative is to use Chrome Sockets
Creating connections
chrome.sockets.tcp.create({}, function(createInfo) {
chrome.sockets.tcp.connect(createInfo.socketId,
IP, PORT, onConnectedCallback);
});
Sending data
chrome.sockets.tcp.send(socketId, arrayBuffer, onSentCallback);
Receiving data
chrome.sockets.tcp.onReceive.addListener(function(info) {
if (info.socketId != socketId)
return;
// info.data is an arrayBuffer.
});
You can use also attempt to use HTML5 Web Sockets (Although this is not direct TCP communication):
var connection = new WebSocket('ws://IPAddress:Port');
connection.onopen = function () {
connection.send('Ping'); // Send the message 'Ping' to the server
};
http://www.html5rocks.com/en/tutorials/websockets/basics/
Your server must also be listening with a WebSocket server such as pywebsocket, alternatively you can write your own as outlined at Mozilla

ws2s project is aimed at bring socket to browser-side js. It is a websocket server which transform websocket to socket.
ws2s schematic diagram
code sample:
var socket = new WS2S("wss://ws2s.feling.io/").newSocket()
socket.onReady = () => {
socket.connect("feling.io", 80)
socket.send("GET / HTTP/1.1\r\nHost: feling.io\r\nConnection: close\r\n\r\n")
}
socket.onRecv = (data) => {
console.log('onRecv', data)
}

See jsocket. Haven't used it myself. Been more than 3 years since last update (as of 26/6/2014).
* Uses flash :(
From the documentation:
<script type='text/javascript'>
// Host we are connecting to
var host = 'localhost';
// Port we are connecting on
var port = 3000;
var socket = new jSocket();
// When the socket is added the to document
socket.onReady = function(){
socket.connect(host, port);
}
// Connection attempt finished
socket.onConnect = function(success, msg){
if(success){
// Send something to the socket
socket.write('Hello world');
}else{
alert('Connection to the server could not be estabilished: ' + msg);
}
}
socket.onData = function(data){
alert('Received from socket: '+data);
}
// Setup our socket in the div with the id="socket"
socket.setup('mySocket');
</script>

In order to achieve what you want, you would have to write two applications (in either Java or Python, for example):
Bridge app that sits on the client's machine and can deal with both TCP/IP sockets and WebSockets. It will interact with the TCP/IP socket in question.
Server-side app (such as a JSP/Servlet WAR) that can talk WebSockets. It includes at least one HTML page (including server-side processing code if need be) to be accessed by a browser.
It should work like this
The Bridge will open a WS connection to the web app (because a server can't connect to a client).
The Web app will ask the client to identify itself
The bridge client sends some ID information to the server, which stores it in order to identify the bridge.
The browser-viewable page connects to the WS server using JS.
Repeat step 3, but for the JS-based page
The JS-based page sends a command to the server, including to which bridge it must go.
The server forwards the command to the bridge.
The bridge opens a TCP/IP socket and interacts with it (sends a message, gets a response).
The Bridge sends a response to the server through the WS
The WS forwards the response to the browser-viewable page
The JS processes the response and reacts accordingly
Repeat until either client disconnects/unloads
Note 1: The above steps are a vast simplification and do not include information about error handling and keepAlive requests, in the event that either client disconnects prematurely or the server needs to inform clients that it is shutting down/restarting.
Note 2: Depending on your needs, it might be possible to merge these components into one if the TCP/IP socket server in question (to which the bridge talks) is on the same machine as the server app.

The solution you are really looking for is web sockets. However, the chromium project has developed some new technologies that are direct TCP connections TCP chromium