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I have 5 Node.js apps. Let us assume that the 5 apps that I have are called:

App A,
App B,
App C,
App D,
App E,

I want App A to connect to Apps B, C, D, E using their IP addresses. I want to use WebSocket protocol (so that B, C, D, E can reply back on certain events). Apps B, C, D, E do not know communicate among themselves, nor do they know of each others existance. Apps B, C, D, E do not know that App A exists. Apps B, C, D, E are running on separate hardware, each having its own IP address. Once again, App A is the one that initiates connection to Apps B, C, D, E via WebSocket.

I tried to use the socket.io and socket.io-client libraries to do achieve this. Since my App A needs to be the one that initiates connection I used the socket.io-client library for it, and for Apps B, C, D, E I used the socket.io library. In usual case Apps B, C, D, E would act as the clients and would use socket.io-client library to connect to App A which would be the server and would use socket.io library. In my case it is opposite, as Apps B, C, D, E do not know of App A, and App A knows their IP addresses.

The problem I have when using the socket.io-client library is how to keep track of socket IDs within App A so that I can emit to a specific socket when I need to by using the socket ID? The socket.io-client library does not seem to keep track (a list) of socket IDs which are connected, as is the case on the server side with socket.io library.

Here is an example of what I tried so far:

App A Code:

const io = require('socket.io-client');
const socketB = io('http://10.11.18.1:3000');
const socketC = io('http://10.11.19.1:3000');
const socketD = io('http://10.11.20.1:3000');
const socketE = io('http://10.11.21.1:3000');

socketB.on('connect', function () 
 if (socket.connected) 
 console.log(socket.id);
 
);

socketC.on('connect', function () 
 if (socket.connected) 
 console.log(socket.id);
 
);

socketD.on('connect', function () 
 if (socket.connected) 
 console.log(socket.id);
 
);

socketE.on('connect', function () 
 if (socket.connected) 
 console.log(socket.id);
 
);

App B, C, D, E Code:

const io = require('socket.io');
const socket = io();
const PORT = 80;

socket.on('connection', client => 
...
);

socket.listen(PORT);

It looks like you will need to explicitly keep track of the servers that app A is connected to. If I understand your problem correctly, you can do something like this and map socket ids to sockets:

const io = require('socket.io-client');

const idToSocket = ;
const sockets = [
 'http://10.11.18.1:3000',
 'http://10.11.19.1:3000',
 'http://10.11.20.1:3000',
 'http://10.11.21.1:3000'
]
 .map(url => io(url))
 .map(socket => 
 socket.on('connect', () => 
 idToSocket[socket.id] = socket;
 );
 return socket;
 );

The sockets array is an array of all the sockets, and the idToSocket is a hash of the socket id to the socket that it relates to.