docs(examples): add example with traefik

Reference: https://doc.traefik.io/traefik/v2.0/

examples/cluster-traefik/README.md

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+
+# Socket.IO Chat with traefik & [redis](https://redis.io/)
+
+A simple chat demo for Socket.IO
+
+## How to use
+
+Install [Docker Compose](https://docs.docker.com/compose/install/), then:
+
+```
+$ docker-compose up -d
+```
+
+And then point your browser to `http://localhost:3000`.
+
+You can then scale the server to multiple instances:
+
+```
+$ docker-compose up -d --scale=server=7
+```
+
+The session stickiness, which is [required](https://socket.io/docs/v3/using-multiple-nodes/) when using multiple Socket.IO server instances, is achieved with a cookie. More information [here](https://doc.traefik.io/traefik/v2.0/routing/services/#sticky-sessions).

examples/cluster-traefik/docker-compose.yml

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+version: "3"
+
+services:
+  traefik:
+    image: traefik:2.4
+    volumes:
+      - ./traefik.yml:/etc/traefik/traefik.yml
+      - /var/run/docker.sock:/var/run/docker.sock
+    links:
+      - server
+    ports:
+     - "3000:80"
+     - "8080:8080"
+
+  server:
+    build: ./server
+    links:
+      - redis
+    labels:
+      - "traefik.http.routers.chat.rule=PathPrefix(`/`)"
+      - traefik.http.services.chat.loadBalancer.sticky.cookie.name=server_id
+      - traefik.http.services.chat.loadBalancer.sticky.cookie.httpOnly=true
+
+  redis:
+    image: redis:6-alpine
+    labels:
+      - traefik.enable=false

examples/cluster-traefik/server/Dockerfile

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+FROM node:14-alpine
+
+# Create app directory
+RUN mkdir -p /usr/src/app
+WORKDIR /usr/src/app
+
+# Install app dependencies
+COPY package.json /usr/src/app/
+RUN npm install --prod
+
+# Bundle app source
+COPY . /usr/src/app
+
+EXPOSE 3000
+CMD [ "npm", "start" ]

examples/cluster-traefik/server/index.js

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+// Setup basic express server
+var express = require('express');
+var app = express();
+var server = require('http').createServer(app);
+var io = require('socket.io')(server);
+var redis = require('socket.io-redis');
+var port = process.env.PORT || 3000;
+var crypto = require('crypto');
+var serverName = crypto.randomBytes(3).toString('hex');
+
+io.adapter(redis({ host: 'redis', port: 6379 }));
+
+server.listen(port, function () {
+  console.log('Server listening at port %d', port);
+  console.log('Hello, I\'m %s, how can I help?', serverName);
+});
+
+// Routing
+app.use(express.static(__dirname + '/public'));
+
+// Chatroom
+
+var numUsers = 0;
+
+io.on('connection', function (socket) {
+  socket.emit('my-name-is', serverName);
+
+  var addedUser = false;
+
+  // when the client emits 'new message', this listens and executes
+  socket.on('new message', function (data) {
+    // we tell the client to execute 'new message'
+    socket.broadcast.emit('new message', {
+      username: socket.username,
+      message: data
+    });
+  });
+
+  // when the client emits 'add user', this listens and executes
+  socket.on('add user', function (username) {
+    if (addedUser) return;
+
+    // we store the username in the socket session for this client
+    socket.username = username;
+    ++numUsers;
+    addedUser = true;
+    socket.emit('login', {
+      numUsers: numUsers
+    });
+    // echo globally (all clients) that a person has connected
+    socket.broadcast.emit('user joined', {
+      username: socket.username,
+      numUsers: numUsers
+    });
+  });
+
+  // when the client emits 'typing', we broadcast it to others
+  socket.on('typing', function () {
+    socket.broadcast.emit('typing', {
+      username: socket.username
+    });
+  });
+
+  // when the client emits 'stop typing', we broadcast it to others
+  socket.on('stop typing', function () {
+    socket.broadcast.emit('stop typing', {
+      username: socket.username
+    });
+  });
+
+  // when the user disconnects.. perform this
+  socket.on('disconnect', function () {
+    if (addedUser) {
+      --numUsers;
+
+      // echo globally that this client has left
+      socket.broadcast.emit('user left', {
+        username: socket.username,
+        numUsers: numUsers
+      });
+    }
+  });
+});

examples/cluster-traefik/server/package.json

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+{
+  "name": "socket.io-chat",
+  "version": "0.0.0",
+  "description": "A simple chat client using socket.io",
+  "main": "index.js",
+  "author": "Grant Timmerman",
+  "private": true,
+  "license": "MIT",
+  "dependencies": {
+    "express": "4.13.4",
+    "socket.io": "^3.1.0",
+    "socket.io-redis": "^6.0.1"
+  },
+  "scripts": {
+    "start": "node index.js"
+  }
+}

examples/cluster-traefik/server/public/index.html

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+<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="UTF-8">
+  <title>Socket.IO Chat Example</title>
+  <link rel="stylesheet" href="style.css">
+</head>
+<body>
+  <ul class="pages">
+    <li class="chat page">
+      <div class="chatArea">
+        <ul class="messages"></ul>
+      </div>
+      <input class="inputMessage" placeholder="Type here..."/>
+    </li>
+    <li class="login page">
+      <div class="form">
+        <h3 class="title">What's your nickname?</h3>
+        <input class="usernameInput" type="text" maxlength="14" />
+      </div>
+    </li>
+  </ul>
+
+  <script src="https://code.jquery.com/jquery-1.10.2.min.js"></script>
+  <script src="/socket.io/socket.io.js"></script>
+  <script src="/main.js"></script>
+</body>
+</html>