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Socket.io

Learn realtime communication with Socket.io.

What is Socket.io?

Socket.io is the most popular real-time communication library for Node.js. It provides two-way data flow between client and server. Chat apps, live notifications and multiplayer games are built with Socket.io.

Socket.io is not built in; you install it with npm install socket.io. On the server you import it with const socketIo = require("socket.io");. Socket.io uses WebSocket underneath, but if WebSocket is unavailable it falls back to long polling automatically. That makes browser compatibility very high. In this group you will learn the server side: socketIo(server), io.on, emit, rooms and namespaces.

Creating io with socketIo(server)

You must attach Socket.io to an HTTP server. First create a server with http.createServer, then obtain an io object with socketIo(server). This object manages all real-time events.

Example: const http = require("http"); const server = http.createServer(); const io = socketIo(server);. After this you listen for connecting clients with io.on("connection", ...). The socketIo(server) call binds the server and the Socket.io layer; the client side uses the Socket.io client library. Without an io object no Socket.io feature works.

Practice Import Socket.io

Import the socket.io module with require. Print "Socket.io ready" with console.log.

Starter code

index.js
// Import the socket.io module
// write here

// Print "Socket.io ready"
// write here

Sample solution

index.js
const socketIo = require("socket.io");
console.log("Socket.io ready");

Expected output

Socket.io ready

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 How is Socket.io imported?

index.js
const socketIo = require("socket.io")
  • const socketIo = new Socket()
  • import socket from "realtime"
  • const socketIo = require("socket.io") Correct answer
  • const socketIo = require("http")

Explanation: Socket.io is imported with require('socket.io').

2 Complete the socketIo call

Question 
const io = socketIo(____);
Answer 
const io = socketIo(server);

Explanation: socketIo(server) creates the io object.

3 Match each piece to its role

Term Meaning
socketIo Library
http.createServer HTTP server
io Socket.io instance
npm install Installs the package

All lessons in Advanced

1 Clustering

  1. cluster module
  2. cluster.fork
  3. Worker IPC
  4. cluster.isMaster
  5. Graceful reload
  6. Summary - Clustering

2 Worker threads

  1. worker_threads
  2. Worker
  3. postMessage
  4. MessageChannel
  5. SharedArrayBuffer
  6. Summary - Worker threads

3 Performance

  1. perf_hooks
  2. Event loop latency
  3. Memory usage
  4. CPU profiling
  5. Optimization
  6. Summary - Performance

4 Security

  1. Helmet
  2. CORS
  3. Rate limiting
  4. Input validation
  5. JWT
  6. Summary - Security

5 MongoDB

  1. Mongoose
  2. Schema
  3. Model
  4. Query
  5. Aggregate
  6. Summary - MongoDB

6 SQL database

  1. pg module
  2. Pool
  3. Parameterized query
  4. Transactions
  5. Sequelize
  6. Summary - SQL database

7 WebSocket

  1. ws module
  2. WebSocket server
  3. Connection
  4. Message
  5. Broadcast
  6. Summary - WebSocket

8 Realtime

  1. Socket.io
  2. io.on
  3. emit and on
  4. Rooms
  5. Namespaces
  6. Summary - Realtime

9 Microservice basics

  1. What are microservices?
  2. Service communication
  3. API gateway
  4. Service discovery
  5. Circuit breaker
  6. Summary - Microservices

10 Message queue

  1. What are message queues?
  2. Bull
  3. Producer
  4. Consumer
  5. Retry and backoff
  6. Summary - Message queue

11 Logging

  1. Winston
  2. Log levels
  3. Transports
  4. Structured logging
  5. Pino
  6. Summary - Logging

12 Configuration

  1. dotenv
  2. Environment variables
  3. Config hierarchy
  4. Validation
  5. Secrets management
  6. Summary - Configuration

13 Advanced testing

  1. Integration tests
  2. Supertest
  3. Test database
  4. Coverage
  5. E2E tests
  6. Summary - Advanced testing

14 Advanced final

  1. Project design
  2. Express + DB
  3. WebSocket
  4. Security
  5. Putting it together