Caching, Queues, and Real-Time Systems

Learn Redis caching, background jobs, message queues, WebSockets, Socket.IO concepts, and event-driven architectures.

Caching with Redis

Caching reduces latency and database load by storing frequently accessed data in a fast key-value store. Redis is commonly used for application caching, distributed locks, counters, sessions, rate limiting, and ephemeral data.

A cache strategy must define expiration, invalidation, consistency expectations, and behavior when the cache is unavailable.

Cache-Aside Pattern

const cached = await redis.get(`user:${id}`);

if (cached) {
  return JSON.parse(cached);
}

const user = await User.findById(id).lean();
await redis.set(`user:${id}`, JSON.stringify(user), { EX: 300 });

return user;

💡 The application first checks the cache and queries the database only when the value is missing.

Key Takeaways

Caching is a trade-off between performance and consistency.
Every cache should have an invalidation or expiration strategy.
The application should define what happens when the cache is unavailable.

Knowledge Check

What is the cache-aside pattern?

Message Queues and Background Jobs

Queues move slow or retryable work out of synchronous HTTP requests. Typical background jobs include sending emails, processing images, generating reports, consuming events, and integrating with third-party APIs.

A queue-based architecture improves responsiveness and provides controlled retries, concurrency, and failure handling.

Queue-Based Architecture

Key Takeaways

Queues decouple request handling from asynchronous background work.
Workers should be idempotent when retries are possible.
Dead-letter or failure handling is important for permanently failing jobs.

Knowledge Check

Why use a background queue for email delivery?

WebSockets and Real-Time Communication

WebSockets provide a persistent, bidirectional connection between a client and server. They are useful for chat, live notifications, dashboards, multiplayer applications, and collaborative experiences.

Socket.IO builds higher-level features around real-time communication, including rooms, events, reconnection behavior, and adapters for distributed deployments.

Simple Socket.IO Server

import { Server } from 'socket.io';

const io = new Server(httpServer, {
  cors: { origin: '[https://example.com](https://example.com)' }
});

io.on('connection', (socket) => {
  socket.on('message', (message) => {
    io.emit('message', message);
  });
});

💡 The server listens for connections and broadcasts received messages to connected clients.

Key Takeaways

WebSockets enable bidirectional real-time communication.
Distributed Socket.IO deployments need an appropriate adapter or shared infrastructure.
Authentication and authorization also apply to socket connections.

Knowledge Check

What is a key characteristic of WebSockets?

Caching with Redis

Message Queues and Background Jobs

WebSockets and Real-Time Communication

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