How Redis Caching Actually Works — A Visual Guide for Backend Developers

TL;DR : Redis caching sits between your application and database, storing frequently accessed data in memory for sub-millisecond reads. This guide walks through cache-aside, write-through, and write-behind patterns with diagrams, working Node.js code, and real-world lessons from building a stock trading platform handling 100K+ concurrent users. The Problem Your API is slow. Users are waiting 200-500ms for responses that hit the database every single time. Your PostgreSQL instance is sweating under load, and your cloud bill is climbing. I hit this exact wall at Mstock — a stock trading platform with 100K+ concurrent users. Every user's dashboard needed real-time portfolio data, watchlist prices, and market depth. Hitting the database for every request? That's a recipe for a crashed production server during market hours. The answer was Redis. Not as a silver bullet, but as a carefully designed caching layer. Here's how it actually works. What Is Redis Caching? Redis is an in-memory data