Databases Lesson 9: Redis Caching

Redis is an in-memory key-value store — lightning fast because it stores data in RAM. Used for caching, sessions, queues, and real-time data.

Core Data Structures

# Strings: simple key-value
SET user:1:name "Alice"
GET user:1:name        # "Alice"
SETC user:1:token "abc123" 3600  # expire in 1 hour

# Lists: ordered, like arrays
LPUSH queue "job1" "job2"   # push to left
RPOP queue                   # pop from right (FIFO)

# Sets: unique values
SADD online:users 1 2 3
SMEMBERS online:users
SISMEMBER online:users 1     # is user 1 online?

# Sorted Sets: set with scores (leaderboards!)
ZADD leaderboard 1500 "Alice"
ZADD leaderboard 2000 "Bob"
ZRANGE leaderboard 0 -1 WITHSCORES  # sorted by score

# Hashes: object-like
HSET user:1 name "Alice" email "alice@x.com" age 30
HGET user:1 name
HGETALL user:1

Caching Pattern

const redis = require("redis");
const client = redis.createClient();

async function getUserById(id) {
  // 1. Check cache
  const cached = await client.get(`user:${id}`);
  if (cached) return JSON.parse(cached);
  
  // 2. Cache miss — fetch from database
  const user = await db.query("SELECT * FROM users WHERE id = $1", [id]);
  
  // 3. Store in cache (expire after 5 min)
  await client.setEx(`user:${id}`, 300, JSON.stringify(user));
  
  return user;
}