Redis Explained: Caching, Sessions, Queues, and Real-Time Data

bash
SET user:123:name "John Doe"
GET user:123:name  # "John Doe"

# With expiration (TTL)
SET session:abc123 "user_data" EX 3600  # Expires in 1 hour

# Atomic counter
INCR page:home:views  # 1, 2, 3, ...

bash
HSET user:123 name "John" email "john@example.com" age 28
HGET user:123 name  # "John"
HGETALL user:123    # Returns all fields

bash
LPUSH queue:emails "email1" "email2"  # Push to left
RPOP queue:emails                      # Pop from right (FIFO queue)
LRANGE queue:emails 0 -1              # Get all items

bash
SADD online:users "user:123" "user:456"
SISMEMBER online:users "user:123"  # true
SCARD online:users                 # 2 (count)

bash
ZADD leaderboard 1500 "player:1" 2300 "player:2" 1800 "player:3"
ZREVRANGE leaderboard 0 9  # Top 10 players
ZRANK leaderboard "player:1"  # Player's rank

typescript
async function getUser(userId: string) {
  // Check cache first
  const cached = await redis.get(`user:${userId}`);
  if (cached) return JSON.parse(cached);
  
  // Cache miss: query database
  const user = await db.users.findOne({ id: userId });
  
  // Store in cache with 1 hour TTL
  await redis.set(`user:${userId}`, JSON.stringify(user), 'EX', 3600);
  
  return user;
}

1. TTL-based: Set expiration, let it auto-expire
2. Write-through: Update cache on every write
3. Write-behind: Batch cache updates
4. Cache-aside: Application manages cache reads/writes

typescript
async function rateLimit(ip: string, limit: number, windowSec: number) {
  const key = `rate:${ip}`;
  const current = await redis.incr(key);
  
  if (current === 1) {
    await redis.expire(key, windowSec);
  }
  
  if (current > limit) {
    throw new Error('Rate limit exceeded');
  }
}

// Usage: 100 requests per 15 minutes
await rateLimit(req.ip, 100, 900);

typescript
// Store session
await redis.set(`session:${sessionId}`, JSON.stringify({
  userId: user.id,
  role: user.role,
  loginAt: Date.now()
}), 'EX', 86400);  // 24 hours

// Retrieve session
const session = JSON.parse(
  await redis.get(`session:${sessionId}`)
);

typescript
// Publisher
redis.publish('notifications', JSON.stringify({
  userId: '123',
  message: 'New comment on your post'
}));

// Subscriber
redis.subscribe('notifications', (message) => {
  const notification = JSON.parse(message);
  sendWebSocket(notification.userId, notification.message);
});