Common Beginner Mistakes

19. Common Beginner Mistakes

Bad vs Good: A Quick Reference

flowchart TB
    Mistakes[Common Redis Mistakes ❌] --> M1[Mistake 1: No TTL on Cache]
    Mistakes --> M2[Mistake 2: Using KEYS * in Production]
    Mistakes --> M3[Mistake 3: Storing Huge Values]
    Mistakes --> M4[Mistake 4: Bad Key Naming]
    Mistakes --> M5[Mistake 5: Not Handling Cache Errors]
    Mistakes --> M6[Mistake 6: Redis as Primary DB]
    Mistakes --> M7[Mistake 7: No maxmemory Set]

    M1 --> F1[Fix: SET key value EX 300]
    M2 --> F2[Fix: Use SCAN instead]
    M3 --> F3[Fix: Store individual items]
    M4 --> F4[Fix: Use user:id:field naming]
    M5 --> F5[Fix: try/catch + DB fallback]
    M6 --> F6[Fix: Pair Redis with durable DB]
    M7 --> F7[Fix: Set maxmemory + eviction policy]

    style Mistakes fill:#ef4444,color:#fff
    style M1 fill:#ef4444,color:#fff
    style M2 fill:#ef4444,color:#fff
    style M3 fill:#ef4444,color:#fff
    style M4 fill:#ef4444,color:#fff
    style M5 fill:#ef4444,color:#fff
    style M6 fill:#ef4444,color:#fff
    style M7 fill:#ef4444,color:#fff
    style F1 fill:#059669,color:#fff
    style F2 fill:#059669,color:#fff
    style F3 fill:#059669,color:#fff
    style F4 fill:#059669,color:#fff
    style F5 fill:#059669,color:#fff
    style F6 fill:#059669,color:#fff
    style F7 fill:#059669,color:#fff

Themes: Every mistake comes from treating Redis like a traditional database or forgetting it’s an in-memory cache with finite RAM.

Mistake 1: No Expiration on Cache Keys

# ❌ BAD — key lives forever
SET cache:products '{"items": [...]}'

# ✅ GOOD — expires in 5 minutes
SET cache:products '{"items": [...]}' EX 300

Why it matters: Without TTL, cache entries pile up indefinitely. Redis will eventually hit its memory limit and start evicting random keys or refusing writes.

Mistake 2: Using KEYS * in Production

# ❌ NEVER do this in production
KEYS *          # Scans ALL keys — blocks Redis!
KEYS user:*

# ✅ Use SCAN instead (non-blocking, cursor-based)
SCAN 0 MATCH user:* COUNT 100

Why it matters: KEYS is O(n) and blocks all other commands while running. On a database with millions of keys, this can freeze Redis for seconds.

Mistake 3: Storing Huge Values

# ❌ BAD — storing entire catalog as one key
SET catalog:all '{...10MB of data...}'

# ✅ GOOD — store individual items
HSET product:1001 name "Laptop" price "45000"
HSET product:1002 name "Mouse" price "1500"

Why it matters: Large values slow down serialization, network transfer, and consume more memory per key.

Mistake 4: Bad Key Naming

# ❌ BAD — unclear, may conflict
SET u42 "Prathamesh"
SET p9001 "Laptop"
SET x "temp"

# ✅ GOOD — structured, readable
SET user:42:name "Prathamesh"
SET product:9001:name "Laptop"
SET cache:homepage:v2 "..."

Mistake 5: Not Handling Cache Errors Gracefully

// ❌ BAD — crashes the app if Redis is down
const data = JSON.parse(await redis.get('cache:key'));
return res.json(data);

// ✅ GOOD — fallback to database if Redis fails
try {
  const cached = await redis.get('cache:key');
  if (cached) return res.json(JSON.parse(cached));
} catch (err) {
  console.warn('Redis unavailable, using DB fallback:', err.message);
}
const data = await db.query('...');
return res.json(data);

Mistake 6: Using Redis as a Primary Database

Redis is not designed to replace your primary relational or document database. It is a complementary tool for:

Caching
Session management
Rate limiting
Real-time data

Always keep your primary data in a durable database (MySQL, PostgreSQL, MongoDB).

Mistake 7: Not Setting maxmemory

Without a memory limit, Redis will consume all available RAM and potentially crash your server.

# In redis.conf
maxmemory 512mb
maxmemory-policy allkeys-lru

# Or via CLI
CONFIG SET maxmemory 512mb
CONFIG SET maxmemory-policy allkeys-lru

Eviction policies:

Policy	Behavior
`noeviction`	Return error when memory full
`allkeys-lru`	Evict least recently used keys
`volatile-lru`	Evict LRU keys with TTL only
`allkeys-random`	Evict random keys
`volatile-ttl`	Evict keys with shortest TTL