Cache That Added Latency

We added Redis cache to speed things up. It added 50ms latency.

What we cached:

• User profile (changes every request)
• Shopping cart (user-specific, volatile)
• Real-time inventory counts

The problem:

• Cache hit rate: 3%
• 97% of requests: cache miss + DB query
• Plus network round-trip to Redis: 2ms
• Plus serialization/deserialization: 5ms
• Net result: slower than no cache

What we should have cached:

• Product catalog (read-heavy, rarely changes)
• Category tree (static)
• Feature flags (global, rarely changes)
• Computed aggregations (expensive to calculate)

Cache math:

Benefit = (hit_rate × db_latency) - cache_overhead
                                    If benefit < 0, cache is hurting you

Fixed version:

• Removed volatile data from cache
• Added static content with long TTL
• Hit rate: 3% → 85%
• P50 latency: 150ms → 40ms

Lesson: Cache read-heavy, rarely-changing data. Not write-heavy, volatile data.

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