#redis

Performance & Scaling Bug Report

Semantic cache returned the answer to a different question at 0.92 similarity

Ryan Pham · Sep 10, 2025 · 6 min read

We added a semantic cache to our help assistant to cut LLM cost. Within days a user asking how to cancel a subscription got instructions for cancelling an order. The two questions were 0.92 similar. We added exact-match first, scoped keys, intent gating and a measured threshold.

Databases Bug Report

Users randomly logged out: Redis maxmemory eviction removed session keys

Daniel Tran · May 6, 2025 · 9 min read

Users were logged out at random, mostly in the afternoon. Sessions and a fast-growing cache shared one Redis with allkeys-lru, so every hourly cache warm-up evicted idle sessions. Separate DB numbers would not have helped; separate instances did.

Databases Bug Report

Cache stampede when a hot Redis key expired and pushed Postgres to 100% CPU

Daniel Tran · Mar 20, 2025 · 8 min read

Every ten minutes our catalog API spiked to 4-second responses and the database hit 100% CPU. A hot Redis key expired and 600 requests rebuilt it at once. Single-flight, a short Redis lock, stale-while-revalidate and TTL jitter fixed it.