At 3:17 AM on a Tuesday in Q3 2024, our p99 API latency hit 4.8 seconds, cache miss rate spiked to 38%, and our on-call engineer was ready to quit. We’d been running Memcached 1.6 as our primary cache for 7 years—this was the breaking point. Three months later, after migrating to Redis 7.2 with a custom sharding layer, our cache miss rate dropped to 15.2%, p99 latency fell to 280ms, and we saved $22,000 per month in unnecessary EC2 and RDS overprovisioning. This is the unvarnished story of how we did it, with every benchmark, code snippet, and mistake included.…