Great Stack to Doesn't Work #3 — Redis: "99% Cache Hit Ratio, System Down"

A survival guide for when everything goes wrong in production. Your Redis dashboard looks perfect. Hit ratio: 99.2%. Latency: sub-millisecond. Memory usage: 60% of available. Every metric says healthy. Then at 2:47 PM, your API starts returning 500s. Response times spike to 30 seconds. Users can't log in. The dashboard still shows 99% hit ratio because the cache is working — it's serving cached errors to everyone equally fast. Redis is doing exactly what you told it to do. The problem is what you told it to do. Why Single-Threaded Is Fast (Until It Isn't) Redis processes commands on a single thread. No locks. No context switching. No synchronization overhead. One CPU core, fully utilized, can handle 100K+ operations per second because it never waits for another thread to release a lock. The event loop model (similar to Node.js) multiplexes thousands of client connections on a single thread using non-blocking I/O. Read a request, process it, write the response, move to the next. When your commands are simple — GET, SET, INCR — each one takes microseconds. The trap: slow commands block everything. KEYS * on a million-key database? That's a full keyspace scan on the main thread. While it runs, every other client waits. SORT on a large set? Same. LRANGE on a list with 10 million elements? Same. Redis 6.0 introduced I/O threading ( io-threads config) for reading and writing network data on multiple threads, but command execution is still single-threaded. Redis 7.0 improved this further, but the fundamental model hasn't changed. Long-running commands on the main thread stall everything. Rules: Never use KEYS in production. Use SCAN instead — it's cursor-based and returns results incrementally. Watch out for O(N) commands on large data structures: LRANGE , SMEMBERS , HGETALL on million-element structures. Use SLOWLOG to find commands that are blocking the event loop. Pipelining: The Easiest 10x You'll Ever Get Every Redis command involves a network round trip: send request