redis

Sessions, locks and pub/sub are three very different contracts with the same server. Knowing which guarantees each one lacks is the difference between a cache and an outage.

Spikes that march in step with BGSAVE are a page-size story, not a network story. Check actual huge-page usage, fork duration and copy-on-write pressure on the deployed kernel.

Everyone names token bucket. The scored part is the rest: drift between nodes, the 429 contract with clients, and whether your quota service should fail open or closed.

The catalog is the easy half. The scored half is stampede dynamics: one hot key expiring can multiply origin load by a thousand in a second, and there are three distinct fixes.

The vocabulary question hiding real depth: which primitive replaces ORDER BY, why sharding by player is wrong, and what a 45,000 updates-per-second storm actually costs.

Size stored sessions from expiry and creation rates, then decide how quickly logout must take effect and what failover may forget.