Redis (in-memory store).

The database is the filing cabinet across the room — reliable, but a walk every time.

Keep the hot items on a sticky-note wall right by your desk: in memory.

Grab from the wall instantly — memory is thousands of times faster than disk.

It is just memory, though: fast, but limited in size and wiped on power loss.

It is more than a cache — counters, queues, leaderboards, rate limits all fit.

So you keep a copy in the cabinet too, and let the wall hold what’s hot.