Sharding

You will learn what a shard key actually commits you to (locality, load distribution, blast radius and rebalancing cost), how range, hash and directory sharding differ, and how to build in an escape hatch before you need one.

One team's shard key change is a maintenance ticket; the other's is a multi-day outage — and the difference was decided months before anyone noticed the traffic shift.

Every failure you're currently blaming on prompt engineering — the bottleneck agent, the re-plan that breaks on scale, the handoff where two agents end up disagreeing — already has a name and a fix, and distributed database engineers wrote it up over a decade ago.

The tenant_id shard that held up for a year under real human traffic didn't survive three weeks of an unattended AI agent — here's the layer that would have turned the outage into a config change.