Scale Is a Sequence, Not a Destination
Systems do not break at scale. They break in a predictable order, and knowing the order is most of the job.
Nobody hits “scale”. You hit a specific bottleneck, fix it, and reveal the next one. The useful skill is not making things fast — it is knowing which wall you are about to hit.
The usual order
Read load saturates first, and it is the cheapest to fix. Then write contention. Then the thing nobody modelled: a single coordination point that every request touches.