Scaling
Growing a system's capacity so it keeps working as users, traffic, or data multiply.
What it tells you about a developer
Scaling stories are among the best signals in an engineering interview. A developer who has scaled a system has watched it strain under real growth, found the bottleneck, and fixed it under pressure. That experience only exists at companies whose products succeeded, and this developer can usually name what broke first as traffic grew: a database, a queue, or a single hot server.
Software that works for a hundred users can collapse at a hundred thousand, because databases, servers, and networks each have limits. Scaling is the work of raising those limits before users hit them. There are two directions: vertical scaling means a bigger machine, horizontal scaling means more machines. Cloud platforms can add capacity automatically, which is called autoscaling.