How would you know a backend system is near capacity, and what would you do before it falls over?

In production I would design capacity controls per bottleneck. If SQL is saturated, adding API instances can make it worse. If a queue is backing up, increasing consumers helps only if the downstream dependency can handle it. I would alert on saturation trends and define emergency controls in runbooks. After the incident, I would review capacity tests, autoscale rules, retry policies, and whether the product needs prioritization between critical and optional work.