Tell me about a Node.js service you built or maintained. What was it responsible for and what was hard about it?

Note: Give scale numbers. "An API" tells the interviewer nothing; "an order service handling about 400 requests a second at peak" tells them what problems you have actually met.

Cover four things in order:

• What it did and who depended on it. One sentence. Was it user-facing, or a background worker other services relied on?
• The shape of the load. Steady traffic, spiky traffic, long-running jobs, or lots of small I/O. This determines every interesting decision that follows.
• The genuinely hard part. Good candidates: keeping the event loop unblocked when a CPU-heavy task appeared, handling a downstream service that became slow rather than failing outright, memory growth you had to trace, or making retries safe when the same message could arrive twice.
• What you would do differently. This is the part that separates a senior answer from a junior one.