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What Stalls an Event Loop?

Every endpoint on a Node service gets slower at the same time, including a health check that does nothing. What do you look for?

The scenario

A Node API's p99 rises from 40ms to 1.4s at 09:00 daily. /healthz, which returns a static object, rises identically. CPU is at 100% on one core of an eight-core box, and a report endpoint that stringifies a 40MB object is called by a cron at 09:00.

What it tests

  • Whether the candidate recognises correlated latency across unrelated endpoints as a shared-resource signature
  • Whether they know what can occupy a single-threaded loop
  • Whether they know how to measure loop lag rather than guess

Answer it out loud first. Recognising a good answer is not the same skill as producing one.