Health Check Endpoint

1. The Pattern

A small endpoint that answers one question: "should this instance receive traffic right now?" It is called every couple of seconds by the machinery that routes, restarts, and scales. It is cheap, constant, and consequential, because the caller's only remedies are drastic.

The load balancer lesson introduced health checks and their two opposite errors.

A shallow check tests only whether the process responds. It can remain green while the instance cannot reach any dependency required for useful work.