Health Check Endpoint

The Pattern

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

The load balancer lesson already used these and left a warning behind: health checks can lie in two directions. A shallow check ("process responds") passes while the server can't reach anything: green but useless