Startup Health Checks: Validate Env & Probe Services Before Your App Accepts Traffic
· 8 min read
Your application deployed. The CI pipeline went green. The health endpoint returned 200 OK. And then, three minutes later, the first real user request exploded in production with ECONNREFUSED because DATABASE_URL was never set in the new environment.
This class of incident is entirely preventable. The fix is not better monitoring or smarter alerting. It is making the application refuse to start when it is not ready to serve traffic.
This post covers exactly how to do that across two runtimes: NestJS and ASP.NET Core. The pattern is the same in both: validate environment variables first, probe every external service second, and only then bind the HTTP server and accept connections.
