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HTTP server: Health checks

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Health endpoints let load balancers and orchestrators decide what to do with a process. Liveness answers "is this process alive at all", readiness answers "can it take traffic right now". This server exposes both.

Readiness depends on the things the server needs to do useful work, such as a database pool or a warmed cache. Here a flag flips to true once the simulated dependency has started up.
let dbConnected = false;

setTimeout(() => {
  dbConnected = true;
  console.log("Database connected, now ready");
}, 2000);
In a real service this would issue a cheap query, like SELECT 1.
function pingDb(): Promise<boolean> {
  return Promise.resolve(dbConnected);
}

Deno.serve(async (req) => {
  const url = new URL(req.url);
Probes must see the current state, so never let a cache answer them.
  const headers = { "cache-control": "no-store" };
Liveness: a 200 whenever the process is serving requests. Orchestrators restart the container when this fails repeatedly, so keep it free of dependency checks. A broken database should not cause a restart loop.
  if (url.pathname === "/healthz") {
    return new Response("ok\n", { status: 200, headers });
  }
Readiness: 200 only when dependencies are usable. On failure the orchestrator takes the instance out of load balancing but does not restart it, and puts it back once it reports ready again.
  if (url.pathname === "/readyz") {
    const ready = await pingDb();
    return new Response(ready ? "ready\n" : "database unavailable\n", {
      status: ready ? 200 : 503,
      headers,
    });
  }

  return new Response("Hello\n", { headers });
});
Right after startup the process is alive but not yet ready: curl -i http://localhost:8000/healthz HTTP/1.1 200 OK cache-control: no-store content-length: 3 date: Thu, 11 Jun 2026 18:55:02 GMT ok curl -i http://localhost:8000/readyz HTTP/1.1 503 Service Unavailable cache-control: no-store content-length: 21 date: Thu, 11 Jun 2026 18:55:02 GMT database unavailable Two seconds later the dependency is up and readiness flips: curl -i http://localhost:8000/readyz HTTP/1.1 200 OK cache-control: no-store content-length: 6 date: Thu, 11 Jun 2026 18:55:04 GMT ready

Run this example locally using the Deno CLI:

deno run -N https://docs.deno.com/examples/scripts/http_server_health_checks.ts

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