Kako implementiraš graceful shutdown?

Question

Accepted Answer

**Graceful shutdown** pomeni čisto zaustavitev strežnika, ko prejme signal za prekinitev — zaključi requests, ki so v teku, zapri povezave (DB, ipd.) in sprosti vire — namesto da ga brusko ubiješ in izgubiš aktivno delo. To je bistveno za zero-downtime deploy in zanesljivost.

## Zakaj je abruptna zaustavitev problem

```text
Without graceful shutdown, when the process is killed (deploy, scale-down, crash):
  ✗ In-flight requests are dropped → users get errors
  ✗ DB transactions left incomplete
  ✗ Connections not closed cleanly → leaks/locks
```

Med deploy-i in avtoskaliranjem se procesi nenehno ustavljajo — početi to čisto je to, kar naredi deploy-e nemotene.

## Implementacija

```js
import http from "http";
const server = http.createServer(app);
server.listen(3000);

async function shutdown(signal) {
  console.log(`${signal} received — shutting down gracefully`);

// 1. Stop accepting NEW connections; finish existing ones
  server.close(async () => {
    // 2. This callback runs once all active requests have completed
    await db.end();            // close database pool
    await redis.quit();        // close other connections
    console.log("clean shutdown complete");
    process.exit(0);           // exit successfully
  });

// 3. Safety net: force-exit if cleanup hangs too long
  setTimeout(() => {
    console.error("forced shutdown — cleanup timed out");
    process.exit(1);
  }, 10_000).unref();
}

// Listen for termination signals
process.on("SIGTERM", () => shutdown("SIGTERM")); // sent by orchestrators (k8s, Docker)
process.on("SIGINT", () => shutdown("SIGINT"));   // Ctrl+C in dev
```

Zaporedje: ujemi signal → `server.close()` (ustavi nove povezave, izprazni obstoječe) → zapri DB/Redis/druge vire → izhod. **Timeout fallback** prisiljen izhod, če se kaj zapne, tako da zamrznjeno povezavo ne more za vedno blokirati zaustavitve.

## Signali

```text
SIGTERM → the standard "please terminate" signal from orchestrators on deploy/scale-down
SIGINT  → Ctrl+C (development)
SIGKILL → cannot be caught (force kill) — graceful shutdown can't help here
```

## V kontejneriziranemu okolju

```text
Kubernetes/Docker send SIGTERM, wait a grace period (e.g. 30s), THEN SIGKILL.
Your app must finish within that window → keep the shutdown timeout shorter.
Readiness probes should mark the pod unready so the load balancer stops sending traffic.
```

## Zakaj je to pomembno

Graceful shutdown je tisto, kar omogoča **zero-downtime deploy** in zanesljivo delovanje v sodobnih (posebej kontejnerizirannih/orkestriranih) okoljih, kjer se procesi rutinsko ustavljajo in ponovno zaženejo.

Brez tega vsak deploy ali scale-down spusti aktivne zahtevke in tvega neskladnost podatkov.

Implementacija zaporedja catch-signal → stop-accepting → drain → close-resources → exit (z timeout varnostnim mrežo) je značilnost produkcijske Node storitve in pogosto spregledano, vendar pomembno operativno vprašanje.