NestJS inatoa lifecycle hooks zipi?

Question

Accepted Answer

NestJS inatoa **lifecycle hooks** — methods unazozitekeleza ili kuendesha code katika nyakati mahususi za mzunguko wa maisha wa programu au wa provider (initialization na shutdown). Ni muhimu kwa setup (kuunganisha na rasilimali) na cleanup (kufunga connections kwa upole).

## Hooks kuu

```ts
@Injectable()
export class DatabaseService
  implements OnModuleInit, OnApplicationShutdown {

async onModuleInit() {
    // runs once the host MODULE's dependencies are resolved → do INITIALIZATION
    await this.connect();         // e.g. open a database connection
  }

async onApplicationShutdown(signal: string) {
    // runs when the app is shutting down → do CLEANUP
    await this.disconnect();      // close connections, flush, release resources
  }
}
```

## Mfuatano wa lifecycle

```text
INITIALIZATION (startup):
  onModuleInit()          → after the module's providers are initialized
  onApplicationBootstrap()→ after ALL modules are initialized (whole app ready)

SHUTDOWN (graceful termination):
  onModuleDestroy()         → cleanup as modules are destroyed
  beforeApplicationShutdown() → before shutdown completes
  onApplicationShutdown(signal) → final shutdown (signal = SIGTERM, etc.)
```

## Matumizi ya kawaida: Prisma service ikiunganisha/ikikata muunganisho

```ts
@Injectable()
export class PrismaService extends PrismaClient
  implements OnModuleInit, OnModuleDestroy {

async onModuleInit() {
    await this.$connect();        // connect on startup
  }
  async onModuleDestroy() {
    await this.$disconnect();      // disconnect on shutdown — clean release
  }
}
```

Huu ni mtindo wa kawaida kwa rasilimali zinazohitaji setup/teardown ya wazi — unganisha katika `onModuleInit`, achilia katika `onModuleDestroy`.

## Kuwezesha shutdown hooks

```ts
// shutdown hooks must be enabled explicitly
const app = await NestFactory.create(AppModule);
app.enableShutdownHooks();        // ❗ required for onApplicationShutdown etc. to fire
```

Shutdown hooks zinaendeshwa tu ikiwa utaita `enableShutdownHooks()` — rahisi kusahau, kisha cleanup haitokei kamwe.

## Kwa nini graceful shutdown ni muhimu

```text
When the app receives SIGTERM (deploy, scale-down, container stop):
  ✗ Without shutdown hooks → connections left open, in-flight work dropped
  ✓ With shutdown hooks → close DB/Redis/queues cleanly, finish work → zero-downtime
```

## Kwa nini ni muhimu

Lifecycle hooks ni muhimu kwa ajili ya kusimamia rasilimali kwa usahihi katika programu za NestJS — zinatoa mahali sahihi pa kutekeleza **initialization** (kufungua database/Redis connections, kupakia data, kujisajili na services) na, muhimu zaidi, **cleanup ya upole** wakati wa shutdown (kufunga connections, ku-flush buffers, kumaliza kazi iliyoko mtiririkoni).

Kuzielewa ni muhimu kwa uaminifu wa production: **initialization hooks** (`onModuleInit`, `onApplicationBootstrap`) zinahakikisha setup inatokea wakati sahihi mara dependencies zikiwa tayari, na **shutdown hooks** (`onModuleDestroy`, `onApplicationShutdown`) zinawezesha **graceful shutdown** — ambayo ni muhimu sana katika deployment ya kisasa (containers/orchestrators hutuma SIGTERM mara kwa mara wakati wa deploys na scaling, na bila cleanup sahihi unavujisha connections na kudondosha maombi yaliyoko hai, ukivunja zero-downtime deployments).

Mtindo wa kawaida wa kuunganisha katika `onModuleInit` na kukata muunganisho katika `onModuleDestroy` (k.m. kwa Prisma/database services) ni maarifa ya kila siku ya NestJS, na kujua hitaji la kisirisiri la kuita `enableShutdownHooks()` (vinginevyo shutdown hooks hazitoendeshwa kamwe kimyakimya) ni jambo la kivitendo, rahisi kukosa.

Kubobea katika lifecycle hooks ni sehemu ya kujenga NestJS services imara, zilizo tayari kwa production zinazoshughulikia rasilimali na deployments kwa usahihi.