Application lifecycle in NestJS

Introduction

The application lifecycle in NestJS helps you control what happens when your app starts, runs, and stops. It makes sure your app sets up and cleans up properly.

When you want to run some code right after the app starts, like connecting to a database.

When you need to clean up resources before the app stops, like closing database connections.

When you want to perform tasks during app shutdown, such as saving logs or clearing caches.

When you want to listen to lifecycle events to manage app behavior smoothly.

Syntax

NestJS

import { Injectable, OnModuleInit, OnModuleDestroy, OnApplicationBootstrap, OnApplicationShutdown } from '@nestjs/common';

@Injectable()
export class MyService implements OnModuleInit, OnModuleDestroy, OnApplicationBootstrap, OnApplicationShutdown {
  onModuleInit() {
    // Called once the module has been initialized
  }

  onApplicationBootstrap() {
    // Called once the entire application has bootstrapped
  }

  onModuleDestroy() {
    // Called before the module is destroyed
  }

  onApplicationShutdown(signal?: string) {
    // Called before the application shuts down
  }
}

Use lifecycle hooks by implementing the interfaces in your service or provider.

Each hook runs at a specific time during app start or stop.

Examples

This runs code right after the module is ready.

NestJS

export class AppService implements OnModuleInit {
  onModuleInit() {
    console.log('Module initialized');
  }
}

This runs code when the app is about to stop, showing the shutdown signal.

NestJS

export class AppService implements OnApplicationShutdown {
  onApplicationShutdown(signal?: string) {
    console.log(`App shutting down due to: ${signal}`);
  }
}

Sample Program

This example shows a service that logs messages when the app module starts and when the app shuts down. It helps you see the lifecycle in action.

NestJS

import { Injectable, OnModuleInit, OnApplicationShutdown } from '@nestjs/common';

@Injectable()
export class LifecycleService implements OnModuleInit, OnApplicationShutdown {
  onModuleInit() {
    console.log('App module has been initialized');
  }

  onApplicationShutdown(signal?: string) {
    console.log(`App is shutting down. Signal: ${signal ?? 'none'}`);
  }
}

// In main.ts
import { NestFactory } from '@nestjs/core';
import { Module } from '@nestjs/common';

@Module({
  providers: [LifecycleService],
})
class AppModule {}

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  await app.listen(3000);
}

bootstrap();

Important Notes

Lifecycle hooks help manage resources like database connections cleanly.

Use the shutdown signal to know why the app is stopping (like Ctrl+C or system shutdown).

Not all hooks are required; implement only what you need.

Summary

Application lifecycle hooks let you run code at key app moments.

Use OnModuleInit to run code after module setup.

Use OnApplicationShutdown to clean up before the app stops.

Practice

1. Which lifecycle hook in NestJS is used to run code right after a module has been initialized?

easy

A. OnModuleInit

B. OnApplicationBootstrap

C. OnApplicationShutdown

D. OnModuleDestroy

Solution

1. Step 1: Understand module initialization timing

   The OnModuleInit hook runs immediately after the module's dependencies are resolved and the module is set up.

2. Step 2: Match the hook to the description

   OnApplicationBootstrap runs after the entire app starts, OnApplicationShutdown runs before app stops, and OnModuleDestroy runs before module is destroyed.

3. Final Answer:

   OnModuleInit -> Option A

4. Quick Check:

   Run code after module setup = OnModuleInit [OK]

Hint: Module init code runs in OnModuleInit hook [OK]

Common Mistakes:

• Confusing OnModuleInit with OnApplicationBootstrap
• Using OnApplicationShutdown for startup code
• Thinking OnModuleDestroy runs after module init

2. Which of the following is the correct way to implement the OnApplicationShutdown lifecycle hook in a NestJS service?

easy

A. export class MyService implements OnApplicationShutdown { onApplicationShutdown(signal?: string) { /* code */ } }

B. export class MyService implements OnModuleInit { onApplicationShutdown() { /* code */ } }

C. export class MyService implements OnApplicationBootstrap { onApplicationShutdown(signal: number) { /* code */ } }

D. export class MyService implements OnModuleDestroy { onApplicationShutdown(signal?: string) { /* code */ } }

Solution

1. Step 1: Identify correct interface and method signature

   The OnApplicationShutdown interface requires implementing onApplicationShutdown(signal?: string) method.

2. Step 2: Check method name and parameter types

   export class MyService implements OnApplicationShutdown { onApplicationShutdown(signal?: string) { /* code */ } } correctly implements OnApplicationShutdown with optional string parameter signal. Others mismatch interface or method name.

3. Final Answer:

   export class MyService implements OnApplicationShutdown { onApplicationShutdown(signal?: string) { /* code */ } } -> Option A

4. Quick Check:

   Correct interface and method signature = export class MyService implements OnApplicationShutdown { onApplicationShutdown(signal?: string) { /* code */ } } [OK]

Hint: Match interface name with method signature exactly [OK]

Common Mistakes:

• Implementing wrong interface for shutdown hook
• Using wrong method name or parameters
• Confusing OnApplicationShutdown with OnModuleInit

3. Consider this NestJS service code snippet:

export class AppService implements OnModuleInit, OnApplicationShutdown {
  onModuleInit() {
    console.log('Module initialized');
  }
  onApplicationShutdown(signal?: string) {
    console.log('App shutting down:', signal);
  }
}

What will be printed if the application starts and then receives a SIGINT signal to stop?

medium

A. Module initialized only

B. App shutting down: SIGINT\nModule initialized

C. App shutting down: SIGINT only

D. Module initialized\nApp shutting down: SIGINT

Solution

1. Step 1: Understand lifecycle hook order on start

   onModuleInit runs after module setup, so 'Module initialized' prints on app start.

2. Step 2: Understand shutdown hook on SIGINT

   When SIGINT signal is received, onApplicationShutdown runs and logs 'App shutting down: SIGINT'.

3. Final Answer:

   Module initialized\nApp shutting down: SIGINT -> Option D

4. Quick Check:

   Start logs init, stop logs shutdown with signal [OK]

Hint: Startup logs OnModuleInit, shutdown logs OnApplicationShutdown [OK]

Common Mistakes:

• Assuming shutdown logs before startup
• Missing signal parameter in shutdown log
• Thinking only one hook runs

4. This NestJS service code has an error:

export class MyService implements OnApplicationShutdown {
  onApplicationShutdown() {
    console.log('Cleaning up');
  }
}

What is the error and how to fix it?

medium

A. Should implement OnModuleDestroy instead of OnApplicationShutdown

B. Wrong method name; should be onShutdown instead

C. Missing optional signal parameter in onApplicationShutdown method

D. No error; code is correct

Solution

1. Step 1: Check method signature for OnApplicationShutdown

   The interface requires onApplicationShutdown(signal?: string) with an optional string parameter.

2. Step 2: Identify missing parameter in method

   The method lacks the optional signal parameter, causing a signature mismatch error.

3. Final Answer:

   Missing optional signal parameter in onApplicationShutdown method -> Option C

4. Quick Check:

   Method signature must match interface including optional parameters [OK]

Hint: Include optional signal parameter in onApplicationShutdown method [OK]

Common Mistakes:

• Omitting optional signal parameter
• Using wrong method name
• Confusing OnApplicationShutdown with OnModuleDestroy

5. You want to perform cleanup tasks only when the app shuts down due to a SIGTERM signal, ignoring other signals. How should you implement this in a NestJS service?

hard

A. Override the main bootstrap function to handle SIGTERM manually

B. Implement OnApplicationShutdown and check if signal === 'SIGTERM' inside onApplicationShutdown method

C. Use OnApplicationBootstrap to detect shutdown signals

D. Implement OnModuleDestroy and check for SIGTERM signal there

Solution

1. Step 1: Choose correct lifecycle hook for app shutdown

   OnApplicationShutdown runs when the app is stopping and receives the shutdown signal.

2. Step 2: Filter signal inside onApplicationShutdown method

   Inside onApplicationShutdown(signal?: string), check if signal === 'SIGTERM' to run cleanup only for that signal.

3. Final Answer:

   Implement OnApplicationShutdown and check if signal === 'SIGTERM' inside onApplicationShutdown method -> Option B

4. Quick Check:

   Filter shutdown signal inside OnApplicationShutdown hook [OK]

Hint: Check signal inside OnApplicationShutdown to filter SIGTERM [OK]

Common Mistakes:

• Using OnModuleDestroy which runs on module destroy, not app shutdown
• Trying to detect shutdown in OnApplicationBootstrap
• Manually overriding bootstrap instead of lifecycle hooks