Application lifecycle in NestJS - Step-by-Step Execution

Concept Flow - Application lifecycle

Start Application

↓

Module Initialization

↓

Dependency Injection Setup

↓

OnModuleInit Hook

↓

Application Ready

↓

Handle Requests

↓

OnApplicationShutdown Hook

↓

Cleanup Resources

↓

Application Stops

This flow shows how a NestJS app starts, initializes modules, sets up dependencies, runs lifecycle hooks, handles requests, and cleans up on shutdown.

Execution Sample

NestJS

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

@Injectable()
export class AppService implements OnModuleInit, OnApplicationShutdown {
  onModuleInit() { console.log('Module initialized'); }
  onApplicationShutdown() { console.log('Application shutting down'); }
}

This code defines a service that logs messages when the module initializes and when the app shuts down.

Execution Table

Step	Action	Hook Called	Console Output	State
1	Start Application	None		App starting
2	Initialize Modules	None		Modules initialized
3	Setup Dependency Injection	None		Dependencies injected
4	Call onModuleInit	onModuleInit	Module initialized	Module ready
5	Application Ready	None		App ready to handle requests
6	Handle Requests	None		Serving requests
7	Trigger Shutdown	onApplicationShutdown	Application shutting down	Cleaning up
8	Cleanup Resources	None		Resources cleaned
9	Stop Application	None		App stopped

💡 Application stops after cleanup and shutdown hooks complete.

Variable Tracker

Variable	Start	After Step 4	After Step 7	Final
appState	starting	moduleReady	shuttingDown	stopped
consoleOutput		Module initialized	Application shutting down	Application shutting down

Key Moments - 2 Insights

Why does onModuleInit run before the app is ready to handle requests?

Because onModuleInit is called right after modules and dependencies are set up (see step 4 in execution_table), ensuring the module is ready before serving requests.

When exactly does onApplicationShutdown run?

It runs during the shutdown process (step 7), before the app fully stops, to allow cleanup of resources.

Visual Quiz - 3 Questions

Look at the execution table, what is the appState after step 4?

A moduleReady

B starting

C shuttingDown

D stopped

💡 Hint

Check variable_tracker row for appState after step 4.

At which step does the application start handling requests?

A Step 3

B Step 5

C Step 7

D Step 9

💡 Hint

See execution_table row where 'Application Ready' and 'Serving requests' state appear.

If onModuleInit was removed, what would change in the execution flow?

A Module initialization would be skipped

B Application would not start

C No console output at step 4

D Shutdown hook would not run

💡 Hint

Look at consoleOutput in execution_table at step 4.

Concept Snapshot

NestJS Application Lifecycle:
- Start app -> initialize modules
- Setup dependencies
- onModuleInit runs after DI setup
- App ready to handle requests
- onApplicationShutdown runs on shutdown
- Cleanup and stop app
Use lifecycle hooks to run code at these stages.

Full Transcript

The NestJS application lifecycle begins when the app starts. Modules initialize and dependencies are injected. Then, the onModuleInit hook runs to signal module readiness. After that, the app is ready to handle requests. When the app shuts down, the onApplicationShutdown hook runs to clean up resources before the app stops. This lifecycle helps organize code to run at the right time during app start and stop.

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