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.