Application lifecycle in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Application Lifecycle in NestJS?

It is the sequence of events from starting the app, running it, to shutting it down, including hooks to run code at each stage.

beginner

Name the lifecycle hook used to run code right after the application has fully started.

The hook is called onApplicationBootstrap().

intermediate

What is the purpose of the onModuleDestroy() hook in NestJS?

It allows you to run cleanup code when a module is about to be destroyed, like closing database connections.

intermediate

How does NestJS handle graceful shutdown of an application?

NestJS listens for shutdown signals and calls lifecycle hooks like onApplicationShutdown() to clean resources before exit.

intermediate

Which interface should a service implement to run code when the application is shutting down?

Implement the OnApplicationShutdown interface and define the onApplicationShutdown() method.

Which lifecycle hook runs after a module is initialized in NestJS?

A onModuleInit()

B onApplicationShutdown()

C onModuleDestroy()

D beforeApplicationStart()

onModuleInit() runs after a module is initialized.

What does the onApplicationBootstrap() hook do?

A Runs code after the entire application has bootstrapped

B Runs code during shutdown

C Runs code when a module is destroyed

D Runs code before the app starts

onApplicationBootstrap() runs after the whole app is ready.

Which interface is used to handle cleanup during app shutdown?

A OnModuleInit

B OnApplicationShutdown

C OnModuleDestroy

D OnBootstrap

OnApplicationShutdown is for cleanup during shutdown.

What signal does NestJS listen for to trigger graceful shutdown?

A SIGBOOT

B SIGSTART

C SIGRUN

D SIGINT

NestJS listens for SIGINT to start shutdown.

Which lifecycle hook is NOT part of NestJS application lifecycle?

A onModuleDestroy

B onApplicationShutdown

C onAppStart

D onModuleInit

onAppStart is not a NestJS lifecycle hook.

Explain the main lifecycle hooks in NestJS and when they run.

Think about the app starting, running, and stopping phases.

Describe how NestJS supports graceful shutdown and why it is important.

Consider what happens when you stop a running app.

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