Event patterns (event-based) in NestJS

Introduction

Event patterns let different parts of your app talk by sending and listening to messages. This helps your app stay organized and react to things happening.

When you want to separate parts of your app so they don't depend on each other directly.

When you need to react to actions happening somewhere else in your app, like a user signing up.

When you want to build features that work with messages from other services or microservices.

When you want to handle tasks asynchronously, like sending emails after a user registers.

Syntax

NestJS

import { Controller } from '@nestjs/common';
import { EventPattern, Payload } from '@nestjs/microservices';

@Controller()
export class MyController {
  @EventPattern('event_name')
  handleEvent(@Payload() data: any) {
    // handle the event data
  }
}

@EventPattern listens for events with a specific name.

@Payload() extracts the data sent with the event.

Examples

This listens for the 'user_created' event and logs the user data.

NestJS

import { Controller } from '@nestjs/common';
import { EventPattern, Payload } from '@nestjs/microservices';

@Controller()
export class UserController {
  @EventPattern('user_created')
  handleUserCreated(@Payload() userData: any) {
    console.log('User created:', userData);
  }
}

This listens for 'order_placed' events and processes orders asynchronously.

NestJS

import { Controller } from '@nestjs/common';
import { EventPattern, Payload } from '@nestjs/microservices';

@Controller()
export class OrderController {
  @EventPattern('order_placed')
  async handleOrder(@Payload() orderData: any) {
    // process order asynchronously
  }
}

Sample Program

This simple controller listens for 'send_notification' events and prints a message to notify a user.

NestJS

import { Controller } from '@nestjs/common';
import { EventPattern, Payload } from '@nestjs/microservices';

@Controller()
export class NotificationController {
  @EventPattern('send_notification')
  handleNotification(@Payload() message: { userId: string; text: string }) {
    console.log(`Notify user ${message.userId}: ${message.text}`);
  }
}

Important Notes

Event names should be clear and descriptive to avoid confusion.

Event handlers can be async if you need to do tasks like database calls.

Use event patterns to keep your app modular and easier to maintain.

Summary

Event patterns help parts of your app communicate by sending and receiving messages.

Use @EventPattern to listen for specific events and @Payload() to get event data.

This approach makes your app more organized and ready for complex tasks.

Practice

1. What is the main purpose of using @EventPattern in a NestJS application?

easy

A. To create database entities

B. To define HTTP routes for REST APIs

C. To listen for specific events and handle their data

D. To style components with CSS

Solution

1. Step 1: Understand the role of @EventPattern

   This decorator is used to mark a method that listens for a specific event in an event-based system.

2. Step 2: Compare with other options

   Options B, C, and D relate to other parts of NestJS or frontend development, not event listening.

3. Final Answer:

   To listen for specific events and handle their data -> Option C

4. Quick Check:

   @EventPattern listens to events [OK]

Hint: Remember: @EventPattern listens to events, not routes or styles [OK]

Common Mistakes:

• Confusing @EventPattern with @Controller for routes
• Thinking @EventPattern creates database models
• Mixing event listening with styling tasks

2. Which of the following is the correct way to receive event data inside an event handler method in NestJS?

easy

A. Use @Payload() decorator on a method parameter

B. Use @Body() decorator on a method parameter

C. Use @Query() decorator on a method parameter

D. Use @Param() decorator on a method parameter

Solution

1. Step 1: Identify the decorator for event data

   In NestJS event handlers, @Payload() extracts the data sent with the event.

2. Step 2: Differentiate from HTTP decorators

   @Body(), @Query(), and @Param() are used in HTTP controllers, not event handlers.

3. Final Answer:

   Use @Payload() decorator on a method parameter -> Option A

4. Quick Check:

   @Payload() gets event data [OK]

Hint: Event data comes from @Payload(), not HTTP decorators [OK]

Common Mistakes:

• Using @Body() in event handlers instead of @Payload()
• Confusing HTTP request decorators with event decorators
• Omitting the decorator and expecting automatic data injection

3. Given the following NestJS event handler code, what will be logged when an event named 'order_created' with payload { id: 42 } is emitted?

@EventPattern('order_created')
handleOrder(@Payload() data: any) {
  console.log(`Order ID: ${data.id}`);
}

medium

A. No output because event name is wrong

B. Order ID: undefined

C. SyntaxError due to missing return

D. Order ID: 42

Solution

1. Step 1: Understand event matching

   The handler listens for 'order_created' events, so it will trigger when such an event is emitted.

2. Step 2: Check payload usage

   The payload has an object with id: 42, so data.id is 42 and will be logged.

3. Final Answer:

   Order ID: 42 -> Option D

4. Quick Check:

   Payload id 42 logs correctly [OK]

Hint: Match event name exactly and use @Payload() to access data [OK]

Common Mistakes:

• Assuming event name is case-insensitive
• Forgetting to use @Payload() and getting undefined
• Expecting a return value to log output

4. Identify the error in this NestJS event handler code:

@EventPattern('user_registered')
handleUser(@Payload data: any) {
  console.log(data.username);
}

medium

A. Missing parentheses in @Payload decorator

B. Incorrect event name format

C. Method name must be 'onUserRegistered'

D. Payload parameter must be last in method

Solution

1. Step 1: Check decorator syntax

   The @Payload decorator requires parentheses even if no arguments are passed, so it should be @Payload().

2. Step 2: Verify other parts

   Event name format is correct, method name can be anything, and parameter order is flexible.

3. Final Answer:

   Missing parentheses in @Payload decorator -> Option A

4. Quick Check:

   @Payload() needs parentheses [OK]

Hint: Always write @Payload() with parentheses, even if empty [OK]

Common Mistakes:

• Omitting parentheses in decorators
• Thinking method names must follow event names
• Assuming parameter order matters for decorators

5. You want to create a NestJS service that listens to two different events: 'payment_success' and 'payment_failed'. Which of the following is the best way to implement this using event patterns?

hard

A. Use @Controller with two routes named after events

B. Create two methods, each decorated with @EventPattern for each event name

C. Create one method with @EventPattern('payment') and check event type inside

D. Use @Payload() decorator twice in one method for both events

Solution

1. Step 1: Understand event pattern usage

   Each event should have its own handler method decorated with @EventPattern specifying the event name.

2. Step 2: Evaluate other options

   Create one method with @EventPattern('payment') and check event type inside is less clear and not idiomatic; Use @Controller with two routes named after events is for HTTP routes, not events; Use @Payload() decorator twice in one method for both events is invalid syntax.

3. Final Answer:

   Create two methods, each decorated with @EventPattern for each event name -> Option B

4. Quick Check:

   Separate methods for separate events [OK]

Hint: Use one @EventPattern per event with separate methods [OK]

Common Mistakes:

• Trying to handle multiple events in one method without checking
• Confusing event handlers with HTTP controllers
• Using multiple @Payload decorators in one method