Event patterns (event-based) in NestJS - Mini Project: Build & Apply

Event Patterns with NestJS

📖 Scenario: You are building a simple NestJS application that listens for user registration events and processes them.This is like a mailroom receiving packages: when a new package (event) arrives, the mailroom (your app) handles it accordingly.

🎯 Goal: Create a NestJS event-based system where a UserRegistered event is defined, a listener subscribes to it, and the event is emitted to trigger the listener.

📋 What You'll Learn

Define an event pattern called UserRegistered

Create an event listener method that listens for UserRegistered events

Emit a UserRegistered event with a payload containing username and email

Use NestJS event-based patterns with @EventPattern and ClientProxy

💡 Why This Matters

🌍 Real World

Event-based communication is common in microservices and distributed systems to decouple components and react to changes asynchronously.

💼 Career

Understanding event patterns in NestJS is valuable for backend developers working with scalable, event-driven architectures.

1

Create the event pattern and payload interface

Create an interface called UserRegisteredEvent with username and email as string properties. Then define a constant USER_REGISTERED_EVENT with the value 'user_registered'.

NestJS

# Define the event interface and event pattern constant
# Your code here

Need a hint?

Think of the event as a message with a name and data. The interface describes the data shape.

2

Create an event listener method with @EventPattern

In a NestJS service class, create a method called handleUserRegistered decorated with @EventPattern(USER_REGISTERED_EVENT). The method should accept a parameter payload of type UserRegisteredEvent.

NestJS

export interface UserRegisteredEvent {
  username: string;
  email: string;
}

export const USER_REGISTERED_EVENT = 'user_registered';

import { Injectable } from '@nestjs/common';
import { EventPattern } from '@nestjs/microservices';

@Injectable()
export class UserEventsService {
  // Create a method handleUserRegistered with @EventPattern(USER_REGISTERED_EVENT) decorator
  // The method should accept a parameter named payload of type UserRegisteredEvent
  
  // Your code here
}

Need a hint?

The @EventPattern decorator listens for events with the given name.

3

Inject ClientProxy and emit the event

In a NestJS service class called UserService, inject ClientProxy named client via the constructor. Create a method registerUser that accepts username and email strings, then emits the USER_REGISTERED_EVENT with a payload matching UserRegisteredEvent.

NestJS

export interface UserRegisteredEvent {
  username: string;
  email: string;
}

export const USER_REGISTERED_EVENT = 'user_registered';

import { Injectable } from '@nestjs/common';
import { EventPattern } from '@nestjs/microservices';

@Injectable()
export class UserEventsService {
  @EventPattern(USER_REGISTERED_EVENT)
  handleUserRegistered(payload: UserRegisteredEvent) {
    // Event handling logic here
  }
}

import { Injectable } from '@nestjs/common';
import { ClientProxy } from '@nestjs/microservices';

@Injectable()
export class UserService {
  constructor(private readonly client: ClientProxy) {}

  // Create a method registerUser(username: string, email: string)
  // Emit USER_REGISTERED_EVENT with payload { username, email }
  
  // Your code here
}

Need a hint?

Use this.client.emit(eventName, payload) to send events.

4

Complete the event handling logic

Inside the handleUserRegistered method, add a line to log the message User registered: {username} ({email}) using the payload data with a template string.

NestJS

export interface UserRegisteredEvent {
  username: string;
  email: string;
}

export const USER_REGISTERED_EVENT = 'user_registered';

import { Injectable } from '@nestjs/common';
import { EventPattern, ClientProxy } from '@nestjs/microservices';

@Injectable()
export class UserEventsService {
  @EventPattern(USER_REGISTERED_EVENT)
  handleUserRegistered(payload: UserRegisteredEvent) {
    // Log user registration info here
    
    // Your code here
  }
}

@Injectable()
export class UserService {
  constructor(private readonly client: ClientProxy) {}

  registerUser(username: string, email: string) {
    const payload: UserRegisteredEvent = { username, email };
    this.client.emit(USER_REGISTERED_EVENT, payload);
  }
}

Need a hint?

Use a template string with backticks to include variables inside the log message.