Building Scalable NestJS Applications with CQRS: Best Practices and Advanced Implementation Techniques

Welcome, esteemed readers! Get ready to delve into the vast universe of Command Query Responsibility Segregation (CQRS) as we explore its implementation in NestJS applications, using advanced techniques and practical examples. This exciting journey aims to equip you with the knowledge and tools to structure your application codebase effectively, design efficient commands and queries, handle domain events, and separate the read and write operations of your system.

Why, you may ask, are we focusing on CQRS? Well, CQRS offers a unique way of structuring your application, one that separates the write operations (Commands) from the read operations (Queries), thereby enhancing the performance and scalability of your application. This pattern, when correctly implemented, can lead to highly maintainable, scalable, and robust applications.

“CQRS is not merely a pattern, but a journey towards creating better, scalable, and maintainable applications."

By the end of our journey, you will not just have a theoretical understanding of CQRS and its principles, but also possess real-world practical knowledge on how to implement it in your NestJS applications. So, fasten your seat belts and get ready for an enlightening adventure!

Understanding the Concepts and Principles of CQRS

CQRS, short for Command Query Responsibility Segregation, is a design pattern that separates write operations (Commands) and read operations (Queries) in an application. This segregation simplifies the design and enhances the performance of complex business systems.

So how does this segregation work? Here's a straightforward explanation:

1. Commands are responsible for all the changing behaviours in your system. They represent the intention to modify data. However, they do not return any data.
2. Queries, on the other hand, are responsible for returning data. They do not affect the state of the system.

In essence, CQRS promotes a clean separation between tasks that modify data and tasks that read data, eliminating any overlap that can complicate the architecture.

Remember, while CQRS can bring us numerous benefits, it's not a silver bullet for all situations. It excels in complex domains where business rules for reading and writing data diverge significantly.

Benefits of CQRS

Now that we understand the principles of CQRS, let's delve into its benefits:

• Improved performance: By segregating commands and queries, we can optimize each independently. This yields a significant performance boost.
• Scalability: With CQRS, we can scale read-side and write-side independently based on the load.
• Flexibility: It allows us to choose different data storage and processing technologies for different needs.
• Simplified development: By separating concerns, it makes our code easier to understand, develop, and maintain.

These benefits make CQRS a compelling choice for complex business systems where high performance, scalability, and maintainability are critical.

Implementing CQRS in NestJS

Initial Setup

Before we start implementing the CQRS pattern, we need to set up our NestJS application. First, install the necessary packages:

npm install --save @nestjs/cqrs

Next, we need to configure our database. In this tutorial, we will use PostgreSQL. Here is the SQL command to create a table named Person:

CREATE TABLE Person(
 Id SERIAL PRIMARY KEY NOT NULL,
 Name Text NULL,
 Age INT NULL
)

After creating the table, we need to define an entity for it in our NestJS application:

import { Column, Entity, PrimaryGeneratedColumn } from 'typeorm';

@Entity({name:'person'})
export class Person {
  @PrimaryGeneratedColumn('increment',{name:'id'})
  id: number;
  @Column({name:'name'})
  name: string;
  @Column({name:'age'})
  age: number;
}

We then need to configure TypeORM, a module in NestJS for handling database operations, to connect to our PostgreSQL database:

import { TypeOrmModule } from '@nestjs/typeorm';
import { Person } from './entities/person';
@Module({
  imports: [
    TypeOrmModule.forRoot({
      type:'postgres',
      host:'localhost',
      port: 5432,
      username: 'postgres',
      password:'secret',
      database:'myworlddb',
      entities:[Person]
    })
  ]
})
export class AppModule {}

Finally, we need to import the CqrsModule in our module:

import { CqrsModule } from '@nestjs/cqrs';
@Module({
  imports:[CqrsModule]
})
export class PersonModule {}

Now that we have set up our application, let's move on to implementing the CQRS pattern.

Implementing Query Handlers

In the CQRS pattern, queries are used to read data from the database. A query handler is responsible for handling a specific type of query.

First, we need to create a query. In this case, our query is to get all persons from the database:

export class GetPersonsQuery {}

Then we need to create a handler for this query:

import { IQueryHandler, QueryHandler } from '@nestjs/cqrs';
import { InjectRepository } from '@nestjs/typeorm';
import { Person } from 'src/entities/person';
import { Repository } from 'typeorm';
import { GetPersonsQuery } from '../impl/get-persons.query';
@QueryHandler(GetPersonsQuery)
export class GetPersonsHandler implements IQueryHandler<GetPersonsQuery> {
  constructor(
    @InjectRepository(Person) private personRepo: Repository<Person>,
  ) {}
  async execute(query: GetPersonsQuery): Promise<Person[]> {
    return await this.personRepo.find();
  }
}

Finally, we need to register this handler in our module:

import { GetPersonsHandler } from './queries/handlers/get-persons.handler';
@Module({
  providers:[GetPersonsHandler]
})
export class PersonModule {}

Implementing Command Handlers

In the CQRS pattern, commands are used to write data to the database. A command handler is responsible for handling a specific type of command.

First, we need to create a command. In this case, our command is to save a person to the database:

export class SavePersonCommand {
  name: string;
  age: number;
}

Then we need to create a handler for this command:

import { CommandHandler, ICommandHandler } from "@nestjs/cqrs";
import { InjectRepository } from "@nestjs/typeorm";
import { Person } from "src/entities/person";
import { Repository } from "typeorm";
import { SavePersonCommand } from "../impl/save-person.command";
@CommandHandler(SavePersonCommand)
export class SavePersonHandler implements ICommandHandler<SavePersonCommand> {
    constructor(
        @InjectRepository(Person) private personRepo: Repository<Person>,
      ) {}
    async execute(command: SavePersonCommand) {
        var person = new Person();
        person.age = command.age;
        person.name = command.name;
        await this.personRepo.insert(person);
    }
}

Finally, we need to register this handler in our module:

import { SavePersonHandler } from './commands/handler/save-person.handler';
@Module({
  providers:[SavePersonHandler]
})
export class PersonModule {}

With the above steps, we have successfully implemented the CQRS pattern in our NestJS application. The GetPersonsHandler handles the read operations, while the SavePersonHandler handles the write operations. Each handler is registered in the PersonModule, making them available for use throughout the application.