🧱 CQRS & Event Sourcing

Welcome to CQRS & Event Sourcing! These are powerful patterns that help you build scalable and maintainable applications. Today, we'll explore how to implement them in NestJS.

💡 What is CQRS?

• CQRS stands for Command Query Responsibility Segregation.
• It separates the application into two parts: one for handling commands (writes) and another for handling queries (reads).
• This separation improves performance, scalability, and maintainability.

// Command handler
@CommandHandler()
export class CreateUserCommandHandler implements ICommandHandler<CreateUserCommand> {
  constructor(private readonly userRepository: UserRepository) {}

  async execute(command: CreateUserCommand): Promise<void> {
    const user = new User(command.username, command.password);
    await this.userRepository.save(user);
  }
}

// Query handler
@QueryHandler()
export class GetUserQueryHandler implements IQueryHandler<GetUserQuery, UserDTO> {
  constructor(private readonly userRepository: UserRepository) {}

  async execute(query: GetUserQuery): Promise<UserDTO> {
    const user = await this.userRepository.findOne(query.id);
    return new UserDTO(user);
  }
}

💡 What is Event Sourcing?

Event Sourcing is a design pattern that records the history of an application's state by storing events. These events can be used to reconstruct the current state of the system.

• Every change to the system is recorded as an event.
• Events are immutable and provide a complete audit trail.
• Event sourcing enables you to rebuild your application's state at any point in time.

// Aggregate root
export class User extends AggregateRoot {
  private _username: string;
  private _password: string;

  constructor(username: string, password: string) {
    super();
    this._username = username;
    this._password = password;
  }

  changePassword(newPassword: string): void {
    const event = new PasswordChangedEvent(this.id, newPassword);
    this.record(event);
  }
}

💡 How to Implement CQRS and Event Sourcing in NestJS?

• Use @nestjs/cqrs package for CQRS implementation.
• Implement command handlers with @CommandHandler() decorator.
• Use query handlers for read operations with @QueryHandler() decorator.
• Store events using Event Store or integrate with external tools like EventStoreDB.

// Command bus configuration
@Module()
export class CqrsModule {
  static forRoot(): DynamicModule {
    return {
      module: CqrsModule,
      imports: [
        CqrsCoreModule.forRoot({
          commandBus: {
            handlers: [CreateUserCommandHandler],
          },
          queryBus: {
            handlers: [GetUserQueryHandler],
          },
        }),
      ],
    };
  }
}

💡 Best Practices for CQRS and Event Sourcing

• Use CQRS only when your application requires high scalability or complex domain logic.
• Keep command handlers simple and focused on a single responsibility.
• Store events in an immutable format for auditing purposes.
• Implement sagas to handle long-running processes.

❌ Common Mistakes to Avoid

• Don't use CQRS for simple CRUD operations.
• Avoid creating overly complex domain models without proper justification.
• Don't mix event sourcing with traditional database patterns unless necessary.

🔌 Plugin Architecture & Multi-Tenancy

Welcome to the world of plugin architecture and multi-tenancy! These advanced techniques allow you to build highly modular, scalable applications that can adapt to various tenants' needs.

💡 Introduction to Plugin Architecture

A plugin architecture enables you to create reusable, dynamic modules that can be registered or unregistered at runtime. This approach is particularly useful for creating flexible systems that support multiple features without tightly coupling them.

• Allows for modular development and deployment.
• Facilitates a microservices-like architecture within a single application.
• Supports dynamic extensions of functionality without restarting the application.

✅ Creating Your First Plugin in NestJS

import { Module } from '@nestjs/common';

@Module({
  providers: [
    {
      provide: 'PLUGIN_SERVICE',
      useValue: () => ({
        greeting: (name: string) => `Hello, ${name}!`,
      }),
    },
  ],
})
export class PluginModule {};

💡 Multi-Tenancy Overview

Multi-tenancy is a key strategy for building SaaS platforms. It allows you to serve multiple tenants from a single application instance while maintaining isolation between them.

• Tenant-based data isolation.
• Shared infrastructure with tenant-specific configurations.
• Scalable architecture for handling varying tenant loads.

✅ Implementing Tenant-Aware Services

import { Injectable, ContextSwitcher } from '@nestjs/core';

@Injectable()
export class UserService {
  constructor(private readonly contextSwitcher: ContextSwitcher) {}

  async getUser(tenantId: string) {
    const tenantContext = await this.contextSwitcher.switchToTenant(tenantId);
    try {
      // Fetch user data specific to the tenant
      return { id: 1, name: 'John Doe', tenantId };
    } finally {
      this.contextSwitcher.restoreContext();
    }
  }
}

💡 Best Practices for Plugin and Multi-Tenancy Architecture

• Keep plugins loosely coupled to avoid dependency conflicts.
• Use tenant-specific contexts to isolate data access layers.
• Implement proper tenant routing strategies to prevent data leakage.
• Consider using schema-based isolation for multi-tenancy in databases.

✅ Real-World Applications of Plugin Architecture and Multi-Tenancy

These techniques are widely used in SaaS platforms, enterprise applications, and systems that require dynamic feature extensions. For example:

• A CRM system with plugins for different payment gateways.
• An e-commerce platform supporting multiple tenants with varying business rules.