CQRS Pattern Flow

This document explains how the CQRS (Command Query Responsibility Segregation) pattern is implemented in MBC CQRS Serverless.

CQRS Overview

Command Flow - Write Path

The flow of write operations.

Command Flow Steps

1. Request Received: Client sends POST/PUT/DELETE request
2. DTO Validation: Controller validates input using class-validator
3. Command Dispatch: Controller creates and dispatches command
4. Business Logic: Command handler executes business rules
5. Persistence: Command service persists to DynamoDB with optimistic locking
6. Event Publishing: Domain events are published to SNS
7. Response: Success response returned to client

Query Flow - Read Path

The flow of read operations.

Query Flow Steps

1. Request Received: Client sends GET request
2. Query Dispatch: Controller creates and dispatches query
3. Data Retrieval: Query handler calls data service
4. Database Query: Data service queries DynamoDB or RDS
5. Response: Data returned to client

Key Components

Command Handler

@CommandHandler(CreateResourceCommand)
export class CreateResourceHandler
  implements ICommandHandler<CreateResourceCommand> {

  constructor(private readonly commandService: CommandService) {}

  async execute(command: CreateResourceCommand): Promise<DataEntity> {
    // 1. Validate business rules
    // 2. Create entity
    // 3. Persist and publish event
    return this.commandService.publish(entity);
  }
}

Query Handler

@QueryHandler(GetResourceQuery)
export class GetResourceHandler
  implements IQueryHandler<GetResourceQuery> {

  constructor(private readonly dataService: DataService) {}

  async execute(query: GetResourceQuery): Promise<DataEntity> {
    return this.dataService.getItem({
      pk: query.pk,
      sk: query.sk,
    });
  }
}

Benefits of CQRS

Adopting the CQRS pattern provides these benefits:

• Scalability: Read and write can be scaled independently
• Optimization: Optimize each side for its specific purpose
• Flexibility: Use different data models for reads and writes
• Performance: Denormalize read models for fast queries
• Auditability: Complete event history for audit trails