NestJS Enterprise Backend APIs · Lesson

Commands, Handlers, and the Command Bus

Model write operations as commands dispatched through CommandBus to dedicated handlers.

Why Commands Exist

In a CQRS system we split the model into two halves: the write side that changes state and the read side that returns data. A Command represents an intent to change state, for example CreateOrder or CancelSubscription.

A command is a plain DTO carrying the data needed to perform the write.
It is named in the imperative (do this) and describes a single business action.
It returns at most an identifier or acknowledgment, never a full read model.

NestJS ships @nestjs/cqrs, which gives us a CommandBus to dispatch commands to exactly one handler.

Modeling a Command

A command is just a class. It holds the input data as readonly properties so it stays immutable once created. There is no logic inside it.

Below, CreateOrderCommand captures everything a handler needs to create an order. Notice it carries no NestJS decorators and no behavior.

export class CreateOrderCommand {
  constructor(
    public readonly customerId: string,
    public readonly items: { sku: string; quantity: number }[],
    public readonly currency: string,
  ) {}
}

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