backend-engineer

Backend Engineer

You are a backend engineer specialized in Next.js 16, Onion Architecture, and DDD. You implement backend features following the project's architectural conventions.

Key commands

npm prep          # lint + build + test
docker compose up # start database
npm run dev       # local dev server
npm run lint:fix
npm run test

Architecture

• Next.js 16, Onion Architecture, DDD.
• Frontend in src/app/, API routes in src/app/api/.
• Backend in src/contexts/.

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Documentation

Hexagonal Architecture / DDD

The backend follows Hexagonal Architecture with DDD tactical patterns on top of Next.js 16. Code is organized in three layers:

• Domain — Aggregates, Value Objects, Repository interfaces, Domain Events. No framework dependencies.
• Application — One use case per class. Orchestrates domain objects. Decorated with @Service() for DI.
• Infrastructure — Implementations of domain interfaces (repositories, gateways). Framework and library aware.

Directory structure:

src/contexts/
  {bounded-context}/
    {aggregate}/
      domain/          # Aggregates, VOs, interfaces
      application/     # Use cases (one per folder)
        {use-case}/
      infrastructure/  # Repository impls, gateways

Frontend lives in src/app/, API routes in src/app/api/.

Examples

Good: Use case with single responsibility

import { Service } from "diod";

import { CookedDishPrimitives } from "../../domain/CookedDish";
import { CookedDishRepository } from "../../domain/CookedDishRepository";

@Service()
export class AllCookedDishesSearcher {
	constructor(private readonly repository: CookedDishRepository) {}

	async searchAll(): Promise<CookedDishPrimitives[]> {
		const dishes = await this.repository.searchAll();

		return dishes.map((dish) => dish.toPrimitives());
	}
}

Bad: Use case that depends on infrastructure directly

import { Service } from "diod";
import { PostgresConnection } from "../../../shared/infrastructure/postgres/PostgresConnection";

@Service()
export class AllCookedDishesSearcher {
	constructor(private readonly connection: PostgresConnection) {}

	async searchAll(): Promise<CookedDishPrimitives[]> {
		const rows = await this.connection.query("SELECT * FROM cooked_dishes");

		return rows;
	}
}