Ports and Adapters for Domain Isolation
Decouple business logic from frameworks by depending on abstract ports instead of concrete infrastructure.
Ports and Adapters for Domain Isolation is a free NestJS Enterprise Backend APIs lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the NestJS Enterprise Backend APIs learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Why Isolate the Domain?
In an enterprise NestJS backend, your most valuable code is the business logic: pricing rules, eligibility checks, workflow state machines. This logic should outlive any specific database, message broker, or HTTP framework.
The Ports and Adapters pattern (also called Hexagonal Architecture) achieves this by inverting dependencies. The domain defines abstract ports (interfaces) describing what it needs. Concrete adapters (Postgres, Redis, Stripe) implement those ports and plug in from the outside.
- The domain depends on nothing external.
- Infrastructure depends on the domain, never the reverse.
The Dependency Rule
The core invariant is a one-way dependency arrow: infrastructure → application → domain. Source-code imports may only point inward.
A domain entity must never import from @nestjs/common, TypeORM, or an HTTP client. If it does, you have coupled your rules to a framework's release cycle and made them hard to test in isolation.
Ports live inside the boundary; adapters live outside it. The interface (the port) is owned by the domain, so the domain dictates the contract and the outside world conforms to it.
Defining a Driven Port
A driven port (a.k.a. secondary port) is an outbound dependency the domain needs but does not own. Example: persisting an order. The application layer declares an interface describing exactly what it requires, in domain terms.
Notice this file imports nothing from NestJS or a database driver. It is pure TypeScript describing a capability.
export interface Order {
id: string;
customerId: string;
totalCents: number;
status: 'PENDING' | 'PAID' | 'CANCELLED';
}
// Driven (outbound) port: owned by the application/domain layer.
export interface OrderRepositoryPort {
save(order: Order): Promise<void>;
findById(id: string): Promise<Order | null>;
}A Driving Port: The Use Case
A driving port (primary port) is the entry point into the application, called by the outside world (controllers, CLI, message consumers). It is typically a use-case interface.
The use case orchestrates domain logic and talks to driven ports only through their interfaces. It never knows whether persistence is Postgres or an in-memory map.
export interface PlaceOrderCommand {
orderId: string;
customerId: string;
totalCents: number;
}
// Driving (inbound) port: the application's public contract.
export interface PlaceOrderPort {
execute(cmd: PlaceOrderCommand): Promise<void>;
}Implementing the Use Case
The use case implements the driving port and depends on driven ports by their interface type. Here it validates a business rule, builds a domain entity, and delegates persistence to the port.
This class is framework-agnostic. We add NestJS's @Injectable() only as a thin decoration so the DI container can manage it; the logic itself does not depend on Nest at all.
import { Injectable, Inject } from '@nestjs/common';
import { PlaceOrderPort, PlaceOrderCommand } from './place-order.port';
import { OrderRepositoryPort } from './order-repository.port';
import { ORDER_REPOSITORY } from './tokens';
@Injectable()
export class PlaceOrderUseCase implements PlaceOrderPort {
constructor(
@Inject(ORDER_REPOSITORY)
private readonly orders: OrderRepositoryPort,
) {}
async execute(cmd: PlaceOrderCommand): Promise<void> {
if (cmd.totalCents <= 0) {
throw new Error('Order total must be positive');
}
await this.orders.save({
id: cmd.orderId,
customerId: cmd.customerId,
totalCents: cmd.totalCents,
status: 'PENDING',
});
}
}Injection Tokens for Interfaces
TypeScript interfaces vanish at runtime, so NestJS cannot use them as DI keys directly. The idiom is to define a string or symbol token and bind the port to a concrete adapter in the module.
Always reference the token with @Inject(TOKEN) at injection sites. Symbols avoid accidental string collisions across modules.
export const ORDER_REPOSITORY = Symbol('OrderRepositoryPort');
export const PLACE_ORDER = Symbol('PlaceOrderPort');
// Usage at an injection site:
// constructor(@Inject(ORDER_REPOSITORY) repo: OrderRepositoryPort) {}Writing a Driven Adapter
An adapter implements a driven port using concrete infrastructure. Here a TypeORM adapter satisfies OrderRepositoryPort. It maps between the persistence model and the domain Order, keeping the database schema out of the domain.
The domain never sees OrderEntity or the repository — only the port. Swap to MongoDB tomorrow by writing a new adapter; the use case stays untouched.
import { Injectable } from '@nestjs/common';
import { InjectRepository } from '@nestjs/typeorm';
import { Repository } from 'typeorm';
import { OrderRepositoryPort, Order } from '../application/order-repository.port';
import { OrderEntity } from './order.entity';
@Injectable()
export class TypeOrmOrderRepository implements OrderRepositoryPort {
constructor(
@InjectRepository(OrderEntity)
private readonly repo: Repository<OrderEntity>,
) {}
async save(order: Order): Promise<void> {
await this.repo.save(this.repo.create(order));
}
async findById(id: string): Promise<Order | null> {
const row = await this.repo.findOne({ where: { id } });
return row ? { ...row } : null;
}
}Wiring Ports to Adapters in a Module
The NestJS module is the composition root where ports meet adapters. Use provide with the token and useClass with the concrete adapter. This is the only place that knows both sides.
Because binding happens here, the entire domain remains ignorant of TypeORM. Tests can supply a different binding without modifying any business code.
import { Module } from '@nestjs/common';
import { TypeOrmModule } from '@nestjs/typeorm';
import { OrderEntity } from './infrastructure/order.entity';
import { TypeOrmOrderRepository } from './infrastructure/typeorm-order.repository';
import { PlaceOrderUseCase } from './application/place-order.usecase';
import { ORDER_REPOSITORY, PLACE_ORDER } from './application/tokens';
@Module({
imports: [TypeOrmModule.forFeature([OrderEntity])],
providers: [
{ provide: ORDER_REPOSITORY, useClass: TypeOrmOrderRepository },
{ provide: PLACE_ORDER, useClass: PlaceOrderUseCase },
],
exports: [PLACE_ORDER],
})
export class OrdersModule {}The Controller is Just Another Adapter
An HTTP controller is a driving adapter: it translates a transport-specific request into a call on the driving port. It holds no business logic — just mapping and delegation.
Replacing REST with gRPC or a Kafka consumer means writing a new driving adapter against the same PlaceOrderPort. The use case never changes.
import { Body, Controller, Inject, Post } from '@nestjs/common';
import { PlaceOrderPort } from '../application/place-order.port';
import { PLACE_ORDER } from '../application/tokens';
@Controller('orders')
export class OrdersController {
constructor(
@Inject(PLACE_ORDER) private readonly placeOrder: PlaceOrderPort,
) {}
@Post()
async create(@Body() body: { orderId: string; customerId: string; totalCents: number }) {
await this.placeOrder.execute(body);
return { status: 'accepted' };
}
}Testing With an In-Memory Adapter
The biggest payoff is testability. Because the use case depends on a port, you test it with a trivial in-memory adapter — no database, no Nest container, no mocking framework.
This snippet is a complete standalone program: it defines the port, a fake adapter, the use case, and runs an assertion. It demonstrates that the domain logic is fully exercisable in isolation.
interface Order {
id: string;
customerId: string;
totalCents: number;
status: 'PENDING' | 'PAID' | 'CANCELLED';
}
interface OrderRepositoryPort {
save(order: Order): Promise<void>;
findById(id: string): Promise<Order | null>;
}
class InMemoryOrderRepo implements OrderRepositoryPort {
private store = new Map<string, Order>();
async save(o: Order) { this.store.set(o.id, o); }
async findById(id: string) { return this.store.get(id) ?? null; }
}
class PlaceOrderUseCase {
constructor(private readonly orders: OrderRepositoryPort) {}
async execute(cmd: { orderId: string; customerId: string; totalCents: number }) {
if (cmd.totalCents <= 0) throw new Error('Order total must be positive');
await this.orders.save({ ...cmd, id: cmd.orderId, status: 'PENDING' });
}
}
async function main() {
const repo = new InMemoryOrderRepo();
const useCase = new PlaceOrderUseCase(repo);
await useCase.execute({ orderId: 'o1', customerId: 'c1', totalCents: 4200 });
const saved = await repo.findById('o1');
console.log(saved?.status === 'PENDING' ? 'PASS' : 'FAIL');
}
main();Anti-Corruption at the Boundary
Adapters double as an anti-corruption layer. External shapes — a Stripe webhook payload, a third-party DTO — must be translated into clean domain types inside the adapter, never leaked inward.
- Keep mapping logic in the adapter, not the use case.
- Never let a vendor's enum or snake_case field reach a domain entity.
- Validate and normalize at the edge so the core trusts its inputs.
This discipline is what keeps the hexagon's interior stable while the messy outside world churns.
import { PaymentPort } from '../application/payment.port';
// Adapter translates a vendor payload into a domain-friendly result.
export class StripePaymentAdapter implements PaymentPort {
async charge(customerId: string, amountCents: number): Promise<{ paid: boolean }> {
const vendorResp = await this.callStripe(customerId, amountCents);
// Map vendor shape -> domain shape (anti-corruption).
return { paid: vendorResp.status === 'succeeded' };
}
private async callStripe(_c: string, _a: number) {
return { status: 'succeeded' as const };
}
}Quick Check
Consider a NestJS use case that must persist data. Where should the persistence interface (the port) be defined, and who implements it?
Recap
You isolated the domain from frameworks using Ports and Adapters:
- Ports are interfaces owned by the domain/application layer — driving (inbound use cases) and driven (outbound dependencies like repositories).
- Adapters live outside the boundary and implement driven ports or call driving ports.
- The dependency rule forces all imports to point inward; the domain depends on nothing external.
- In NestJS, bind ports to adapters in the module using injection tokens (symbols), since interfaces disappear at runtime.
- The payoff: swap infrastructure freely and test business logic in isolation with simple in-memory adapters.
Frequently asked questions
Is the “Ports and Adapters for Domain Isolation” lesson free?
Yes — the full text of “Ports and Adapters for Domain Isolation” is free to read here on the web, and the NestJS Enterprise Backend APIs course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the NestJS Enterprise Backend APIs course, upgrade to CoddyKit PRO.
What will I learn in “Ports and Adapters for Domain Isolation”?
Decouple business logic from frameworks by depending on abstract ports instead of concrete infrastructure. You practise NestJS Enterprise Backend APIs with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start NestJS Enterprise Backend APIs?
No prior experience is required. NestJS Enterprise Backend APIs on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Ports and Adapters for Domain Isolation” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this NestJS Enterprise Backend APIs lesson?
Yes. Every NestJS Enterprise Backend APIs lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Ports and Adapters for Domain Isolation
- Dynamic Provider Registration with DiscoveryService
- Lazy-Loaded Modules and Feature Toggles
- Extension Points with the Module Reference API