Implement and add examples for Adapters and Providers from Infrastructure folder? about domain-driven-hexagon

[Sairyss]

For example, If you need to get a user, you can create a port:

interface GetUserPort {
  getUser(id: string): Promise<User>
}

And adapter (repository in this case is an adapter for the database):

class UserRepository implements GetUserPort {
  async getUser(id: string): Promise<User> {
    const user = await this.database.query('SELECT * FROM users WHERE id = $1', [id]);
    return user;
}

If you need to get a user from an external api, the principle is the same:

class UserAdapter implements GetUserPort {
  async getUser(id: string): Promise<User> {
    const user = await fetch('http://some-external-api.com', { userId: id });
    return user;
}

In the service where you need to use this adapter you would do something like this:

export class UserService {
  constructor(
    @Inject(UserAdapter) // <- inject adapter
    private user: GetUserPort; // <- service itself depends on a port
  ) {}

  async doSomething(id: string) {
     const user = await this.user.getUser(id);
     // ...
  }

[WildEgor]

It's ok for external DB or HTTP API, but what about calling other microservices (Kafka, Rabbit)? Because I saw "microservice adapters" part of the diagram.

[Sairyss]

Same principles can be applied for any out-of-process communications. Your domain doesn't need to know how your microservices communicate: via HTTP, RabbitMQ, Kafka, etc. Domain doesn't care about that details, it just calls a port to do something, and adapter gets care of that. This is simple abstraction principle.

For example, using RabbitMQ RPC

class UserAdapter implements GetUserPort {
  async getUser(id: string): Promise<User> {
    await this.rabbitMQ.sendToQueue('GET_USER', { userId: id }, {
      replyTo: queue
    });
    const user = await channel.consume(queue, /* ... listen for RPC response */)
    return user;
}

[WildEgor]

@Sairyss thanks, great project!

[ErlanBazarov]

What if some part of user is stored in database and other part is stored in 3rd party storage? How can we get whole user from the application service?

[Sairyss]

What if some part of user is stored in database and other part is stored in 3rd party storage? How can we get whole user from the application service?

@ErlanBazarov You can combine a call to your db and a call to external service into a single method getUser, but I don't see many reasons to do that. 3rd party storage is not something you can control, what if it becomes unaccessible for any reason? All calls to getUser will fail. Or what if it goes out of business? You will lose your users data.

What you usually should do in that case is create a copy of that data stored in 3rd party service into your own database, storage is cheap nowadays. If data gets updated frequently in the 3rd party storage, you can use periodic jobs to keep it fresh in your db. This way you will not depend that much on a 3rd party.