Adapter

An adapter is a port with its methods filled in. You pass the port class to Adapter and implement every abstract method. start and stop are already there for connecting and disconnecting.

class InMemoryPosts extends Adapter(PostRepository) {
  override async get(id: string): Promise<Post> { /* … */ }
  override async save(post: Post): Promise<void> { /* … */ }
}

Why it exists

The use case asks a PostRepository to save. Something has to actually keep the post: a Map while you are sketching the feature, a database when the API runs, a fake in a test that records the calls. Those are adapters of one port. Swapping them changes the constructor arguments at the composition root, and execute stays put.

start and stop give a connection a home that is still outside the use case. The process opens the database before it serves traffic, and closes it when it shuts down.

How it’s used

An in-memory repository is enough to walk the publish story. The API process can later substitute a database class that extends the same port. The composition root constructs the adapter, starts it, and hands it to PublishPost.

class InMemoryPosts extends Adapter(PostRepository) {
  private readonly rows = new Map<string, Post>()

  override async get(id: string): Promise<Post> {
    const post = this.rows.get(id)
    if (post === undefined) throw BlogError.PostMissing()
    return post
  }

  override async save(post: Post): Promise<void> {
    this.rows.set(post.props.id, post)
  }
}

const posts = new InMemoryPosts()
await posts.start()
const publishPost = new PublishPost(posts, events)

The defaults of start and stop resolve immediately, so this adapter can leave them alone. A database adapter overrides start to connect and stop to disconnect. The application calls them. The use case calls get and save.

API

Adapter(PortClass) returns an abstract class that extends that port. Implement the port’s methods. The compiler reports a missing one on the class.

Member	Role
token	The port’s token. An adapter keeps it.
Port methods	Implement each abstract method from the port.
start()	Called by the application before the adapter is used. Override it to connect. The default resolves immediately.
stop()	Called by the application when it is finished with the adapter. Override it to disconnect. The default resolves immediately.

The token is fixed by the port. InMemoryPosts.token is PostRepository.token.