How was the contract between internal services and the adapter arranged?

The contract between internal services and the adapter is usually built on a clearly defined interface — a set of methods and data formats that the services are obliged to support. This can be a REST API with endpoint descriptions and a JSON schema, gRPC with protobuf messages, or an internal communication protocol.

For example, the adapter might expect the service to provide a method get_user(id: int) -> User, where User is an object with specific fields. It is important that both parties agree on the versions and data formats to avoid incompatibilities.

In Python, this might look like:

class UserServiceAdapter:
    def get_user(self, user_id: int) -> dict:
        # Call the internal service and transform the response into the expected format
        response = internal_service.fetch_user(user_id)
        return {
            "id": response.id,
            "name": response.name,
            "email": response.email
        }

Thus, the contract is an agreement on what data and methods are available, and in what format they are transmitted.