Layered & hexagonal architecture

As a system grows, the thing that decides whether it stays changeable is where the boundaries are. This pillar is about shaping code into services — and it starts with the oldest lever there is: depend on interfaces, not concretes. This module has you refactor a tangled module into ports & adapters.

The move, in shape: the core stops constructing its dependency and takes it by injection.

from typing import Protocol

class Notifier(Protocol):          # the port: what, not how
    def send(self, message: str) -> None: ...

class OrderService:                # the core depends on the port
    def __init__(self, notifier: Notifier):
        self._notifier = notifier  # injected adapter

With clean boundaries in place, the next module puts a boundary on the outside of the system — an API other code calls.