README.md

@@ -12,6 +12,8 @@ Subclass an adapter in your own repository to add domain-specific methods while
 | **Adapters**     | Technology-specific base classes (`RedisAdapter`, `MinioAdapter`) |
 | **Your project** | Subclass an adapter and add domain methods                        |
 
+Each public interface is a `@runtime_checkable` `Protocol` with `@abstractmethod` members. **Subclass an adapter** when you need connection management and shared behavior — explicit subclasses get runtime instantiation guards and inherited default methods (e.g. `scan_keys`). **Type-annotate against an interface** when you want loose coupling — any object with the right methods satisfies the contract for mypy and `isinstance()` checks, without inheriting from this package.
+
 Connection adapters expose `connect()`, `disconnect()`, and `is_connected()`. The latter verifies backend reachability with a cached health probe (default TTL: 1 second). Subclasses may override `health_check_ttl_seconds`. `connect()` is idempotent: calling it while already connected and healthy is a no-op.
 
 ## Future direction

python_repositories/adapters/minio_adapter.py

@@ -19,7 +19,7 @@ except ImportError as exc:
     ) from exc
 
 
-class MinioAdapter(ObjectRepositoryInterface, ConnectionAwareAdapter):
+class MinioAdapter(ConnectionAwareAdapter, ObjectRepositoryInterface):
     """Minio adapter exposing basic CRUD functionality."""
 
     endpoint_env_var_name: str = "MINIO_ENDPOINT"

python_repositories/adapters/redis_adapter.py

@@ -21,7 +21,7 @@ except ImportError as exc:
     ) from exc
 
 
-class RedisAdapter(JsonRepositoryInterface, ConnectionAwareAdapter):
+class RedisAdapter(ConnectionAwareAdapter, JsonRepositoryInterface):
     """Redis adapter exposing basic CRUD functionality."""
 
     uri_env_var_name: str = "REDIS_URI"

python_repositories/interfaces/connection_aware_interface.py

@@ -1,9 +1,11 @@
-"""Definition of ConnectionAwareInterface abstract base class."""
+"""Definition of ConnectionAwareInterface protocol and abstract base class."""
 
-from abc import ABC, abstractmethod
+from abc import abstractmethod
+from typing import Protocol, runtime_checkable
 
 
-class ConnectionAwareInterface(ABC):
+@runtime_checkable
+class ConnectionAwareInterface(Protocol):
     """Interface that defines connection-related methods."""
 
     @abstractmethod

python_repositories/interfaces/context_aware_interface.py

@@ -1,12 +1,13 @@
-"""Definition of ContextAwareInterface abstract base class."""
+"""Definition of ContextAwareInterface protocol and abstract base class."""
 
 from __future__ import annotations
 
-from abc import ABC, abstractmethod
-from typing import Self
+from abc import abstractmethod
+from typing import Protocol, Self, runtime_checkable
 
 
-class ContextAwareInterface(ABC):
+@runtime_checkable
+class ContextAwareInterface(Protocol):
     """Interface that defines context-related methods."""
 
     @abstractmethod

python_repositories/interfaces/json_repository_interface.py

@@ -1,11 +1,12 @@
-"""Definition of JsonRepositoryInterface abstract base class."""
+"""Definition of JsonRepositoryInterface protocol and abstract base class."""
 
-from abc import ABC, abstractmethod
+from abc import abstractmethod
 from collections.abc import Iterator
-from typing import Any
+from typing import Any, Protocol, runtime_checkable
 
 
-class JsonRepositoryInterface(ABC):
+@runtime_checkable
+class JsonRepositoryInterface(Protocol):
     """Interface that defines JSON document CRUD methods."""
 
     @abstractmethod

python_repositories/interfaces/object_repository_interface.py

@@ -1,10 +1,12 @@
-"""Definition of ObjectRepositoryInterface abstract base class."""
+"""Definition of ObjectRepositoryInterface protocol and abstract base class."""
 
-from abc import ABC, abstractmethod
+from abc import abstractmethod
 from io import BytesIO
+from typing import Protocol, runtime_checkable
 
 
-class ObjectRepositoryInterface(ABC):
+@runtime_checkable
+class ObjectRepositoryInterface(Protocol):
     """Interface that defines binary object CRUD methods."""
 
     @abstractmethod

tests/unit/structural_typing_test.py

@@ -0,0 +1,134 @@
+"""Unit tests for structural typing of public interfaces."""
+
+from __future__ import annotations
+
+from collections.abc import Iterator
+from io import BytesIO
+from typing import Any
+
+from python_repositories.interfaces import (
+    ConnectionAwareInterface,
+    ContextAwareInterface,
+    JsonRepositoryInterface,
+    ObjectRepositoryInterface,
+)
+
+
+class InMemoryJsonRepo:
+    """Plain class that satisfies JsonRepositoryInterface without inheritance."""
+
+    def get(self, key: str) -> dict[str, Any] | None:
+        return None
+
+    def set(self, key: str, data: dict[str, Any]) -> None:
+        pass
+
+    def delete(self, key: str) -> None:
+        pass
+
+    def list_keys(self, pattern: str) -> list[str]:
+        return []
+
+    def scan_keys(
+        self,
+        pattern: str,
+        *,
+        count: int | None = None,
+    ) -> Iterator[str]:
+        del count
+        yield from self.list_keys(pattern)
+
+
+class InMemoryObjectRepo:
+    """Plain class that satisfies ObjectRepositoryInterface without inheritance."""
+
+    def get(self, object_name: str) -> BytesIO | None:
+        return None
+
+    def put(
+        self,
+        object_name: str,
+        data: BytesIO,
+        content_type: str = "application/octet-stream",
+    ) -> None:
+        pass
+
+    def delete(self, object_name: str) -> None:
+        pass
+
+    def list_objects(self, prefix: str = "") -> list[str]:
+        return []
+
+
+class FakeConnection:
+    """Plain class that satisfies ConnectionAwareInterface without inheritance."""
+
+    def connect(self) -> None:
+        pass
+
+    def disconnect(self) -> None:
+        pass
+
+    def is_connected(self) -> bool:
+        return True
+
+
+class FakeContextManager:
+    """Plain class that satisfies ContextAwareInterface without inheritance."""
+
+    def __enter__(self) -> FakeContextManager:
+        return self
+
+    def __exit__(
+        self, exc_type: type | None, exc_val: object | None, exc_tb: object | None
+    ) -> None:
+        pass
+
+
+def accepts_json_repo(repo: JsonRepositoryInterface) -> None:
+    """Type-checking hook for JsonRepositoryInterface structural subtyping."""
+    repo.get("key")
+
+
+def accepts_object_repo(repo: ObjectRepositoryInterface) -> None:
+    """Type-checking hook for ObjectRepositoryInterface structural subtyping."""
+    repo.get("object")
+
+
+def accepts_connection_aware(connection: ConnectionAwareInterface) -> None:
+    """Type-checking hook for ConnectionAwareInterface structural subtyping."""
+    connection.is_connected()
+
+
+def accepts_context_aware(context: ContextAwareInterface) -> None:
+    """Type-checking hook for ContextAwareInterface structural subtyping."""
+    with context:
+        pass
+
+
+def test_json_repository_structural_subtyping() -> None:
+    """Test that a plain class satisfies JsonRepositoryInterface structurally."""
+    repo: JsonRepositoryInterface = InMemoryJsonRepo()
+    accepts_json_repo(repo)
+    assert isinstance(repo, JsonRepositoryInterface)
+
+
+def test_object_repository_structural_subtyping() -> None:
+    """Test that a plain class satisfies ObjectRepositoryInterface structurally."""
+    repo: ObjectRepositoryInterface = InMemoryObjectRepo()
+    accepts_object_repo(repo)
+    assert isinstance(repo, ObjectRepositoryInterface)
+
+
+def test_connection_aware_structural_subtyping() -> None:
+    """Test that a plain class satisfies ConnectionAwareInterface structurally."""
+    connection: ConnectionAwareInterface = FakeConnection()
+    accepts_connection_aware(connection)
+    assert isinstance(connection, ConnectionAwareInterface)
+
+
+def test_context_aware_structural_subtyping() -> None:
+    """Test that a plain class satisfies ContextAwareInterface structurally."""
+    context: ContextAwareInterface = FakeContextManager()
+    accepts_context_aware(context)
+    assert isinstance(context, ContextAwareInterface)