content: #  Copyright (c) ZenML GmbH 2025. All Rights Reserved.
  #
  #  Licensed under the Apache License, Version 2.0 (the "License");
  #  you may not use this file except in compliance with the License.
  #  You may obtain a copy of the License at:
  #
  #       https://www.apache.org/licenses/LICENSE-2.0
  #
  #  Unless required by applicable law or agreed to in writing, software
  #  distributed under the License is distributed on an "AS IS" BASIS,
  #  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
  #  or implied. See the License for the specific language governing
  #  permissions and limitations under the License.
  """Base app extension interface."""
  
  from abc import ABC, abstractmethod
  from typing import TYPE_CHECKING
  
  if TYPE_CHECKING:
      from zenml.deployers.server.app import BaseDeploymentAppRunner
  
  
  class BaseAppExtension(ABC):
      """Abstract base for app extensions.
  
      Extensions provide advanced framework-specific capabilities like:
      - Custom authentication/authorization
      - Observability (logging, tracing, metrics)
      - Complex routers with framework-specific features
      - OpenAPI customizations
      - Advanced middleware patterns
  
      Subclasses must implement install() to modify the app.
      """
  
      @abstractmethod
      def install(
          self,
          app_runner: "BaseDeploymentAppRunner",
      ) -> None:
          """Install extension into the application.
  
          Args:
              app_runner: The deployment app runner instance being used to build
                  and run the web application.
  
          Raises:
              RuntimeError: If installation fails.
          """
  