Implements a multi-layer caching system with Redis, APCu, and filesystem backends. Features intelligent key formatting, automatic serialization/deserialization, cache statistics tracking, and specialized handling for different data types (auth tokens, progress indicators). Demonstrates skill in gracefully handling backend failures and providing a unified caching interface.

Implements a factory pattern that attempts to generate summaries using multiple AI providers (Ollama, Google Gemini, OpenAI, etc.) in order of preference, falling back to the next available provider if one fails. Provides a standardized interface for summarization and encapsulates provider-specific error handling. Shows good use of polymorphism and error resilience.

Implements a secure Server-Sent Events endpoint for real-time progress tracking with referer-based validation, IP-based rate limiting, and proper SSE headers. Includes timeout handling, progress caching, and cleanup mechanisms. Demonstrates thoughtful security considerations and robust real-time communication patterns.