Provides a generic provider selection mechanism that allows the application to swap implementations for CDN, email, media storage, search, image resizing, authentication, and support ticketing without altering business logic. Providers are registered via setuptools entry points and selected at runtime using a base class with abstract methods. This pattern is cleanly isolated, documented, and could be extracted as a standalone library for any Python application needing pluggable backend services.

Implements a multi-tier cache chain that combines local memory, memcached, a hard cache, and a persistent permacache (Cassandra). The key innovation is the `StaleCacheChain` which serves stale data while asynchronously refreshing the cache, and a `TransitionalCache` that handles gradual migration between cache backends. This pattern is well-tested in production for handling millions of requests and could be extracted as a general-purpose caching library.

Defines the `FeatureState` class that manages A/B test and feature flag state loaded from live configuration (ZooKeeper). It supports user bucketing (logged-in/out), page experiments, multi-variant experiments with deterministic bucket assignment, and eventing for analytics. The design decouples experiment logic from request handling via a `World` proxy. This component is self-contained (only depends on a few internal utilities for hashing) and could be extracted as a reusable feature flag library for Python web applications.