Implements a cache fetcher with CAS (compare-and-swap) operations and optional fill locks to prevent thundering herd problems. It provides fetch, fetch_multi, write, delete, and clear methods, with a sophisticated lock mechanism that allows one client to fill the cache while others wait or fall back to a versioned key. This component is self-contained and could be extracted as a generic cache client with fill-lock support.

A generic cache key loader that supports single and batch loading of cache entries with a pluggable cache fetcher interface. It handles cache key generation, encoding/decoding, and batch miss resolution. The load_batch method efficiently groups cache keys by fetcher and resolves misses in bulk. This module is reusable for any caching scenario requiring batch loading with custom fetchers.

Defines eager and lazy load strategies for batch cache loading. The Eager module loads all requested keys immediately, while the Lazy module defers loading until load_now is called, allowing multiple load requests to be batched together. This pattern is useful for prefetching associations and can be extracted as a generic batching mechanism for cache or database loads.