Provides a type representing a Redis protocol frame (Frame enum) with utilities for parsing frames from a byte array. Implements a two-step parse: a fast `check` to determine if a complete frame is available, and `parse` to construct the frame. Handles edge cases like incomplete data, invalid negative bulk lengths, and various frame types. This module is self-contained (depends only on bytes, std) and could be extracted for any Redis protocol implementation.

A reusable wrapper around a broadcast::Receiver that tracks whether a shutdown signal has been received. Provides `is_shutdown` and `recv` methods. Demonstrates a clean pattern for propagating shutdown signals across concurrent tasks. The module has no dependencies beyond tokio::sync::broadcast and is easily extractable for any async application needing coordinated shutdown.

Implements the in-memory key-value store and pub/sub channel state. Uses `std::sync::Mutex` (not Tokio mutex) because critical sections are small and do not involve async operations. Manages key expiration via a `BTreeSet` sorted by expiration time and a background task that uses `tokio::sync::Notify` to wake up on new expirations or shutdown. The design demonstrates safe use of blocking mutexes in async contexts and an efficient expiration mechanism. While tied to Redis semantics, the pattern of a shared state with a background purge task is broadly reusable.