Souin is a distributed HTTP caching middleware library written in Go, designed to be embedded into reverse proxies and web frameworks. It implements RFC 7234 (HTTP caching) with support for stale-while-revalidate, request coalescing, Vary, and ETag validation. The system provides a surrogate-key based cache invalidation mechanism (IETF draft) that allows tag-based purging, with built-in providers for Akamai, Cloudflare, and Fastly CDNs. It exposes a REST API for cache management (list, purge, flush) and Prometheus metrics for monitoring. The architecture is plugin-based: a core caching engine in `pkg/middleware/` is wrapped by 17+ framework-specific adapters (Caddy, Traefik, Tyk, Gin, Echo, Fiber, Chi, etc.) and a standalone reverse-proxy server. Each plugin translates framework-specific request/response objects into a common context, invokes the core `SouinBaseHandler.ServeHTTP`, and writes cached responses via a `CustomWriter` that supports range requests. Storage is abstracted behind a `Storer` interface with implementations for Redis, Badger, NutsDB, Olric, Etcd, NATS, Otter, SimpleFS, and an in-memory default with LZ4 compression. Configuration is parsed from YAML, JSON, or framework-native formats (Caddyfile, Traefik YAML) into shared data structures. Key data flows: an incoming request is first checked against the cache (fresh/stale). On a miss, the upstream is called, the response is stored with its cache key (computed from method, URI, headers, body, and GraphQL query), and the response is served. Surrogate keys are extracted from `Surrogate-Key` headers and stored for tag-based invalidation. The system also supports GraphQL mutation detection for automatic cache invalidation. External dependencies include the storage backends (Redis, etcd, etc.) and CDN APIs for purge propagation. The codebase demonstrates strong engineering practices: a clean separation of concerns between caching logic, storage abstraction, RFC compliance utilities, and plugin adapters. The RFC modules (`pkg/rfc/`) are self-contained implementations of HTTP caching standards (RFC 9111 age calculation, RFC 9211 Cache-Status header, RFC 8941 structured fields) that could be extracted and reused in other HTTP tools. The surrogate key system and storage interface provide extensible patterns for custom invalidation strategies and backends.