Iodine is a high-performance Ruby web server and application framework built on the facil.io C library. It provides a Rack/NeoRack compatible server with WebSocket, Server-Sent Events, Pub/Sub, static file serving, TLS termination, and response compression. The framework also includes a rich set of utilities: modern cryptographic primitives (ChaCha20-Poly1305, Ed25519, X25519, post-quantum X25519MLKEM768, Argon2, etc.), JSON parsing, Mustache templating, compression (Deflate, Gzip, Brotli), and a RESP3 Redis client for scalable Pub/Sub. Architecture: Multi-process master-worker model. The master process monitors workers and handles Pub/Sub message routing. Each worker runs a C reactor thread for I/O and a Ruby thread pool for application code, achieving GVL-free concurrency. Configuration is via CLI flags, environment variables, or config.ru/config.nru files. Supports hot restarts (SIGUSR1) and cluster mode (multi-process with fork). Key entry points: exe/iodine (main executable), lib/iodine.rb (Ruby-side boot), and the C extension ext/iodine/iodine.c. The Iodine::Connection class handles HTTP/WebSocket/SSE/TCP connections. Iodine::Listener manages server sockets and URL routing. Iodine::PubSub provides pluggable engines (LOCAL, CLUSTER, RESP3) with history replay. Iodine::Base::Crypto and Iodine::Base::Compression offer self-contained utility modules. External dependencies: facil.io C library (bundled), optional OpenSSL for TLS, optional Brotli for compression. The gem is self-contained but requires a C compiler. The codebase includes a comprehensive test suite and benchmarks. Caveats: embedded TLS 1.3 is unaudited, Windows support is experimental and single-process only, and HTTP/2 is not supported. The project is actively maintained with recent commits and support for Ruby 3.2–4.0.