Framework bundled with Rails for real-time web applications. Composed of four parts: a WebSocket server (runs inside the Rails process, Rack-compatible), a broadcaster that pushes messages to subscribed streams, a channels framework (controller-like DSL with callbacks), and clients (built-in JavaScript plus third-party). Strengths: zero-setup via `rails new`, no separate Ruby process needed (unlike earlier EventMachine solutions), and a smart JS client that handles reconnect/resubscribe. Weaknesses: WebSocket-only (no fallbacks, problematic for ~3–6% of users on old browsers), high memory usage vs Erlang/Go equivalents (improved somewhat in Ruby 2.5), and broadcast latency that grows quickly past a few thousand subscribers — even with 8 workers on an 8-CPU machine. Can saturate a 16-core c4.4xlarge at moderate load.