Newer Ruby concurrency framework based on fibers. Not available when David Halasz started his VNC proxy project; he notes he would probably use it today. Adam Okoń recommends async at wroclove.rb 2026 for LLM-heavy IO workloads: switching Active Job's adapter to async lets Rails jobs run on fibers, and the async gem exposes a barrier abstraction that schedules concurrent IO tasks until the system reaches ~80% load and cancels unbounded tasks past that. It also supports manual concurrency management via a semaphore for throttling outbound service calls. In Adam's production case async cut a workflow from above 50 seconds to 15–20 seconds.