Rails on Kino, and the state of Rails-on-Ractors

examples/rails-hello runs edge Rails on Kino.

What works as of Kino 0.1.0: :threaded mode

Rails 8.2.0.alpha boots and serves with mode :threaded (see the example's kino.rb; just bundle exec kino in that directory). Measured on the hello-world (c7a.2xlarge, 8 cores, production mode, 8×5): ~2.6k req/s in 92 MB PSS, single process. The 8-worker Puma cluster reaches ~12.1k by parallelizing across forks, at 389 MB PSS (794 MB RSS, but its forks share the framework copy-on-write, so PSS is the fair figure)—Rails-on-Ractors is interesting precisely because it could offer that ~4.6× parallelism at ~1/4th of the memory.

Pair it with production-style Rails settings: eager load, no code reloading, database pool ≥ workers × threads, logger to stdout or another thread-safe device.

What doesn't (yet): :ractor mode—with the exact blockers

examples/rails-hello/ractor_probe.rb re-tests this against whatever Rails is bundled; when it prints SUCCEEDED and a 200 ractor-mode response (sharing the app is only the first blocker), flip mode :ractor. As of June 2026 (rails main):

  1. Sharing the booted app fails. Ractor.make_shareable(Rails.application) raises Ractor::IsolationError: Proc's self is not shareable at railties/lib/rails/application/finisher.rb:151—a routes-reloader hook lambda Rails registers at boot (in the development path) whose self is the unshareable application instance.
  2. Calling the app from a worker ractor fails even where make_shareable succeeds on a class: Ractor::IsolationError: can not get unshareable values from instance variables of classes/modules from non-main Ractors (@instance from HelloApp)—Rails.application is a class-level ivar read on the dispatch path.
  3. Every workaround is closed by one VM rule (verified on Ruby 4.0.5): class instance variables are main-ractor-only—non-main ractors can neither read them (when the value is unshareable) nor set them, even inside a Ruby::Box (RUBY_BOX=1). Box isolates constant tables, but Ractor isolation still applies to box-local classes:
   Ractor::IsolationError: can not set instance variables of
   classes/modules by non-main Ractors

So booting a private Rails per worker ractor (boxed or not) dies on the first class-ivar write, and sharing one boot dies on reads.