Compatibility

This guide gives an overview of the compatibility of Async with Ruby and other frameworks.

Ruby

Async has two main branches, stable-v1 and main.

Stable V1

The stable-v1 branch of async is compatible with Ruby 2.5+ & TruffleRuby, and partially compatible with JRuby.

Because it was designed with the interfaces available in Ruby 2.x, the following limitations apply:

class Async::Task implements context switching using Fiber.yield and Fiber.resume. This means that module Async may not be compatible with code which uses fibers for flow control, e.g. Enumerator.
class Async::Reactor is unable to intercept blocking operations with native interfaces. You need to use the wrappers provided by Async::IO.
DNS resolution is blocking.

Main

The main branch of async is compatible with Ruby 3.0.2+, and partially compatible with TruffleRuby. JRuby is currently incompatble.

Because it was designed with the interfaces available in Ruby 3.x, it supports the fiber scheduler which provides transparent concurrency.

class Async::Task uses Fiber#transfer for scheduling so it is compatible with all other usage of Fiber.
class Async::Reactor implements the Fiber scheduler interface and is compatible with a wide range of non-blocking operations, including DNS, Process.wait, etc.
External C libraries that use blocking operations may still block.
Ruby <= 3.0.2 has some bugs in its non-blocking thread primitives. These should be fixed in 3.0.3+.

Rails

Rails itself is generally compatible with Async and the fiber scheduler, but certain parts of Rails are not compatible with Async and have ossified ("accidental standardization") around thread-per-request as a general model. These issues are fully addressed in Rails v7.1+, which supports Rack 3 and fiber-per-request.

Enabling Fiber Per Request

Ensure you have the following configuration in your application:

class Application < Rails::Application
	config.active_support.isolation_level = :fiber
end

Pain Points

ActiveRecord with the latest version of the pg gem supports concurrent database queries.
ActiveRecord with mysql2 gem does not support asynchronous queries. Potentially fixed by https://github.com/trilogy-libraries/trilogy.
ActiveSupport::CurrentAttributes is per-isolated execution context. This means that child threads or fibers won't share the state. If you desire this, use Fiber.storage instead.