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New I/O for Ruby (nio4r): cross-platform asynchronous I/O primitives for scalable network clients and servers. Modeled after the Java NIO API, but simplified for ease-of-use.

nio4r provides an abstract, cross-platform stateful I/O selector API for Ruby. I/O selectors are the heart of "reactor"-based event loops, and monitor multiple I/O objects for various types of readiness, e.g. ready for reading or writing.

Projects using nio4r

  • ActionCable: Rails 5 WebSocket protocol, uses nio4r for a WebSocket server
  • Celluloid: Actor-based concurrency framework, uses nio4r for async I/O
  • Async: Asynchronous I/O framework for Ruby
  • Puma: Ruby/Rack web server built for concurrency


  • Expose high-level interfaces for stateful IO selectors
  • Keep the API small to maximize both portability and performance across many different OSes and Ruby VMs
  • Provide inherently thread-safe facilities for working with IO objects

Supported platforms

Supported backends

  • libev: MRI C extension targeting multiple native IO selector APIs (e.g epoll, kqueue)
  • Java NIO: JRuby extension which wraps the Java NIO subsystem
  • Pure Ruby: backend that should work on any Ruby interpreter


Please see the nio4r wiki for more detailed documentation and usage notes:

  • Getting Started: Introduction to nio4r's components
  • Selectors: monitor multiple IO objects for readiness events
  • Monitors: control interests and inspect readiness for specific IO objects
  • Byte Buffers: fixed-size native buffers for high-performance I/O

See also:


nio4r is not a full-featured event framework like EventMachine or Instead, nio4r is the sort of thing you might write a library like that on top of. nio4r provides a minimal API such that individual Ruby implementers may choose to produce optimized versions for their platform, without having to maintain a large codebase.


Bump the version first:

bundle exec bake gem:release:version:patch


rake clean
rake release


You might need to delete Gemfile.lock before trying to bundle install.

# Ensure you have the correct JDK:
pacman -Syu jdk-openjdk
archlinux-java set java-19-openjdk

# Ensure you are using jruby:
chruby jruby
bundle update

# Build the package:
rake clean
rake compile
rake release