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NOTE: This is the 2.x stable branch of nio4r. For the 1.x legacy branch, please see:

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::IO: Actor-based concurrency framework, uses nio4r for async I/O
  • Socketry Async: Asynchronous I/O framework for Ruby


  • 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

  • Ruby 2.2.2+
  • Ruby 2.3
  • Ruby 2.4
  • JRuby 9000

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


For discussion and general help with nio4r, email [email protected] or join on the web via the Google Group.

We're also on IRC at ##socketry on


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.


Copyright (c) 2011-2017 Tony Arcieri. Distributed under the MIT License. See LICENSE.txt for further details.

Includes libev 4.24. Copyright (c) 2007-2016 Marc Alexander Lehmann. Distributed under the BSD license. See ext/libev/LICENSE for details.