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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Ractor incompatibility: DefaultFactory not shareable, C extension methods marked ractor-unsafe","articleBody":"The `msgpack` gem (1.8.0) cannot be used inside [Ractors](https://docs.ruby-lang.org/en/master/Ractor.html) on Ruby 4.0. Every operation - `MessagePack.pack`, `.unpack`, creating `Packer`/`Unpacker`/`Factory` instances - fails. This makes it impossible to use Ractors for parallel MessagePack serialization/deserialization.\n\n```ruby\n#!/usr/bin/env ruby\n# frozen_string_literal: true\n\n# POC: msgpack gem 1.8.x is incompatible with Ruby 4.0 Ractors\n\nrequire \"bundler/inline\"\n\ngemfile(true) do\n  source \"https://rubygems.org\"\n  gem \"msgpack\", \"~\u003e 1.8\"\nend\n\nrequire \"msgpack\"\n\nputs \"Ruby #{RUBY_VERSION}, msgpack #{MessagePack::VERSION}\"\nputs \"-\" * 50\n\ntests = {\n  \"MessagePack.pack\"       =\u003e -\u003e { Ractor.new { MessagePack.pack({\"a\" =\u003e 1}) }.value },\n  \"MessagePack.unpack\"     =\u003e -\u003e { Ractor.new(MessagePack.pack(\"hi\")) { |d| MessagePack.unpack(d) }.value },\n  \"Packer.new in Ractor\"   =\u003e -\u003e { Ractor.new { MessagePack::Packer.new.write(\"x\").to_s }.value },\n  \"Unpacker.new in Ractor\" =\u003e -\u003e { Ractor.new(MessagePack.pack(1)) { |d| u = MessagePack::Unpacker.new; u.feed(d); u.read }.value },\n  \"Factory.new in Ractor\"  =\u003e -\u003e { Ractor.new { MessagePack::Factory.new }.value },\n  \"Send Packer to Ractor\"  =\u003e -\u003e { Ractor.new(MessagePack::Packer.new) { |p| p.to_s }.value },\n  \"Access DefaultFactory\"  =\u003e -\u003e { Ractor.new { MessagePack::DefaultFactory }.value },\n}\n\ntests.each do |name, test|\n  print \"#{name}: \"\n  begin\n    test.call\n    puts \"OK\"\n  rescue Ractor::RemoteError =\u003e e\n    puts \"FAILED — #{e.cause.class}: #{e.cause.message}\"\n  rescue =\u003e e\n    puts \"FAILED — #{e.class}: #{e.message}\"\n  end\nend\n```","author":{"url":"https://github.com/mensfeld","@type":"Person","name":"mensfeld"},"datePublished":"2026-05-08T15:05:03.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/390/msgpack-ruby/issues/390"}