Thread-safety analysis for your projects, as an extension to RuboCop.

Installation and Usage

Installation into an application

Add this line to your application's Gemfile:

gem 'rubocop-thread_safety'

Install it with Bundler by invoking:

$ bundle

Add this line to your application's .rubocop.yml:

require: rubocop-thread_safety

Now you can run rubocop and it will automatically load the RuboCop Thread-Safety cops together with the standard cops.

Scanning an application without adding it to the Gemfile

Install the gem:

$ gem install rubocop-thread_safety

Scan the application for just thread-safety issues:

$ rubocop -r rubocop-thread_safety --only ThreadSafety,Style/GlobalVars,Style/ClassVars,Style/MutableConstant


There are some added configuration options that can be tweaked to modify the behaviour of these thread-safety cops.

Correcting code for thread-safety

There are a few ways to improve thread-safety that stem around avoiding unsynchronized mutation of state that is shared between multiple threads.

State shared between threads may take various forms, including:

  • Class variables (@@name). Note: these affect child classes too.
  • Class instance variables (@name in class context or class methods)
  • Constants (NAME). Ruby will warn if a constant is re-assigned to a new value but will allow it. Mutable objects can still be mutated (e.g. push to an array) even if they are assigned to a constant.
  • Globals ($name), with the possible exception of some special globals provided by ruby that are documented as thread-local like regular expression results.
  • Variables in the scope of created threads (where is called).

Improvements that would make shared state thread-safe include:

  • freeze objects to protect against mutation. Note: freeze is shallow, i.e. freezing an array will not also freeze its elements.
  • Use data structures or concurrency abstractions from concurrent-ruby, e.g. Concurrent::Map
  • Use a Mutex or similar to synchronize access.
  • Use ActiveSupport::CurrentAttributes
  • Use RequestStore
  • Use Thread.current[:name]


After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at

Copyright (c) 2016-2021 CoverMyMeds. See LICENSE.txt for further details.