Worked on the Shopify/ci-queue repository to enhance concurrency safety and reliability in the CI queueing system. Addressed thread-safety concerns by replacing the standard Set with Concurrent::Set, leveraging the concurrent-ruby gem to improve concurrent state management. Fixed a bug in the worker reservation logic to prevent nil reservations when no tests are available, ensuring more robust handling of test assignments. Focused on backend development using Ruby and Ruby on Rails, with an emphasis on CI/CD workflows and concurrency control. The work resulted in improved throughput and reliability of the CI queue under concurrent load, demonstrating careful attention to state mutations.