Rails 6 Parallel Tests

Rails 6 applications will run tests in parallel by default. The number of parallel workers to run tests can be customised. Moreover, all the parallel worker processes will have their own temporary database to run tests.

The feature is available from Rails 5.1.5+ applications. Rails applications having huge number of tests face the problem of huge runtime. Gems like Parallel Tests are used in older Rails applications to improve speed of test suite.

Need of Parallel testing in Rails

  • Parallel testing is necessary in order to reducer the run time of test suite in Rails applications
  • Tests run in single process don’t use multiple cores of the server (which are usually available these days!)


To be able to use parallel tests, call parallelize helper in test_helper.rb.

class ActiveSupport::TestCase
  parallelize(workers: 3)
  • This will split tests in 3 worker processes.
  • Each worker will use it’s own temporary database to run tests

This can be configured in Rails 5.1.5+ applications with helper described above.

Setup and Teardown Hooks

Two hooks parallelize_setup and parallelize_teardown are provided.

parallelize_setup hook is run after processes are forked, before tests are run. This can be used to perform actions that need to be done before tests are run such as create databases.

  parallelize_setup do
    # create databases

parallelize_teardown hook is run after tests are run. This can be used to clean up e.g. drop database.

  parallelize_teardown do
    # drop databases

Parallel testing with processes / threads

class ActiveSupport::TestCase
  parallelize(workers: 3)

When we set workers as described above, Rails 6 by default uses processes to run tests in parallel.

class ActiveSupport::TestCase
  parallelize(workers: 3, with: :threads)

Pass an option with and values as threads to run tests in parallel using threads.

Find out more about process / thread option in Rails Guides

Customize number of parallel workers

We can customize number of worker processes to run by settings an environment variable PARALLEL_WORKERS.

PARALLEL_WORKERS=10 bin/rails test
  • This will fork 10 processes and run tests in parallel.
  • This will override default number set in test_helper.rb



Akshay Mohite

Hi there! I am a Ruby on Rails & ReactJS Enthusiast, building some cool products at DTree Labs.

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