---
title: "Rails 6 adds parallel testing"
description: "Rails 6 adds parallel testing"
canonical_url: "https://www.bigbinary.com/blog/rails-6-adds-parallel-testing"
markdown_url: "https://www.bigbinary.com/blog/rails-6-adds-parallel-testing.md"
---

# Rails 6 adds parallel testing

Rails 6 adds parallel testing

- Author: Amit Choudhary
- Published: April 29, 2019
- Categories: Rails 6, Rails

We frequently think about how good it would be if we could run tests in parallel
on local so there would be less wait time for tests to be completed. Wait times
increase considerably when the count of tests are on the higher side, which is a
common case for a lot of applications.

Though CI tools like [CircleCi](https://circleci.com/) and
[Travis CI](https://travis-ci.org/) provide a feature to run tests in parallel,
there still wasn't a straightforward way to parallelize tests on local before
Rails 6.

Before Rails 6, if we wanted to parallelize tests, we would use
[Parallel Tests](https://github.com/grosser/parallel_tests).

Rails 6 adds the parallelization of tests by default. Rails 6 added
[parallelize](https://github.com/rails/rails/pull/31900) as a class method on
[ActiveSupport::TestCase](https://api.rubyonrails.org/v5.2/classes/ActiveSupport/TestCase.html)
which takes a hash as a parameter with the keys `workers` and `with`. The
`worker` key is responsible for setting the number of parallel workers. The
default value of the `worker` key is `:number_of_processors`, which finds the
number of processors on the machine and sets it as the number of parallel
workers. `with` takes two values - `:processes`, which is the default one, and
`:threads` as a value.

Rails 6 also added two hooks - `parallelize_setup`, which is called before the
processes are forked, and `parallelize_teardown`, which is called after the
processes are killed. Rails 6 also handles creation of multiple databases and
namespacing of those databases for parallel tests out of the box.

If we want to disable parallel testing, we can set the value of `workers` as 1
or less.

#### Rails 6.0.0.beta2

```ruby
class ActiveSupport::TestCase
  parallelize_setup do |worker|
    # setup databases
  end
   parallelize_teardown do |worker|
    # cleanup database
  end

  # Run tests in parallel with specified workers
  parallelize(workers: :number_of_processors)

  # Setup all fixtures in test/fixtures/*.yml for all tests in alphabetical order.
  fixtures :all

  # Add more helper methods to be used by all tests here...
end
```

Rails 6 also provides an environment variable `PARALLEL_WORKERS` to set the
number of parallel workers on runtime.

```bash
$ PARALLEL_WORKERS=10 bin/rails test
```

Here is the relevant [pull request](https://github.com/rails/rails/pull/31900)
for adding `parallelize` and
[pull request](https://github.com/rails/rails/pull/34735) for setting number of
processors as default workers count.

## Links

- [Human page](https://www.bigbinary.com/blog/rails-6-adds-parallel-testing)