---
title: Test Parallelization Best Practices
description: >-
  Optimize Test Parallelization planning and test discovery for Ruby and Rails
  test suites.
breadcrumbs: >-
  Docs > Test Optimization in Datadog > Test Parallelization > Test
  Parallelization Best Practices
---

# Test Parallelization Best Practices

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## Optimize the planning step{% #optimize-the-planning-step %}

Test Parallelization adds a planning step that discovers tests before execution. For example, RSpec projects use dry-run discovery. Keep this step lightweight so the time saved by parallel execution is not offset by planning overhead.

### Preinstall system dependencies with Docker{% #preinstall-system-dependencies-with-docker %}

If your tests need operating system packages, include them in a CI base image instead of installing them during every CI run.

In the `ci/Dockerfile.test` file:

```dockerfile
FROM ruby:3.3
RUN apt-get update && DEBIAN_FRONTEND=noninteractive \
    apt-get install -y --no-install-recommends imagemagick libpq-dev \
 && rm -rf /var/lib/apt/lists/*
WORKDIR /app
```

### Cache project dependencies{% #cache-project-dependencies %}

Use your CI provider dependency cache. For example, GitHub Actions can cache Bundler dependencies with `ruby/setup-ruby`:

```yaml
- uses: ruby/setup-ruby@v1
  with:
    ruby-version: 3.3
    bundler-cache: true
```

### Skip database setup during discovery{% #skip-database-setup-during-discovery %}

Discovery does not execute tests, so database setup, migrations, seeds, and fixtures are often unnecessary during the planning step.

During discovery, `DD_TEST_OPTIMIZATION_DISCOVERY_ENABLED` is set to `1`. Use this variable to skip expensive setup code during planning.

For example, in Rails:

```ruby
# in seeds.rb
return if ENV["DD_TEST_OPTIMIZATION_DISCOVERY_ENABLED"].present?
# your seeds here

# in rails_helper.rb
ActiveRecord::Migration.maintain_test_schema! unless ENV["DD_TEST_OPTIMIZATION_DISCOVERY_ENABLED"].present?

RSpec.configure do |config|
  unless ENV["DD_TEST_OPTIMIZATION_DISCOVERY_ENABLED"].present?
    config.use_transactional_fixtures = true
  else
    config.use_transactional_fixtures = false
    config.use_active_record = false
  end
end
```

After these changes, test discovery can run faster and avoid failures when the database is unavailable during planning.

## Configure Minitest in non-Rails projects{% #configure-minitest-in-non-rails-projects %}

For non-Rails Minitest projects, `ddtest` uses `bundle exec rake test` and passes selected files in the `TEST_FILES` environment variable. Configure your `Rake::TestTask` to read `TEST_FILES`:

```ruby
Rake::TestTask.new(:test) do |test|
  test.test_files = ENV["TEST_FILES"] ? ENV["TEST_FILES"].split : ["test/**/*.rb"]
end
```

## Further reading{% #further-reading %}

- [Set up Test Parallelization](https://docs.datadoghq.com/tests/test_parallelization/setup.md)
- [Configure Test Parallelization](https://docs.datadoghq.com/tests/test_parallelization/configuration.md)
- [Troubleshooting Test Parallelization](https://docs.datadoghq.com/tests/test_parallelization/troubleshooting.md)