Add test parallelization to Rails

Provides both a forked process and threaded parallelization options. To
use add `parallelize` to your test suite.

Takes a `workers` argument that controls how many times the process
is forked. For each process a new database will be created suffixed
with the worker number; test-database-0 and test-database-1
respectively.

If `ENV["PARALLEL_WORKERS"]` is set the workers argument will be ignored
and the environment variable will be used instead. This is useful for CI
environments, or other environments where you may need more workers than
you do for local testing.

If the number of workers is set to `1` or fewer, the tests will not be
parallelized.

The default parallelization method is to fork processes. If you'd like to
use threads instead you can pass `with: :threads` to the `parallelize`
method. Note the threaded parallelization does not create multiple
database and will not work with system tests at this time.

parallelize(workers: 2, with: :threads)

The threaded parallelization uses Minitest's parallel exector directly.
The processes paralleliztion uses a Ruby Drb server.

For parallelization via threads a setup hook and cleanup hook are
provided.

```
class ActiveSupport::TestCase
  parallelize_setup do |worker|
    # setup databases
  end

  parallelize_teardown do |worker|
    # cleanup database
  end

  parallelize(workers: 2)
end
```

[Eileen M. Uchitelle, Aaron Patterson]

1 files changed, 83 insertions, 0 deletions

diff --git a/guides/source/testing.md b/guides/source/testing.md
index af6127f4a5..5d2b8bc5d6 100644
--- a/guides/source/testing.md
+++ b/guides/source/testing.md
@@ -462,6 +462,89 @@ Rails options:
     -c, --[no-]color                 Enable color in the output
 ```
 
+Parallel Testing
+----------------
+
+Parallel testing allows you to parallelize your test suite. While forking processes is the
+default method, threading is supported as well. Running tests in parallel reduces the time it
+takes your entire test suite to run.
+
+## Parallel testing with processes
+
+The default parallelization method is to fork processes using Ruby's DRb system. The processes
+are forked based on the number of workers provided. The default is 2, but can be changed by the
+number passed to the parallelize method. Active Record automatically handles creating and
+migrating a new database for each worker to use.
+
+To enable parallelization add the following to your `test_helper.rb`:
+
+```
+class ActiveSupport::TestCase
+  parallelize(workers: 2)
+end
+```
+
+The number of workers passed is the number of times the process will be forked. You may want to
+parallelize your local test suite differently from your CI, so an environment variable is provided
+to be able to easily change the number of workers a test run should use:
+
+```
+PARALLEL_WORKERS=15 bin/rails test
+```
+
+When parallelizing tests, Active Record automatically handles creating and migrating a database for each
+process. The databases will be suffixed with the number corresponding to the worker. For example, if you
+have 2 workers the tests will create `test-database-0` and `test-database-1` respectively.
+
+If the number of workers passed is 1 or fewer the processes will not be forked and the tests will not
+be parallelized and the tests will use the original `test-database` database.
+
+Two hooks are provided, one runs when the process is forked, and one runs before the processes are closed.
+These can be useful if your app uses multiple databases or perform other tasks that depend on the number of
+workers.
+
+The `parallelize_setup` method is called right after the processes are forked. The `parallelize_teardown` metod
+is called right before the processes are closed.
+
+```
+class ActiveSupport::TestCase
+  parallelize_setup do |worker|
+    # setup databases
+  end
+
+  parallelize_teardown do |worker|
+    # cleanup database
+  end
+
+  parallelize(workers: 2)
+end
+```
+
+These methods are not needed or available when using parallel testing with threads.
+
+## Parallel testing with threads
+
+If you prefer using threads or are using JRuby, a threaded parallelization option is provided. The threaded
+parallelizer is backed by Minitest's `Parallel::Executor`.
+
+To change the parallelization method to use threads over forks put the following in your `test_helper.rb`
+
+```
+class ActiveSupport::TestCase
+  parallelize(workers: 2, with: :threads)
+end
+```
+
+Rails applications generated from JRuby will automatically include the `with: :threads` option.
+
+The number of workers passed to `parallelize` determines the number of threads the tests will use. You may
+want to parallelize your local test suite differently from your CI, so an environment variable is provided
+to be able to easily change the number of workers a test run should use:
+
+```
+PARALLEL_WORKERS=15 bin/rails test
+```
+
 The Test Database
 -----------------