# Scaling Rails

- Test suite should not block production
- Application should be able to handle heavier load of traffic on application naturally

Goal is to make Rails 6 scalable by default. An intuitive software to automatically scale in places that often require a lot of configuration.

## Parallel Testing

- Allow forked processes or threads to run concurrent tests
- Speed up test suite
- No longer run tests linearly

Forked processes will work with dRB (distributed Ruby). `parallelize` method from the `Parallelization` class.
`def start` in `Parallelization` creates a worker pool and `DRb` object to split the work for each process.

A `reporter` collects results from all tests. Will be automatically included in Rails 6 test framework. Rails will automatically parallelize with two workers.
databases and workers are deleted after testing in `run_cleanup`. Rails will automatically create and delete databases in tests.

## Threaded testing

- Windows will require threads (also JRuby)
- I/O bound tests will also need threads
- _Threaded Parallelizer_ is `parallelize(workers: 2, with: :threads)` simply adding that threaded parallization is used.

## Multiple databases

- Improvements to using multiple databses
- If your application is crashing due to read/writes to a single database, you may want to cluster. Multiple databses can be used in Rails 5, but could be easier.
- Currently there's no documentation fo rmulti-DB set up for YAML.
- Should be grouped in tiers:

```
production:
  primary:
    <<: *default
    database: db_production
  animals:
    <<: *default
    database: db_production_animals
```
- As convention, create migrations for each database into their own directory.
- Currently, Rails doesn't support migrations for multiples DBs
-