> ## Documentation Index
> Fetch the complete documentation index at: https://docs.webapp.io/llms.txt
> Use this file to discover all available pages before exploring further.

# SPLIT

## Parallelizing directive

Webapp.io provides a utility to run tests in parallel -
[SPLIT 5](/docs/layerfile-reference#split) duplicates the entire VM 5 times at
the point it executes. In practice this means that you can run tests in parallel
without worrying about race conditions causing flaky tests.

### Rails: knapsack

See [knapsack pro](https://github.com/KnapsackPro/rails-app-with-knapsack)

1. Install the gem
2. Run `KNAPSACK_GENERATE_REPORT=true bundle exec rspec spec` on your local
   computer
3. `git add knapsack_rspec_report.json && git commit -m 'knapsack' && git push origin master`

Your Layerfile will look something like this:

```docker Layerfile theme={null}
# install ruby, bundle install, etc

COPY . .
SPLIT 5
ENV CI_NODE_TOTAL=$SPLIT_NUM CI_NODE_INDEX=$SPLIT
RUN bundle exec knapsack:rspec
```

### Go: custom test runner

See
[this file](https://github.com/distributed-containers-inc/layer-dag-example/blob/master/service-one/parallel-go-test.sh)
for an example parallel test runner for go.

The Layerfile from that example:

```docker Layerfile theme={null}
FROM ../base/Layerfile

COPY . .
SPLIT 5
RUN ./parallel-go-test.sh
```