Download artifacts from circle-ci [Part 3]

2 minute read

We will explore how to optimize your test suite using Crystalball test selection library, to reduce the test run time, down to a minute. This blog post is an attempt to document my own experience of setting up crystalball alongside circle-ci parallel runs.

This is Part 3 of this series.

You can find all the links to this series here

Download crystalball data

Generally to use crystalball, all you need to do is run bundle exec crystalball. Crystalball has a custom RSpec runner. It builds a prediction and runs it.

But we want to leverage circle-ci’s ability to do test splitting based on timing data. So we can not use crystalball rspec runner as is.

We will use crystalball to build the prediction, generate the minimal number of spec files that need to be run. And then pass that list of files to circle-ci so that it can split the files based on timing data and parallelize the test run across multiple containers.

To do so, we can write a rake task to generate the minimal subset of test needed to be run

TESTFILES=$(bundle exec rake circleci:spec_files | circleci tests split --split-by=timings)

Let us extend our circle-config file from Part-2.

    executor: rspec-executor
    parallelism: 3
      - run:
          name: Run rspec in parallel
          command: |
            bundle exec rake circleci:spec_files > /tmp/spec.files
            if [ -s /tmp/spec.files ]; then
              TESTFILES=$(circleci tests split --split-by=timings --show-counts /tmp/spec.files)
              echo $TESTFILES
              [[ ! -z "${TESTFILES}" ]] && bundle exec rspec --format progress \
                                --no-color \
                                --format RspecJunitFormatter \
                                --out ~/test-results/rspec/rspec.xml \

Let us write the rake task to run the crystalball predictor and generate the test files.

namespace :ci do
  desc 'List spec files'
  task spec_files: :environment do
    original_stdout = $stdout.clone
    $stdout.reopen'/dev/null', 'w')

    _error_stream =
    output_stream =

    crystalball_ci_service =
    files = if crystalball_ci_service.runner == 'crystalball'
    ['--dry-run', '--format=json'],
              data = JSON.parse(output_stream.string)
              data['examples'].map do |example|

    original_stdout.write files.join("\n")

All this task does is

  1. checks the current branch.
  2. if it is master, returns all the test files.
  3. if it is not master, we are using to generate the test files.

Since this rake task is piping the output to circleci cli, it is important to keep the stdout in check.

Ok now we have this setup, next up we have to tackle the most important thing. Retrieving crystalball_data.yml files which store the mapped data between tests and the application code.

To do this, we will be using the env variable CRYSTALBALL_BUILD_NUM stored on the job store_crystalball_build_num discussed in Part-2.

Let’s look at the code.

class CrystalballCiService
  CRYSTALBALL_FILE_PATH = 'crystalball/crystalball_data'.freeze
  CRYSTALBALL_DIR_PATH  = Rails.root.join('crystalball')

  def initialize
    @base_url = ""\

  def download_crystalball_file
    artifacts = circleci_request(url: "#{@base_url}/#{ENV['CRYSTALBALL_BUILD_NUM']}/artifacts",
                                 method: :get)

    files = JSON
              .select { |artifact| artifact['path'].include? CRYSTALBALL_FILE_PATH }

    Dir.mkdir(CRYSTALBALL_DIR_PATH) unless File.exist?(CRYSTALBALL_DIR_PATH)

    files.each do |file|
      raw = circleci_request(url: CGI.unescape(file['url']),
                             method: :get,
                             opts: { raw_response: true })
      File.rename(raw.file.path, File.expand_path(file['path']))

  def store_crystalball_build_num
    body = { name: 'CRYSTALBALL_BUILD_NUM',
             value: ENV['CIRCLE_BUILD_NUM'] }
    circleci_request(url: "#{@base_url}/envvar", method: :post, body: body)


  def circleci_request(url:, method:, body: nil, opts: {})
      method: method,
      url: url,
      headers: { 'Circle-Token' => ENV['CIRCLE_CI_TOKEN'] },
      payload: body,

And that is it!

Now we have enough code to

  1. Run entire spec suite in master branch.
  2. Run only the minimal set of tests required for a non-master branch.

In the next part Part-4, which will be a bonus post. I will discuss why and how we should/can run the entire test suite once a PR is ready for code review.

Stay tuned! :heart: