CircleCI 2.0 with parallelism & simplecov for Rails

config.yml

# Ruby CircleCI 2.0 configuration file
#
# Check https://circleci.com/docs/2.0/language-ruby/ for more details

defaults: &defaults
  working_directory: ~/split_app
  parallelism: 2

  docker:
    - image: circleci/ruby:2.5.0-node-browsers
      environment:
        PGHOST: localhost
        PGUSER: split
        RAILS_ENV: test

    - image: circleci/postgres:9.6
      environment:
        POSTGRES_USER: split
        POSTGRES_DB: split_test
        POSTGRES_PASSWORD: ""

version: 2
jobs:
  test:
    <<: *defaults
    steps:
      - checkout

      - save_cache:
          key: v2-repo-{{ .Environment.CIRCLE_SHA1 }}
          paths:
            - ~/split_app

      - restore_cache:
          keys:
            - gem-cache-{{ arch }}-{{ .Branch }}-{{ checksum "Gemfile.lock" }}
            - gem-cache-{{ arch }}-{{ .Branch }}
            - gem-cache
      - run: bundle install --path vendor/bundle
      - save_cache:
          key: gem-cache-{{ arch }}-{{ .Branch }}-{{ checksum "Gemfile.lock" }}
          paths:
            - ~/split_app/vendor/bundle

      - restore_cache:
          keys:
            - yarn-cache-{{ arch }}-{{ .Branch }}-{{ checksum "yarn.lock" }}
            - yarn-cache-{{ arch }}-{{ .Branch }}
            - yarn-cache
      - run:
          name: Yarn Install
          command: yarn install --cache-folder ~/.cache/yarn
      - save_cache:
          key: yarn-cache-{{ arch }}-{{ .Branch }}-{{ checksum "yarn.lock" }}
          paths:
            - ~/split_app/.cache/yarn

      - run: cp .env.test .env
      - run: bundle exec rubocop
      - run: bundle exec brakeman -z
      - run: dockerize -wait tcp://localhost:5432 -timeout 1m
      - run: bundle exec rake db:schema:load

      - run: 
          name: Parallel Rspec
          environment:
            - RAILS_ENV: test
            - RACK_ENV: test
          command: |
            mkdir -p /tmp/rspec
            TEST_FILES="$(circleci tests glob "spec/**/*_spec.rb" | circleci tests split --split-by=timings)"
            bundle exec rspec --profile 10 \
                              --format RspecJunitFormatter \
                              --out /tmp/rspec/rspec.xml \
                              --format progress \
                              -- \
                              $TEST_FILES
      - store_test_results:
          path: /tmp/rspec

      - run:
          name: Stash Coverage Results
          command: |
            mkdir coverage_results
            cp -R coverage/.resultset.json coverage_results/.resultset-${CIRCLE_NODE_INDEX}.json
      - persist_to_workspace:
          root: .
          paths:
            - coverage_results

  coverage:
    <<: *defaults
    steps:
      - attach_workspace:
          at: .

      - restore_cache:
          key: v2-repo-{{ .Environment.CIRCLE_SHA1 }}

      - restore_cache:
          keys:
            - gem-cache-{{ arch }}-{{ .Branch }}-{{ checksum "Gemfile.lock" }}
            - gem-cache-{{ arch }}-{{ .Branch }}
            - gem-cache

      - run: bundle install --path vendor/bundle

      - run:
          name: Merge and check coverage
          command: |
            RUN_COVERAGE=true bundle exec rake simplecov:report_coverage
      - store_artifacts:
          path: ~/split_app/coverage
          destination: coverage

workflows:
  version: 2
  build_and_test:
    jobs:
        - test
        - coverage:
            requires: 
              - test

rails_helper.rb

# frozen_string_literal: true

# spec/rails_helper.rb
require 'active_support/inflector'
require 'simplecov'

SimpleCov.start 'rails' do
  add_filter '/spec/'
  add_filter '/config/'
  add_filter '/vendor/'
  Dir['app/*'].each do |dir|
    add_group File.basename(dir).humanize, dir
  end
  minimum_coverage 100 unless ENV['CIRCLE_JOB']
end

# ...

simplecov_helper.rb

# frozen_string_literal: true

# spec/simplecov_helper.rb
require 'active_support/inflector'
require "simplecov"

class SimpleCovHelper
  def self.report_coverage(base_dir: "./coverage_results")
    SimpleCov.start 'rails' do
      skip_check_coverage = ENV.fetch("SKIP_COVERAGE_CHECK", "false")

      add_filter '/spec/'
      add_filter '/config/'
      add_filter '/vendor/'

      Dir['app/*'].each do |dir|
        add_group File.basename(dir).humanize, dir
      end

      minimum_coverage(100) unless skip_check_coverage
      merge_timeout(3600)
    end
    new(base_dir: base_dir).merge_results
  end

  attr_reader :base_dir

  def initialize(base_dir:)
    @base_dir = base_dir
  end

  def all_results
    Dir["#{base_dir}/.resultset*.json"]
  end

  def merge_results
    results = all_results.map { |file| SimpleCov::Result.from_hash(JSON.parse(File.read(file))) }
    SimpleCov::ResultMerger.merge_results(*results).tap do |result|
      SimpleCov::ResultMerger.store_result(result)
    end
  end
end

simplecov_parallel.rake

# frozen_string_literal: true

# lib/tasks/simplecov_parallel.rake
if Rails.env.test?
  require_relative "../../spec/simplecov_helper"
  namespace :simplecov do
    desc "merge_results"
    task report_coverage: :environment do
      SimpleCovHelper.report_coverage
    end
  end
end

Thanks @trev :), I referred this and implemented for Github action using artifacts