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Manually Setting Up Parallelism

If you want the benefits of parallel testing, and you’re not using one of our automatically supported test runners, or if you’ve overridden our test commands, you’ll still be able to set up parallelism and reduce your test run-times.

To begin with, you’ll need to turn on parallelism from your project’s settings page. Go to Project Settings > Parallelism to adjust the settings.

Splitting your test suite

When you use CircleCI’s parallelization, we run your code on multiple separate VMs. To use parallelism, you make your test runner run only a subset of tests on each VM.

There are two mechanisms for splitting tests among nodes: using the files configuration modifier (a very simple and straightforward way for most use cases), or using parallelism environment variables (aimed at more complex scenarios).

Either way, there are a few things to keep in mind:

  • test and deploy sections in your circle.yml are single-node only by default and will only run on node 0. However, you can override test sections per command (add : to the end of each command you’d like to parallelize, then add a double-indented parallel: true below that).
  • Inferred test commands will automatically parallelize.
  • Currently, commands that only run in node 0 will erroneously show up in the UI as running on all nodes.

Using the files configuration modifier

Parallelizing test runners that accept file names is straightforward! The files modifier can list paths to the test files, and CircleCI will run the test runners with different test files in each node. For example, to parallelize an rspec command, you can set the following:

    - bundle exec rspec:
        parallel: true
          - spec/unit/sample.rb   # can be a direct path to file
          - spec/**/*.rb          # or a glob (ruby globs)

The ** pattern matches multiple directory layers with Ruby globs. You can test glob patterns locally with e.g. ruby -e "puts Dir.glob('**/*.py')".

In this example, we will run bundle exec rspec in all nodes appended with roughly 1/N of the files on each VM (where N is the number of nodes).

By default, the file arguments will be appended to the end of the command.

Using environment variables

For more control over parallelism, we use environment variables to denote the number of VMs and to identify each one, and you can access these from your test runner:

is the total number of parallel VMs being used to run your tests on each push.
is the index of the particular VM. CIRCLE_NODE_INDEX is indexed from zero.

A simple example

If you want to run the two commands rake spec and npm test in parallel, you can use a bash case statement:

    - case $CIRCLE_NODE_INDEX in 0) rake spec ;; 1) npm test ;; esac:
        parallel: true

Note the final colon, and parallel: true (double-indented) on the next line.

This is a command modifier which tells CircleCI that the command should be run in parallel on all test machines. It defaults to true for commands in the machine, checkout, dependencies and database build phases, and it defaults to false for commands in the test phases.

Please note that since we do not support parallel deployment, specifying parallel: true in the deployment phase will cause an error.

Obviously, this is slightly limited because it’s hard-coded to only work with two nodes, and the test time might not balance across all nodes equally.

Automatic balancing with manual parallel setup

With some of the inferred test commands, we automatically balance the tests so that the total execution time is minimised.

If you are overriding the default test command as explained above and still want the tests to be automatically balanced, you’ll need to make sure that the test metadata is collected properly. For example, you would modify the initial RSpec command like this to include the RSpec Junit formatter:

    - bundle exec rspec --format progress --format RspecJunitFormatter --out $CIRCLE_TEST_REPORTS/rspec.xml:
        parallel: true
          - spec/unit/sample.rb   # can be a direct path to file
          - spec/**/*.rb          # or a glob (ruby globs)

The same principle applies to Cucumber tests — to enable automatic test balancing when you manually specify the Cucumber command, you would include the following in your circle.yml:

    - mkdir -p $CIRCLE_TEST_REPORTS/cucumber:
        parallel: true
    - bundle exec cucumber --format json --out $CIRCLE_TEST_REPORTS/cucumber/tests.cucumber:
        parallel: true
          - spec/feature/*.feature


A more powerful version evenly splits all test files across N nodes. We recommend you write a script that does something like:


testfiles=$(find ./test -name '*.py' | sort | awk "NR % ${CIRCLE_NODE_TOTAL} == ${CIRCLE_NODE_INDEX}")

if [ -z "$testfiles" ]
    echo "more parallelism than tests"
    test-runner $testfiles

This script partitions the test files into N equally sized buckets, and calls “test-runner” on the bucket for this machine. Note that you will still need to include parallel: true in circle.yml with this script.

Test suite split with knapsack gem

You can parallelize tests for RSpec, Cucumber, Minitest, Spinach and Turnip with knapsack gem. It will split tests across CI nodes and it makes sure that tests will run comparable time on each CI node. Knapsack gem has built in support for CircleCI.

Build matrix support

CircleCI 1.0 does not have native support for build matrices. However, there is a community project not maintained by CircleCI called circleci-matrix which can be used to produce build matrix behaviour on CircleCI. Please see their documentation for more details. Note that in CircleCI 2.0 you can use parallel jobs to accomplish build matrix behaviors.